Return to Resistance

Breeding Crops to Reduce Pesticide Dependence

Raoul A. Robinson

Copyright © 1996 by Raoul A. Robinson
All rights reserved. No part of this book may be reproduced
in any form without the written consent of the publisher.

Published in the United States of America by
agAccess, 603 Fourth St., Davis CA 95616
and
Published in Canada by the
International Development Research Centre
P.O. Box 8500, Ottawa, Canada K1G 3H9

ISBN 0-932857-17-5 (agAccess)
ISBN 0-88936-774-4 (IDRC)

Canadian Cataloguing-in-Publication Data

Robinson, R.A.

Return to resistance: breeding crops to reduce pesticide dependence. Ottawa, ON, IDRC; Davis, CA, agAccess, 1995. 500p.

/Plant breeding/, /plant genetics/, /disease resistance/, /pest control/ - / Green Revolution/, /crops/, /parasites/, /farmers' associations/, /case studies/, / evaluation/, references.

UDC: 631.52

A microfiche edition is available.

Cover design by DelRae Roth
Book design by Timothy Rice

Printed in the United States of America

This book is dedicated to

Luigi Chiarappa
and
Roberto Garcia Espinosa

Contents

Acknowledgments

Much of this book results from some ten years of teaching at Colegio de Postgraduados, Montecillos, Mexico. In its turn, most of this teaching was made possible by a grant from the Canadian International Development Research Centre, which generously provided this College with funds for both the breeding of beans for comprehensive horizontal resistance, and for participation by Canadian scientists, including myself. I am grateful to IDRC for their generosity, and to the faculty and students of this College for encouragement and inspiration that would have been difficult to find anywhere else.

In addition, I have been co-operating with Universidad Autónoma de Chapingo, Mexico, and I am grateful to the faculty and students of this University also, for comparable support and stimulation.

Introduction

To anyone who is concerned about the environment, it is obvious that all is not well with modern crop husbandry. One problem is that pests and diseases are destroying about one fifth of all crop production. A second problem is that these losses occur in spite of an extravagant use of chemical insecticides and fungicides that cost billions of dollars each year, worldwide. Indeed, in the industrial countries, the use of some kinds of crop protection chemicals has increased nearly tenfold since World War II. Crop production has increased also, very considerably, but so have the crop losses due to parasites, in spite of this increased use of crop protection chemicals.

This kind of parasite damage obviously does not occur in wild ecosystems. After all, we do not spray wild plants, and the world is still green. So why should such appalling pest and disease losses occur in agro-ecosystems, in spite of all this spraying with crop protection chemicals?

This book is addressed mainly to readers who are concerned about the world food supply, and the pollution of our environment with chemical pesticides, but who lack detailed scientific knowledge about these matters. It is also addressed to people who are not scientists, but who are prepared to make an effort to study a new subject that is outside their own fields of expertise. It presents a somewhat complicated and technical topic, but it is written in plain English which, I believe, will be readily comprehensible to anyone who is reasonably willing to persevere. I also believe that readers who do persevere will be excited by their new knowledge, and will feel amply rewarded for their trouble. The book is also addressed to activists who want to put things right, and it explains a possible way of doing this.

The Carrying Capacity of the Environment

A biologist has a rather special way of looking at human history, based on environment, and the carrying capacity of that environment. For any wild species, the carrying capacity of the environment is strictly limited. One square mile of land in a given area can carry only so many members of a species, and no more. It is also a fundamental law of nature that every species tends to reproduce beyond the carrying capacity of its environment. In any species, there is always a surplus of individuals which the environment cannot support, and it is always the weak that must go to the wall. This is the very basis of evolution, and it is the mechanism of natural selection, and the survival of the fittest. Indeed, it is probably more accurate to describe natural selection as the elimination of the least fit, rather than the survival of the most fit.

In the whole history of evolution, only one species has been able to increase the carrying capacity of its environment to any significant extent. That species is us. We did this with a series of cultural developments that are far ahead of anything achieved by the proto-cultures of wild primates. First we developed stone tools that turned a plant gathering species into a scavenger, and later into a skilled, indeed a devastating, hunter of wild animals. Humankind then became a hunter-gatherer in an environment which, until then, had required up to twenty five square miles of territory to support one human adult.

Because these new hunter-gatherers continued to reproduce beyond the carrying capacity of their environment, there was always a surplus of people. This surplus often survived by migrating to a new, uninhabited area. Humans could do this more readily than other species because they had the cultural developments of tools, animal skin clothing, fire, and artificial shelters. Eventually, our ancestors colonised all the habitable land surfaces of the planet. No one knows for sure what the size of the total human population was at that time, but it is estimated at only a few million.

When there was no spare land left to colonise, pressures of population began to be felt, and it was at this point that humankind began the process of domestication. Animals were domesticated first. People began to live with herds of wild herbivores, much as modern Lapps live with herds of reindeer. The people protected the herd from wild carnivores, but they also culled the herd of unwanted males to provide meat, as well as leather, horn, and bone, for the manufacture of tents, clothing, and tools. These people were herders, and their population density was higher than that of the more backward hunter-gatherers. Herders occupied much of Africa and Asia for many millennia, and modern cattle, sheep, and goats are descended from their herds.

The next major development was the domestication of plants. People discovered that they could increase the density of edible plants in their environment by sowing the seeds of these plants. They also discovered that they could choose which seeds to sow. By sowing only seeds taken from the best plants, with the highest yield, and highest quality of food, they tended to improve both the yield and the quality of their crops. In the course of time, this process changed some species of cultivated plants so much that their wild progenitors are now difficult to recognise. This domestication of plants was the basis of agriculture, because crops provide food for both people and domestic animals. Non-food, or industrial crops were also domesticated. These include fibre crops, such as cotton and hemp, as well as various medicinal, narcotic, perfume, and oil plants.

This series of agricultural breakthroughs during the past ten thousand years has increased the human carrying capacity of Planet Earth by several hundred fold. But, unfortunately, the human species still continues to reproduce beyond the carrying capacity of its environment. Very recently, during the present century, a series of medical breakthroughs has complicated this situation even further. Medical science has produced some dramatic reductions in the human death rate, particularly the infant mortality rate. As a result, some three billion people are now alive who would otherwise have died. This must surely rank as one of the greatest achievements ever accomplished by humankind.

Unfortunately, there has not been a corresponding reduction in the human birth rate. Medical science has produced the techniques necessary for reducing the human birth rate to levels commensurate with the reduced death rate, but much of humankind either cannot, or will not, use them. As a consequence, our population has been doubling every thirty years. This means that agriculturists have had to double the carrying capacity of our total environment every thirty years also. So far, they have succeeded, but what of the next thirty years? And the thirty years after that? This is quite a predicament. It is known as the world food problem.

The crisis of population growth and food supply is frightening and, if our population growth is not stabilised soon, we may yet see a wave of malnutrition, and death from starvation, that would make the contemporary epidemic of AIDS seem trivial in comparison. The problem is increased by the fact that even our current levels of agricultural production are possible only with an extravagant use of chemical pesticides. It appears that, if we are to reduce pesticide pollution, by reducing pesticide use, we can do so only at the expense of the world food supply, because reduced pesticide use will lead to increased crop losses from pests. And, conversely, if we are to increase the world food supply, to feed an increasing world population, we shall have to use additional pesticides, or more powerful pesticides. Environmentalists who abhor the use of crop protection chemicals must realise that there is a very real dilemma. We may be compelled to choose between food and pollution, on the one hand, or purity and famine, on the other.

In fact, there may be a solution to this dilemma, and that is what this book is about. There is a very real possibility that we can have both adequate food and freedom from crop protection chemicals, but few people seem to be aware of this. The purpose of this book, therefore, is to make public some rather specialised information that has remained obscure, indeed secret, because of its technical nature. I do not suggest that anyone has been secretive, or that any attempt at a cover-up has been made. There is no conspiracy. But the topic is both scientific and complicated, and it has remained hidden from the general public for this reason only. In writing this book, my task has been to explain this situation in terms intelligible to the scientific layperson. If I have been successful in this explanation, readers should have little difficulty in comprehending it, scientific and complicated though it may appear at first sight.

Readers are accordingly offered a brief description of crop science and crop parasites. They are then asked to study ten pairs of biological contrasts, and some general conclusions and specific examples.

Anyone requiring greater scientific detail is referred to appendices at the end of this book. Readers who require technical descriptions and scientific references are referred to a technical book of mine, as well as some of the writings of J.E. Vanderplank (see bibliography). These technically informed readers will appreciate that the present account involves some deliberate over-simplification. This is essential because there is a limit to the scientific complexity that non-scientists can be expected to absorb. At the opposite extreme, readers who are willing to accept the scientific aspects of this account unread, may safely skip to Part II, making use of the glossary as necessary. The same comment applies to any reader who attempts Part I, but finds it too complex.

First, however, a disclaimer is necessary. This book may give the impression of being highly critical of modern plant breeding, but such an impression is not strictly true. Plant breeding has four broad objectives. These are to improve the yield, the quality of crop product, the agronomic suitability, and the resistance to pests and diseases, of the crop in question. Plant breeding has been spectacularly successful in the first three of these objectives. This is demonstrated by very large increases in agricultural production, and the fact that the world is still able to feed itself in spite of massive increases in the size of the human population. However, the last of these objectives, the breeding of crops for resistance to their parasites, has been spectacularly unsuccessful. This is why we use chemical pesticides on our crops in such large quantities. This book is accordingly critical only of plant breeding for resistance to pests and diseases.

Crop Science & Crop Parasites

The scientific study of agriculture is divided into a number of sub-disciplines, based on animals, plants, climate, and soils. The various disciplines that deal with crops are collectively known as crop science. They include plant breeding, plant physiology, plant pathology (i.e., plant diseases), crop entomology (i.e., insect pests of plants), and weed science.

Plant pathologists study plant diseases, which are mostly caused by pathogens, such as microscopic fungi, bacteria, and viruses. Crop entomologists study the insect pests that eat our crops. All the pests and diseases of crops are collectively known as crop parasites. The chemicals that are used to control crop parasites are generally known as pesticides, and they include fungicides and insecticides.

Weeds are competitors, not parasites, and the use of the term "parasite" specifically excludes weeds from the discussion. This book is not concerned with the control of weeds, even though weeds are often included in the term "pest". Nor is this book concerned with the group of chemicals known as weed-killers, or herbicides, even though these substances are often included in the term "pesticide".

Parasites are organisms which feed on other organisms, known as their hosts, while these hosts are still alive, but usually without killing them. In this respect, they differ from predators which kill, and entirely consume, their prey. Throughout our discussion of parasites, the crop plant is the host, and the pest or pathogen is the parasite. The terms host and parasite may be applied to an individual or to a population.

No one is quite sure how much damage parasites are doing to our crops because this happens to be an exceptionally difficult measurement to make. Most crop scientists accept the general estimate that crop parasites are destroying about thirty percent of all crop produce, worldwide. This loss includes both pre-harvest and post-harvest damage. That is, it includes the losses in both the field and the store. This book is about pre-harvest losses only, and these are thought to be about two thirds of the total. So, very approximately, pre-harvest parasites are destroying about twenty percent of our total crop production. In terms of food crops alone, pre-harvest crop parasites may be destroying enough food to feed about one billion people. What makes this sad story even more sad is that we are losing this crop produce in spite of an extravagant use of chemical pesticides on our crops. It is difficult to escape the impression that all is not well with modern crop science.

Let us now examine those ten pairs of biological contrasts, which are summarized in the figures that open each of the next ten chapters.

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PART ONE
Explanations

CHAPTER ONE
Genetics:
Biometricians and Mendelians

This story begins in 1900, which is a convenient date, being both easy to remember, and the start of the century. In that year, three European scientists simultaneously made an important discovery. These scientists were Hugo de Vries in The Netherlands, Carl Correns in Germany, and Erich Tschermak von Seysenegg in Austria. They discovered the now famous genetic work of Gregor Mendel. Within a year, Mendel's neglected paper had been re-published in German, French, and English, and biology would never be the same again. In particular, there were now two schools of thought in the study of genetics.

Members of the older school called themselves biometricians. They studied the inheritance of characters that are quantitatively variable. These are characters that differ in degree, with every grade of difference between a minimum and a maximum. For example, in flowers, the colour pink can show every degree of difference, and every shade of pink, between the maximum, which is pure red, and the minimum, which is pure white. This was the kind of genetics studied by most of the great biological thinkers of the nineteenth century, such as Charles Darwin, Thomas Huxley, and Francis Galton. Their "bio-metrics" (i.e., life-measurements) assessed quantitative data of many different variables, with continuous scales of measurement.

These variable data are usually analysed by a branch of mathematics called statistics, and their basis is the Gaussian, or bell-shaped, curve (Appendix A). The term 'statistics' has a pejorative use also, illustrated by the phrase "Lies, damn lies, and statistics", which is discussed further in Chapter 14. However, in a mathematical context, the term is entirely respectable.

Typically, if two different parent plants, such as a red-flowered and a white-flowered, were crossed (i.e., cross-pollinated, or mated), the progeny would show all degrees of pinkness, but most of them would be a mid-pink, about halfway between the two parents. The proportion of each degree of pinkness in a large progeny would be represented by the bell-shaped curve shown in Appendix A, and it would be called a normal distribution.

In 1900, the biometricians did not understand the mechanism of their genetics. They thought that inherited characters would blend, or merge, with each other, in much the same way as milk and chocolate would blend in a cup of hot cocoa. They could not see any possibility of the discrete units of inheritance, which we now call genes. This was where Mendel came into the picture.

The new school of genetics called themselves Mendelians. They studied the inheritance of characters that are qualitatively variable. These are characters that differ in kind, being either present or absent, with no intermediates. Thus, in seeds, the character of blackness is either showing or not showing. A bean seed, for example, would be either black or white, and there would be no grey seeds at all. The importance of Mendel's laws of inheritance is that they postulate discrete units of inheritance, and they successfully predict the proportion of the progeny which will either show, or not show, a qualitative character.

Each discrete unit of inheritance is called a gene. Each gene is a unit of DNA code on a microscopic chromosome, and each chromosome occurs twice in an individual. One chromosome comes from the male parent, and the other from the female parent, because each of the reproductive cells, the pollen and the ovules, has only one set of chromosomes. Each chromosome has a copy of the gene, and each of these copies is called an allele.

A gene might control seed colour, which is either white or black. Conventionally, such a gene would be represented by a capital letter, such as W. The capital letter represents a dominant allele that eclipses the effects of a recessive allele which is represented by the lowercase letter, w. A plant that is WW has two dominant alleles for blackness, with one coming from each parent. A plant that is Ww has one dominant and one recessive allele. And a plant that is ww has two recessive alleles. A plant that is WW is black-seeded. So is a plant that is Ww, because the dominant allele eclipses the recessive allele. Only a plant that is ww is white-seeded.

Two other technical terms, and one further point, should be mentioned. A plant that is either WW or ww has two alleles that are the same. They are either both dominant, or both recessive. Such a plant is described as homozygous. However, a plant that is Ww has two different alleles, one dominant and one recessive, and it is described as heterozygous. These terms are derived from the Greek root zygo, meaning a yoke, as in the yoke that links two oxen pulling a cart, while homo = same, and hetero= different. A zygote is produced by the fusion of two sex cells.

Homozygous thus means that the two alleles, coming from the male and female sex cells, were the same genetically, while heterozygous means that they were different. The terms are normally used in this way, and are applied to a single pair of alleles. However, in plants, they can be applied to the entire genetic make-up of an individual. It is usual for all living things to be heterozygous in most of their genetic make-up, because this is the basis of variation, natural selection, survival of the fittest, and evolution. But it is possible for plants to be homozygous in their entire genetic make-up. This is an artificial situation resulting from deliberate agricultural practices, and its importance will become apparent shortly.

If a homozygous white-seeded plant (ww), is crossed with a homozygous black-seeded plant (WW) the progeny will all be heterozygous (Ww), and they will all be black-seeded, because black is dominant. If two of these heterozygous Ww plants are then crossed, their progeny will segregate as:

Ww × Ww→l WW+2 Ww + 1 ww

and the ratio of black to white seeds will be 3:1. This is the famous Mendelian ratio. It is also a clear indication that inheritance is controlled by discrete., independent factors, without any mixing, merging, or blending, as was thought by the biometricians.

Mendel published his results in 1865, in a paper called Experiments With Plant Hybrids but he won no recognition whatever. We shall probably never discover whether the great biological thinkers of that time, including Darwin, Huxley, and Galton, either saw the paper and decided that it was not important, or never saw it at all. The former alternative is the more probable, for several reasons. First, the so-called "Mendel's Laws of Inheritance" were not explicitly stated by Mendel himself. They were formulated by later workers who generously attributed them to Mendel. Consequently, the importance of Mendel's original paper was far from obvious.

Second, these nineteenth century biologists were steeped in the biometrical tradition, and the geological concept of gradualism. This concept had first been proposed by James Hutton in the late eighteenth century, and it was later developed by Charles Lyell, who was one of the most influential of all geologists. Darwin was profoundly inspired by the concept of gradualism, and it became the foundation of his theory of evolution. This concept postulated that all geological and evolutionary changes were slow, gradual, and quantitative. Mendel's laws of qualitative inheritance would have appeared irrelevant in this context, even if they had been explicitly stated.

Third, a fundamentally important discovery in science is often disturbing and disruptive and, for this reason, there will be a very natural human tendency to reject and deny it. If an important discovery forces a scientist to re-think all his ideas and, even worse, threatens much of his published work with obsolescence, that scientist can be forgiven if he has difficulty in accepting it.

Fourth, many people fall innocently into the error of judging new information on the basis of its source, rather than judging the information itself, on its own merits. If new information comes from a famous scientist, working in a famous university, and published in a famous journal, it is likely to be accepted uncritically, even though it might occasionally be downright wrong. And if the new scientific information comes from an utterly obscure monk, working in a scientifically unknown Augustinian abbey in Central Europe, and published in an unimportant local journal of natural history, it is likely to be ignored, even though it may be of seminal significance. Gregor Mendel was this monk.

Finally, it is likely that Mendel sent copies of his paper to many famous scientists. This, after all, was the custom of his time.

So Gregor Mendel, who had made a scientific discovery of fundamental importance, and knew it, and who longed for recognition, died a disappointed man, unrecognised, in 1884, at the age of sixty two. This was nineteen years after the publication of his work, and a further sixteen years were to elapse before it was recognised. In fact, a Russian scientist, I.F. Schmalhausen had recognised the importance of Mendel's work soon after it was published, but he was ignored also. Mendel and Schmalhausen were more than thirty years ahead of their time. Which brings us back to the start of our story, in the year 1900.

With the recognition of Mendel's laws of inheritance, the two schools of genetics not only came into existence; they came into conflict. In those days, it seemed obvious to everyone that, if one school was right, the other must be wrong. The members of the Mendelian school believed, quite correctly, that Mendel's laws were fundamental, and that they would eventually explain the whole of genetics. The biometricians argued that virtually every inherited character of human, agricultural, or evolutionary importance was inherited quantitatively. They contended, with considerable justice, that qualitative, Mendelian characters were of little practical, economic, or evolutionary significance.

The chief protagonists of the Mendelian school were William Bateson and the same Hugo de Vries who had helped to rediscover Mendel's laws. They went so far as to claim that Mendelian genetics had proved that Darwin's theory of evolution, based on gradualism, was wrong. They postulated that all evolutionary change resulted from major mutations and that, as a consequence, evolution progressed erratically, in leaps and bounds that were separated by long periods of stagnation.

Karl Pearson was the chief protagonist of the biometricians, and of gradualism, and he used Darwin's favourite dictum "Natura no facit saltum" (Nature makes no jumps). As with so many famous scientific conflicts, the argument was conducted in print. It grew increasingly heated, and the writing became positively offensive, as the authors stooped to personal insult.

It is difficult to exaggerate the importance of this conflict because it is the foundation of the current, apparent dilemma between either food and pesticide pollution, on the one hand, or no pesticide pollution but famine, on the other hand. It will become apparent that the members of the Mendelian school wrought incredible damage on twentieth century crop science. However, I do not wish to imply any criticism of Gregor Mendel, when I criticise the members of the Mendelian school of genetics. Mendel himself was far too good a scientist, and far too modest a man, to have behaved like the members of the school named after him.

The resolution of this dilemma between adequate food and freedom from pesticides will be discussed in a moment. At this point, we must consider the resolution of the conflict between the two schools of genetics.

The members of the Mendelian school were studying characters whose inheritance was controlled by single genes. As we have seen, a gene that conferred redness in flowers might be either present or absent. Accordingly, the flowers would be either pure red or pure white, and there were no intermediates, no pink flowers. This qualitative redness is now known as a single-gene (or monogenic) character.

It was then discovered that two-gene characters are possible. There could then be red and white flowers and, in addition, there could be pink flowers, halfway between red and white. If there were three, or possibly four or five genes controlling redness, there would be various different shades of pink. And if redness was controlled by many genes, each making a small contribution to either redness or whiteness, there would be every shade of pink between the two extremes of pure red and pure white. If the frequency of these many grades of redness is plotted on a graph, it produces the familiar bell-shaped curve, the normal distribution of the biometricians. In contrast to the single-gene character, this quantitative variable is known as a many-gene (or polygenie) character. So, the members of the Mendelian school were dealing with single-gene characters, while the biometricians were dealing with many-gene characters.

It seemed, therefore, that the battle was over. Both sides were right, and both sides had won. But, in fact, the conflict had left a scar, a distortion, that can be felt to this day. This brings us to the next pair of contrasts.

CHAPTER TWO
Plant Breeding: Pedigree Breeding and Population Crossing

It was perhaps inevitable that the two schools of genetics would produce two entirely different methods of plant breeding. The members of the Mendelian school, it will be remembered, were dealing with single-gene characters that were either present or absent. They developed methods of plant breeding that are now known as pedigree breeding, and which involve gene-transfer techniques. The biometricians, on the other hand, were dealing with many-gene characters that were continuously variable. They were looking at all the degrees of difference between the extremes of a normal distribution. They developed methods of plant breeding that are now known as population breeding, and which involve changes in polygene frequency.

The problem that usually faced the members of the Mendelian school was that a single-gene character, which they wanted to utilise in a cultivated plant, would occur in a useless wild plant. The difficulty was in getting it transferred out of the wild plant, and into the cultivated plant. A gene, after all, is only a small piece of a DNA molecule. It is far too small to be seen, even with the most powerful electron microscope. Consequently, there was no question of being able to pluck it out of one plant, with a micro-dissector, and put it into another plant. Nevertheless, the members of the Mendelian school solved this problem in a way that is both ingenious and elegant.

Let us suppose that the single-gene character was resistance to a fungus disease called "blight". (Plant diseases usually have the most colourful names, such as blight, mildew, wilt, blast, rust, smut, smudge, wart, streak, blister, and scorch). The wild plant carries this gene, and it is apparently immune to blight. Unfortunately, the yield of this wild plant is so low that it is not worth cultivating, and the quality of its product is so poor that no one would buy it anyway. The cultivated plant has a huge yield of an excellent product but, unfortunately, it is highly susceptible to blight, and it can be cultivated only if it is routinely sprayed with a fungicide. The crop yield and the crop quality are both many-gene characters, while the resistance to blight is a single-gene character.

The first thing the members of the Mendelian school would do was to hybridise the wild plant with the cultivated plant. The progeny were mostly about halfway between the two parents in their many-gene, quantitatively variable characters. The yield and quality were thus medium; not too bad, but not very good either. Some of the progeny carried the single gene for resistance while others did not, and the progeny accordingly segregated into individuals that were either resistant or susceptible.

This is the beauty of Mendelian genetics. It is possible to tell at a glance which plants are carrying the gene for resistance, because they are not diseased. This is a qualitative character which is either present or absent. The Mendelian breeder would throw out all the blighted plants and keep all the blight-free plants. As these resistant plants approached maturity, the breeder would select the best one, in terms of its yield, and the quality of its product. The breeder would then cross this best plant with the original cultivated parent. This is a process known as back-crossing.

The progeny of this back-cross would have approximately three quarters of the yield and quality of the original cultivated parent, and only one quarter of the poor yield and quality of the wild parent. This progeny would also be segregating into resistant and susceptible individuals. The breeder would again throw out the susceptibles, and keep the best resistant individual for a second generation of back-crossing. This process of back-crossing can be continued for as many generations as are necessary to restore the yield and quality of the hybrids. Finally, the best of them will have a yield and quality as good as, or possibly even better than, the original cultivated parent. And it will also be carrying the gene for resistance. This gene-transfer technique is so beautiful, and so clever, that it captured the imagination of plant breeders all over the world.

The biometricians' technique of population breeding is entirely different. In principle, it is merely a refinement of the methods that farmers have been using since the dawn of agriculture. As the term implies, biometricians work with populations of plants, and these populations are usually large. They screen the entire population for a small minority of the best plants. These are randomly cross-pollinated among themselves, and they become the parents of the next generation. Each generation is a little better than its predecessor, and this process of small, quantitative improvements by recurrent mass selection can continue until no further progress is possible.

A classic example of population breeding occurred with fodder beet, which are cultivated to feed farm animals. These "roots" contain about 4% of sugar. During the Napoleonic wars, the British blockade deprived most of continental Europe of sugar which, at that time, was produced exclusively from sugarcane, mainly in the West Indies. This shortage prompted the use of fodder beet for sugar extraction. The sugar content of fodder beet is a quantitative variable, controlled by polygenes. By population breeding methods, the sugar content of fodder beet was eventually increased to 16%, and the total yield of roots was also increased very considerably. The result was an entirely new crop called sugar beet.

Let us now consider the method of pollination, which is one of the more important practical differences between pedigree breeding and population breeding. All flowering plants can be classified into one of two categories according to their natural method of pollination. The so-called outbreeders are cross-pollinating plants in which the seed-producing, female parent is normally fertilised with pollen that comes from a different plant. The so-called inbreeders are self-pollinating plants in which the female part of the flower can be successfully fertilised with pollen from the same plant, usually the same flower. Cross-pollination can and does occur among the inbreeders but, normally, it occurs at quite a low frequency.

Pedigree breeders, as their name implies, work with carefully controlled crosses in which the parents of each cross are known and recorded. These crosses are made by hand, by artificial pollination, and this can be labour-intensive, depending on the species of plant being pollinated. With chickpeas (Cicer arietinum), for example, one successful hand pollination will produce only one seed, and only sixty percent of hand-pollinations are successful. With potatoes, one hand-pollination will produce two or three hundred seeds. And with tobacco, it will produce about two hundred thousand seeds. One of the advantages of pedigree breeding is that relatively few crosses are necessary and, consequently, hand-pollination is feasible.

Population breeding, as we have just seen, uses large numbers of pollinations. This difference in technique had an important influence on the development of plant breeding, following the re-discovery of Mendel's laws.

The members of the Mendelian school, working with relatively few, carefully controlled, hand-pollinations, were unaffected by this difference in pollination. With inbreeding plants, they would have to prevent self-pollination by removing the immature male parts of each flower to be pollinated. However, this was not difficult.

The biometricians, on the other hand, depended on large numbers of natural cross-pollinations. With inbreeding species, the frequency of cross-pollination was usually so low that this method of plant breeding was slow, difficult, and often entirely impractical. The biometricians consequently found it difficult to work with inbreeding species, and this gave a clear advantage to the members of the Mendelian school. It so happens that most of the important food crops of the world, such as wheat, rice, peas, and beans, are inbreeders. During the conflict between the two schools, the Mendelian plant breeders were not slow to exploit this advantage.

Nowadays, this difficulty is no longer a problem because there are various techniques for overcoming it. One of them employs a substance called a male gametocide which will make an inbreeding species, such as wheat, male-sterile. The flowers of treated plants are then unable to pollinate themselves, and they must accept pollen from an outside source. Population breeders can now achieve millions of crosses in a crop such as wheat by spraying part of their screening population with a male gametocide (Chapter 25). That part then becomes the male-sterile, seed-producing component, while the unsprayed part becomes the male-fertile, pollen-producing component. However, in the days of the genetic conflict, these alternative techniques were not available. In terms of practical plant breeding, the members of the Mendelian school appeared to be winning.

Then, in 1903, a Danish botanist, W.L. Johannsen, discovered the pure line, which is discussed later in the seventh pair of contrasts (Chapter 7). All we need note at the moment is that this was a technique for making seed-propagated crops breed "true to type". Normally, seed-propagation leads to genetic variation, and this is a nuisance because agriculturally valuable characteristics, such as high yield and high quality of crop product, tend to be lost. Johannsen's pure lines meant that these valuable characteristics could be preserved indefinitely, in spite of propagation by seed. This eventually increased the yield of many crops very considerably. It turned out to be a big boost for the members of the Mendelian school, and a further advantage in their conflict with the biometricians.

However, it still seemed that all the practical applications of genetics belonged to the biornetricians. This made the members of the Mendelian school unneccessarily assertive. Then, in 1905, a British scientist, R.H. Biffin, made a discovery that was the best help the members of the Mendelian school could possibly have wanted. He published his discovery in a famous paper called Mendel's Laws of Inheritance and Wheat Breeding. Like Johannsen's pure lines, this discovery was truly seminal in the sense that it changed the course of history.

Biffin was working with a disease of wheat called rust. He showed that resistance to this disease was inherited in a Mendelian fashion, and nothing could have pleased the members of the Mendelian school more. Suddenly, they had a single-gene character of economic significance, and it quickly transpired that the inheritance of resistance to other plant diseases was controlled by single genes. It must be remembered that, at that time, the members of the Mendelian school had no other single-gene characters of any economic significance whatever. They followed up this discovery of single-gene resistances with such vigour and zeal that they have dominated plant breeding ever since.

At this point, it is perhaps instructive to compare plant breeding with animal breeding. Although single-gene characters do occur in farm animals, none of them are economically important. As a consequence, animal breeding has remained quantitative, and in the hands of the biometricians, for the whole of this century. But for these single-gene resistances to crop parasites, plant breeding would undoubtedly have remained quantitative also.

As a result of Biffin's work in England, and similar work by W.A. Orton in the U.S.A., as well as energetic promotion from the Mendelian school of genetics, it was not long before most crop scientists began to assume, quite incorrectly, that all resistances to all crop parasites were inherited by single genes. Crop scientists also concluded that, if you wanted to breed plants for resistance to a parasite, you must first find a gene for resistance, in order to use the back-crossing technique of gene-transfer. They spoke of "first finding a genetic source of resistance". It will become apparent later that this became a shibboleth, a myth, that has both dominated and plagued the whole of twentieth century crop science.

CHAPTER THREE
Resistance: Vertical and Horizontal

With hindsight, we can now appreciate that it was inevitable that the two kinds of plant breeding would reveal two entirely different kinds of resistance to the parasites of plants. However, few scientists recognised this until J.E. Vanderplank, the most original of all plant pathologists, published a classic book in 1963. This book is called Plant Diseases: Epidemics and Control and, in it, Vanderplank distinguished between single-gene (monogenic) and many-gene (polygenic) resistances. He used the term vertical resistance to describe the single-gene resistance, and the term horizontal resistance to describe the many-gene resistance. However, this description is a deliberate simplification which will be elaborated in a moment.

Vertical resistance is the resistance of the Mendelian school. It is normally qualitative resistance in the sense that it is either present or absent, and there are no intermediates. However, there are a few exceptions to this rule (see Glossary: quantitative vertical resistance). Horizontal resistance is the resistance of the biometricians. It is quantitative resistance in the sense that it can occur at every level between a minimum and a maximum. These terms are very important and three comments about them are necessary.

First, these are abstract terms that are intended to label a concept so new that words to describe it do not exist. The terms are not intended to be interpreted literally, and they have nothing to do with standing up or lying down. Vanderplank could equally have chosen other neutral words, such as hard and soft resistance, or alpha and beta resistance. As the original author of the concept, he had the privilege of choosing its terms, and we should respect his precedence.

Second is the question of why abstract terms were needed at all. Could not Vanderplank have used descriptive terms such as monogenic and polygenic resistance? Unfortunately, these descriptive terms are not accurate because there is rather more to the definition of the two kinds of resistance than just the number of genes controlling their inheritance. This will be explained more fully in a moment.

Third, the terms vertical and horizontal are derived from two classic diagrams that are described in Appendix B. Differences in vertical resistance are parallel to the vertical axis of the diagram, while differences in horizontal resistance are parallel to the horizontal axis of the diagram. So the terms do have a minor descriptive connotation, and this makes them a little easier to remember.

With vertical resistance, there are single genes for resistance in the host plant, and there are also single genes for parasitic ability in the parasite. This is a very important phenomenon known as the gene-for-gene relationship, and it is the definitive character of vertical resistance. The gene-for-gene relationship was discovered in 1940 by the American scientist H.H. Flor, who was working with a disease of flax (Linum usitatissimum) called rust (Melampsora lini). This discovery was later elucidated mathematically by my old, and very dear friend, the late Clayton Person, in Canada.

Flor showed that, for every resistance gene in the host, there was a corresponding, or matching, gene in the parasite. This relationship is an approximate botanical equivalent of the human system of antigens and antibodies. It is common knowledge that any person who catches a cold develops an antibody to that strain of the cold virus. The antibody provides protection against future infections with that strain of the virus, because the virus has an antigen which activates the antibody. Unfortunately, there are many strains of the cold virus, and we are often infected by a strain for which we have no antibody. This is why we keep catching new colds, although we tend to get fewer colds as we grow older, and as we accumulate more and more antibodies. Roughly speaking, each resistance gene in the plant host corresponds to an antibody, and each parasitism gene in the parasite corresponds to an antigen.

It is now realised that the gene-for-gene relationship evolved in plants to operate as a system of locking. Each resistance gene in the host corresponds to a tumbler in a lock. And each parasitism gene in the parasite corresponds to a notch in a key. An individual plant host may have several of these resistance genes, these tumblers, which collectively constitute a biochemical lock. And an individual parasite may have several of these parasitism genes, these notches, which collectively constitute a biochemical key.

When a parasite individual is infecting a host individual, its biochemical key either does, or does not, fit the biochemical lock. If the key fits, the infection is described as a matching infection, and it is a successful infection, because the "door" of resistance has been unlocked and "opened". When this happens, the vertical resistance is described as having broken down. If the parasite key does not fit the host lock, the infection is described as a non-matching infection. It fails because the "door" of resistance remains "locked and barred", and the parasite is denied entry. This system of locking is the definitive characteristic of the gene-for-gene relationship, and the Mendelian, single-gene, vertical resistances to crop parasites.

Horizontal resistance is the resistance of the biometricians. Its definitive characteristic is that it does not involve a gene-for-gene relationship. However, its most prominent characteristic is that it is usually, but not invariably, inherited polygenically. It can occasionally be inherited in a Mendelian fashion, but these Mendelian genes are not part of a gene-for-gene relationship. This means that horizontal resistance is normally quantitative in both its inheritance and its effects, and it exhibits every degree of difference between a minimum and a maximum.

Perhaps the best way of understanding horizontal resistance is to think of it as the resistance which invariably remains after a vertical resistance has been matched. When a parasite succeeds in unlocking a vertical resistance, it then comes up against a second line of defence which is the horizontal resistance. To use a military analogy, vertical resistance corresponds to the coastal defence that prevents a beach-head from being established. The invading forces are either destroyed or thrown back into the sea. Horizontal resistance corresponds to the defence that operates after a beach-head has been established. The invading forces must be prevented from breaking out of their beach-head.

What is so economically important about horizontal resistance is that it operates equally against all strains of the parasite, regardless of what biological keys they may have. In fact, horizontal resistance operates against matching strains of the parasite. Consequently, it does not fail, like vertical resistance, on the appearance of a matching parasite. Horizontal resistance begins to function at the moment a matching infection occurs, and at the moment the vertical resistance breaks down. This means that horizontal resistance cannot be matched, in the way that vertical resistance is matched, and it cannot break down, in the way that vertical resistance breaks down.

This is the main practical difference between the two kinds of resistance. Vertical resistance operates only against non-matching strains of the parasite. Because some matching always occurs, vertical resistance is certain to break down sooner or later. It is temporary resistance. Horizontal resistance operates against matching strains of the parasite, and it never breaks down. It is durable resistance.

Horizontal resistance completely escaped the attention of the members of the Mendelian school. They were not interested in quantitative variation. They were working with qualitative resistances, inherited by single genes. A gene for vertical resistance is either present or absent. For the members of the Mendelian school, a plant was either resistant or susceptible and, normally, there were no intermediates. As we have seen, this is one of the attractions of the Mendelian pedigree breeding method. It is possible to decide at a glance whether the resistance is present or absent. Obviously, the resistant plants in a screening population were parasite-free, and they were kept, and studied. The susceptible plants were parasitised, and they were discarded.

The Mendelian breeders never did notice that there were very considerable differences in the levels of parasitism among the discarded plants. These differences represented quantitative variation in the level of horizontal resistance. But the Mendelian breeders were not interested in such differences. In their view, a plant was either diseased or disease-free, and they treated the diseased plants as rubbish. Why waste time studying rejects?

When Vanderplank published his new ideas in 1963, an immediate dispute arose concerning the relative merits of vertical resistance and horizontal resistance. But the conflict was very one-sided. There was vociferous and almost universal opposition to the very idea of horizontal resistance. I myself have witnessed respectable scientists so angry at the mere mention of horizontal resistance that they showed all the symptoms of incipient apoplexy. The Mendelian techniques of pedigree breeding, back-crossing, pure lines, and vertical resistance dominated the whole of crop science. To even question this "received wisdom" was to invite trouble.

The dominance of the Mendelian school is vividly illustrated by the point that, until Vanderplank published his book, very few crop scientists had even realised that there were, in fact, two kinds of resistance to the parasites of crops. Indeed, many crop scientists vigorously denied the very existence of horizontal resistance. A few of them still deny it, and most of them are still quite unwilling to employ it, or even to investigate it.

It is now clear that the conflict over vertical and horizontal resistance was actually a revival of the original genetic conflict between the members of the Mendelian school and the biometricians. What is depressing about this story is that the original genetic conflict started in 1900. It was resolved scientifically about thirty years later. The two kinds of resistance were recognised by Vanderplank about thirty years later still. And, thirty years after that, in the 1990s, the whole of crop science is still dominated by the Mendelian school of genetics, the Mendelian methods of plant breeding, and the Mendelian resistances to crop parasites.

We must now enquire why the two kinds of resistance to plant parasites should have evolved in plants in the first place.

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CHAPTER FOUR
Infection:
Allo-Infection and Auto-Infection

The word infection has many shades of meaning in the English language. In medicine, it is sometimes taken to mean the disease itself, and we speak of a patient having a "nasty infection". In its adjectival form of "infectious", it usually means a contagious disease that is caused by a biological agent, such as a virus or bacterium. However, we frequently speak of a laugh, or a yawn, being infectious.

Throughout this book, the term infection is defined quite strictly. It means the contact made by one parasite individual, with one host individual, for the purposes of parasitism. And there are two kinds of infection, just as there are two kinds of pollination.

It will be remembered that cross-pollination means that a plant is pollinated by pollen from another plant, while self-pollination means that a plant is pollinated by its own pollen. The technical term for cross-pollination is allogamy, while self-pollination is autogamy. These terms are derived from ancient Greek. Allo means other, or different; auto means self; and gamy means marriage or reproduction.

The two kinds of infection are called allo-infection and auto-infection. Allo-infection is equivalent to cross-pollination, and it means that a host plant is infected by a parasite individual that has arrived from another, different host, or from an independent, dormant state. The parasite had to travel to its new host. Conversely, auto-infection is equivalent to self-pollination, and it means that a host is infected by a parasite individual that was born on, or in, that same host. The parasite had no need to travel.

There is a close analogy with travelling people. Think of the individual host plant as an island, surrounded by sea. Allo-infection is then equivalent to an immigrant arriving on that island, by boat or plane, from somewhere else. Auto-infection is equivalent to the colonisation of the island by the descendants of that immigrant.

This people analogy can also embrace the two kinds of resistance. Think of Ellis Island, in New York, in the bad old days. The parasite genes of a gene-for-gene relationship correspond to the immigration papers of an immigrant, and the host genes correspond to the immigration laws of the U.S.A. These papers and laws either match, or they do not match. The immigrant is accordingly allowed in, or is denied entry, as the case may be.

Horizontal resistance, on the other hand, is represented by the living conditions in the immigrant's new land, which make it either easy or difficult for that immigrant to prosper.

Three further points are worth making. If the island is deserted, the first person to inhabit it must come from outside. The first infection of any plant host must be an allo-infection. Second, colonisation can proceed only after a successful immigration. Auto-infection of a plant host can occur only after there has been a matching allo-infection. Third, whenauto-infection, or colonisation, has continued for some considerable time, possibly for many generations of colonisers, the island becomes crowded. Some individuals may then leave the island in search for another, less crowded island, somewhere else. These explorers will be migrants, and they will allo-infect their new host, their new island.

Two real-life examples will further illustrate this difference between the two kinds of infection, which is critically important. Most people are familiar with the small insects known as aphids, green flies, or green bugs. Anyone who has grown roses will know what a pest they can be. Aphids have several, morphologically different forms, and each form has a special function. Among others, there is both a winged form, and a wingless form. The function of the winged individuals is clearly that of allo-infection, which is possible only by flying. The function of the wingless individuals is obviously that of auto-infection, which is possible by walking.

If a rose bush is completely free of aphids, it is the equivalent of a deserted island. The only possible infection is allo-infection, and this requires a winged aphid. Once it arrives, this allo-infecting aphid, which is invariably a female, will feed on its host and begin to reproduce. Unlike most other insects, it will reproduce without sex, and with live births rather than the laying of eggs. The sexless reproduction is the equivalent of vegetative propagation in plants, and all the progeny are genetically identical to their mother. They constitute a clone. The loss of the egg stage saves time, because the young are born alive. They are also born without wings, because flying is not necessary for auto-infection. The young are all female, and they grow very rapidly as a result of sucking the rich juices of their host. Soon, they too start their own sexless and eggless reproduction. There is then a population explosion of aphids, all auto-infecting the same host plant. All rose growers know how quickly a rose bush can become crowded with aphids. Eventually, over-crowding stimulates the birth of winged individuals, which then fly away to allo-infect a rose bush somewhere else.

Ecologists have a special term for this kind of reproduction. They call it r-strategy. An r-strategist species is one that reproduces very rapidly and cheaply, with large numbers of very small offspring. It is a quantity breeder. It can exploit an ephemeral food supply very effectively by producing a population explosion. This explosion is followed by a population extinction when the food supply disappears, usually with the onset of an adverse season. Only a very few individuals survive the winter, or the tropical dry season, but there are enough of them to produce another population explosion in the following favourable season. Most of the serious pests and diseases of our crops are r-strategists, and it is their population explosions that can be so alarming, so damaging, and so very difficult to control.

The second real-life example concerns a disease of coffee trees called rust. This fungus parasite, like its coffee host, is a native of Africa. In 1970, coffee leaf rust appeared for the first time in Brazil, which is the world's largest coffee producer, and a chill of fear spread among everyone in the coffee trade. Fortunately, the disease was riot nearly as serious in the New World as people had feared, and all of us can still have our morning cup of coffee.

Coffee rust is caused by a microscopic fungus which reproduces by means of spores so small that they are invisible. These spores are similar in size and shape to the pollen cells of flowering plants. When pollen cells are seen en masse, they are yellow, and when rust spores are seen en masse, they are the colour of rusty iron. Just as iron rust will leave an orange smudge on your finger or clothing, so will coffee rust. Hence its name.

Scientists in East Africa discovered that the spores of coffee rust are sticky, and that they are highly resistant to becoming air-borne, and to being dispersed by wind. But they are freely dis-persed in water, and every coffee tree gets wet when it rains. Shortly after this discovery was made, it became obvious that the newly introduced disease in Brazil was spreading at a rate of hun-dreds of miles each year. Brazilian scientists showed that the rust spores were wind-borne. One of those silly scientific disputes arose, with everyone assuming that, if one side were right, the other must be wrong. The spores had to be either water-borne, or wind-borne, and that was that. In fact, both sides were right.

It is now clear that coffee rust spores have two physically different states, and that they can apparently switch freely from one to the other. In one state, they are sticky, and resistant to wind dispersal, but freely dispersed in water. In the other state, they are not sticky, and they are freely dispersed by wind. No one has yet discovered what makes them change from one state to the other, but the most likely factor is atmospheric humidity.

What is important is that the function of the non-sticky state is obviously allo-infection, by wind, from one coffee tree to another. These two coffee trees, the infector and the infected, may be hundreds of miles apart. The function of the sticky state is obviously auto-infection, by rain splash, from one leaf to another leaf, within one coffee tree.

The analogy between the two kinds of pollination and the two kinds of infection is a close one. However, there is one important difference, and it is a historical one. The distinction between autogamy and allogamy has dominated crop science for the whole of this century. Self-pollinating (autogamous) plants were tailor-made for Johannsen's pure lines, Mendelian breeding methods, and vertical resistance. Other scientists modified these techniques to suit cross-pollinating (allogamous) plants, and produced the so-called hybrid varieties, of which the hybrid maize in the corn belt of the United States (Chapter 20) is the most famous example. As a result, the Mendelian school dominated the breeding of allogamous plants also. And the scientists working with vegetatively propagated crops, such as potatoes, pineapples, and sugarcane, also adopted the breeding techniques of the Mendelian school, perhaps unwisely. What matters here is that the distinction between cross-pollination and self-pollination was well recognised.

The distinction between allo-infection and auto-infection should also have dominated crop science for most of this century, because it is just as important. In fact, the distinction between the two kinds of infection was made only recently, and its importance is far from obvious. We must now examine that importance.

CHAPTER FIVE
Host-Parasite Interaction: Matching and Non-Matching

It was mentioned briefly, in the comparison of the two kinds of resistance, that there are two kinds of host-parasite interaction, defined in terms of the gene-for-gene relationship. It will be remembered that each host has a biochemical lock, and that each parasite has a biochemical key. When a single parasite individual is infecting a single host individual, its biochemical key either does, or does not, fit the biochemical lock of the host. If the key fits, both the infection, and the host-parasite interaction, are described as matching. If the key does not fit, they are described as non-matching. With a matching infection, the lock of resistance is opened, the infection is successful, and the parasitism proceeds. With a non-matching infection, the lock remains secure, the infection fails, and the parasitism is prevented.

It is now necessary to consider a system of locking. For the purposes of discussion, we may suppose that there are ten different locks, which occur randomly, and with an equal frequency, in a host population consisting of many thousands of individuals. We may also suppose that there are ten different keys, which occur randomly, and with an equal frequency, in a parasite population consisting of many thousands of individuals. If one parasite individual is allo-infecting one host individual, the probability that its key will fit the lock of that host is then only one in ten.

Now suppose there are one hundred different locks and keys, occurring randomly, and with equal frequency in the two populations. The probability of a key fitting a lock is now only one in a hundred. And, if there are one thousand different locks and keys, the probability of a key fitting a lock is only one in a thousand. Clearly, the greater the diversity of locks and keys, the more effective the system of locking becomes.

So long as we think in terms of a system of locking, operating in populations of the host and the parasite, the gene-for-gene relationship makes a lot of sense. If only one allo-infection in a thousand is successful, the entire epidemic will be slowed down, and stabilised, very considerably. Mathematically, this turns out to be the perfect method of controlling the population explosion of an r-strategist parasite.

The system of locking is also a very economical one. Simple mathematical models (Appendix C) reveal that a gene-for-gene relationship with only twelve pairs of genes will produce 924 different locks and keys, provided that each lock and key has exactly half of the available genes (i.e., six genes in this example). The probability of one matching infection in a thousand could almost be achieved with only twelve pairs of Mendelian genes. On the same basis, sixteen pairs of genes would provide 12,870 locks and keys, and twenty pairs of genes would provide 184,756 locks and keys. Both the diversity of locks and keys, and the effectiveness of the system, increase geometrically with only small increases in the numbers of pairs of genes.

One plant host, or one parasite, has thousands of genes, although bacteria and viruses have fewer. Nevertheless, for such an incredible effect to be produced with a mere dozen pairs of genes is truly remarkable. When compared with the complexity of a living cell, or a single chromosome, the simplicity, the beauty, and the elegance of this system of locking are profoundly suggestive of scientific truth. We must remember also that evolution has a knack of finding the best solution within the existing possibilities.

So far, the discussion has concerned allo-infection. It will be remembered that allo-infection involves travel from a distance by an airborne parasite. (Occasionally, the parasite does not travel, but remains dormant and immobile in the soil; the host and parasite come together when a growing root finds the parasite. But this is still allo-infection).

We must now consider auto-infection which involves a flightless parasite, such as a wingless aphid, or a water-borne rust spore. Let us consider a model epidemic in which there are one thousand biochemical locks and keys. If each host is allo-infected once, one host individual in every thousand will have been matched, and successfully allo-infected. Parasitism can begin in these matched individuals. The parasite draws nutrients from its host and it begins to reproduce. Both the aphid and the rust reproduce without sex. This sexless, r-strategist reproduction is very rapid. Biologically, it is also very economical, and it produces very large numbers of progeny very cheaply. It has a further advantage for the parasite in that all the progeny are genetically identical to each other, and to their parent. They all belong to the same clone. This means that they all have the same biochemical key. And it is also the key that matches the lock of the host that they are auto-infecting. All parts of the one host individual are also genetically identical. The innumerable microscopic cells, in the many leaves, stems, roots, bracts, flowers, and fruit of one plant, all have the same lock. It follows that all auto-infection is matching infection. Vertical resistance cannot control auto-infection. It can control allo-infection only. And it can control non-matching allo-infections only. To put this another way, vertical resistance cannot control any of the consequences of a matching allo-infection. And auto-infection is invariably a consequence of a matching allo-infection.

Equally, it follows that auto-infection can be controlled only by horizontal resistance. It follows also that all the consequences of a matching allo-infection, including all auto-infection, and all the processes of parasitism, can be controlled only by horizontal resistance. To postulate that there is no such thing as horizontal resistance, as some Mendelians still do, is to postulate an absolute susceptibility, once a matching allo-infection has occurred. It need hardly be added that such an absolute susceptibility has never been observed.

It is clear, therefore, that the actual parasitism can be controlled only by horizontal resistance. This parasitism is the process by which the parasite steals nutrients from its host, and both grows and reproduces at its host's expense. Vertical resistance cannot control this parasitism once it has started. It can only prevent the parasitism from starting, and it occasionally fails to do even this, because some matching always occurs. The sole function of vertical resistance is to control the epidemic, and to protect the host population as a whole, by slowing down the population explosion of an r-strategist parasite. It does this by greatly reducing the proportion of allo-infections that are matching infections.

Now consider the subsequent development of the epidemic. When crowding produces winged aphids, or the rust spores become non-sticky and wind-borne, parasite individuals can leave their parent host and travel to another host. They are allo-infecting that new host and, because of the system of locking in our model, the chances are again a thousand to one against their new host having the same lock as their parent host. The probability that their biochemical key will match the biochemical lock of the new host is still only one in a thousand. Vertical resistance continues to control allo-infection throughout the epidemic, during the many rounds of allo-infection that can occur during a single season.

Finally, we come to an odd biological fact. Not all species of plant have vertical resistances. Furthermore, species of host plant which do have vertical resistances have them against only some of their species of parasite. This has been the bane of Mendelian plant breeding. Before their resistance breeding can start, Mendelian breeders must find a gene for resistance. If this genetic source of resistance cannot be found, for the simple reason that it does not exist, Mendelian plant breeders cannot breed for resistance. The breeding cannot even begin.

Conversely, every plant has horizontal resistance to every one of its parasites. This is one of the main advantages of this kind of resistance. The biometricians can breed for resistance to any species of plant parasite. We must now enquire why horizontal resistance is universal, but vertical resistance is not.

CHAPTER Six
Epidemics:
Discontinuous and Continuous

An epidemic is just parasitism, or disease, at the systems level of the population. Some scientists consider that the term epidemic should be confined to people and medicine, on the grounds that its Greek derivation refers to people (demos = people). They argue that epidemics in populations of plants and animals should be called epiphytotics and epizootics respectively. However, this is a matter of taste. My own view is that "epidemic" is an English word derived from the Greek, and that present usage is more important than ancient derivation. I also happen to think that the use of different terms for the same thing in people, animals, and plants is an entirely superfluous jargon.

Unlike people, and other mammals, plants have two quite different kinds of epidemic. They are called discontinuous and continuous and they are defined by the nature of the plants themselves.

Discontinuous epidemics occur typically with annual plants, and with the leaf parasites of deciduous trees and shrubs. With a discontinuous epidemic, the parasitism is intermittent. It stops completely during an adverse season, such as a tropical dry season, or a temperate winter, for the simple reason that there is no host tissue available to the parasite. Discontinuity thus involves seasonal host tissue. This discontinuity creates three difficult problems for the parasite.

First, the parasite must survive until host tissue again becomes available. Most species of plant parasite survive the adverse season by becoming dormant, but other mechanisms also exist. For example, the parasite might migrate to another region, with a different climate, where host tissue is available. Or it might find an alternative host species. Or it might change to a non-parasitic phase, and consume dead plant material.

The second problem is that the parasite must find a new host when the favorable season starts, and when host tissue again becomes available for parasitism. It will be recalled (Chapter 4) that the first infection of this new host tissue must be an allo-infection. Think of a host population consisting of millions of newly emerged seedlings of an annual species. If the epidemic is to develop fully, each one of those millions of plants must be allo-infected.

The third problem is that each parasite individual must match the biochemical lock of the host that it does manage to find. At the beginning of the epidemic, therefore, the parasite population must have many individuals that are going to be wasted, either because they could not find a host, or because they found a host that they did not match. It is obvious that allo-infection is much more important than auto-infection in discontinuous epidemics. It is equally obvious that the system of locking provided by the gene-for-gene relationship is a very valuable stabilizing factor in discontinuous epidemics.

A continuous epidemic occurs with evergreen trees and many tropical herbs, in which there is no interruption in the supply of host tissue. The parasitism can then continue indefinitely, and life becomes much easier for the parasite. A Californian redwood, for example, is an evergreen tree that can live for more than two thousand years. An individual redwood need be allo-infected only once, and auto-infection can then continue without a break for many centuries. Obviously, auto-infection is more important than allo-infection in continuous epidemics.

It is a matter of observed fact that a gene-for-gene relationship has never been found in a plant host species that has continuous epidemics in its wild state. This is because allo-infection is relatively unimportant in continuous epidemics, and vertical resistance can control allo-infection only. The vertical resistance has too little survival value to evolve in a continuous epidemic.

It also transpires that discontinuity is essential to the proper functioning of the gene-for-gene relationship and the system of biochemical locking. A gene-for-gene relationship cannot function in a continuous epidemic. This is because a system of locking cannot operate on a basis of unclocking only. If every door in the town could be unlocked, but not locked again, the system of locking would quickly become useless.

Plant hosts cannot re-lock their biochemical locks, but they solve this problem in another way. They regularly destroy all tissue that has a biochemical lock, and that has probably been matched by the end of a discontinuous epidemic. The only host tissue that has a lock is seasonal tissue, and it is discarded at the end of each season. All the locks that have been unlocked by the parasite are destroyed by leaf-fall in a deciduous tree, or the death of all tissues, except the seed, in an annual herb. Come the end of the season, the parasite is out in the cold, and on its own.

The biochemical locks are not re-locked but, in the new season, they are replaced with new tissues that are both parasite-free, and have locks that are unmatched and functioning. This is the importance of discontinuity. In each new epidemic, there has to be a successful infection of each host individual, if the epidemic is to develop fully. That successful infection must be an allo-infection. And it must be a matching infection. At the beginning of each new season, the system of locking is fully functional again.

The loss of seasonal tissue represents the "recovery" of vertical resistance, and is the converse of the "breakdown." In the course of one complete seasonal cycle, the state of the vertical resistance can change from being unmatched and functioning, to being matched and broken down, to being unmatched and recovered. This corresponds to a system of both unlocking and re-locking. And the system of locking can endure indefinitely.

For example, the system of locking continues to function as young deciduous trees replace old deciduous trees in a forest that might endure for millions of years. The only criterion is that the diversity of locks and keys must be maintained, and there are various genetic mechanisms that can ensure this. The system of locking will also endure indefinitely in an ecosystem of annual plants, as new unmatched plants replace the dead, matched plants of the previous season.

It seems that discontinuous epidemics are always caused by r-strategist parasites. They have to be r-strategists, if they are to exploit a food supply that appears very suddenly at the beginning of a favourable season, and then disappears, equally suddenly, a few weeks later, at the end of that season. Small organisms, such as microscopic parasites, and tiny insects, can take full advantage of such an abundant, but short-lived, food supply only if they have a population explosion.

However, there is a serious problem with population explosions. Like chemical explosions, they are tricky things. They are thoroughly unreliable, and they can very easily get completely out of hand. They are difficult to stop, once they have started, and they equally difficult to curb and restrain. And they can do a great deal of damage if they are not restrained. In an abnormal season that favored the parasite, there could be a population explosion so vast that the very survival of the host population was seriously threatened. And, if the survival of the host is threatened, the survival of the parasite is threatened with it.

This, then, suggests the function of the system of locking conferred by vertical resistance. It is to slow down the population explosion of an r-strategist parasite. It is to stabilize an otherwise unstable, unreliable, unpredictable, and thoroughly dangerous situation. The host population simply cannot afford to be periodically devastated by a parasite population explosion. And the parasite simply cannot afford to devastate its host population because, to do so, would threaten its own survival. So, the two species have evolved an incredibly elegant system of locks and keys that prevents damaging population explosions and, at the same time, ensures the survival of the parasite without excessive damage to the host.

Support for this conclusion comes from the vertical resistance to Hessian fly (Mayetiola destructor,) which is a stem borer of wheat. This resistance is exceptional in that it is quantitative vertical resistance. Although its inheritance is qualitative (i.e., Mendelian), its effects are quantitative. That is, it confers incomplete resistance to non-matching strains of the insect, and no protection whatever against matching strains. This means that a non-matching strain of the fly can allo-infect a wheat stem, and survive within it.

With quantitative vertical resistance, a non-matching infection does not kill the parasite. It merely slows the growth of the parasite, and prevents it from reaching maturity. At first sight, this is ludicrous because this kind of resistance does not control the parasitism. Quantitative vertical resistance appears to have no evolutionary survival value. And, if it has no evolutionary survival value, why should it evolve at all?

The answer appears to be that quantitative vertical resistance did not evolve to prevent allo-infection, or even to prevent parasitism. It evolved to prevent damaging population explosions, and it does this by controlling the reproduction of the parasite. And this is probably the ultimate function of all vertical resistances. A few infections, and a little damage to the host population, are quite unimportant compared with the disaster of an uncontrolled population explosion in the parasite.

We have seen that vertical resistances appears to reduce parasitism by reducing the frequency of matching allo-infection. And, at first sight, this reduction of parasitism appears to be the obvious function of vertical resistance. In fact, the ultimate function of vertical resistance is probably to reduce reproduction in the parasite and, hence, the control of population explosions in the parasite. Most vertical resistances achieve this by the simple expedient of controlling allo-infection. A few do it by allowing allo-infection, allowing some parasitism, and some growth of the parasite, but by either preventing, or greatly reducing, parasite reproduction.

But this is a digression. Let us return to the two kinds of epidemic. In practice, this difference between continuous and discontinuous epidemics is crucial to the functioning of vertical resistance. Consider the epidemics of a leaf parasite of a hypothetical tree. If the tree is deciduous, the epidemic is discontinuous, and the vertical resistance will function at the start of every new epidemic. If the tree lives for, say, five hundred summers, its vertical resistance will protect it through five hundred epidemics. By chance, in a few of these epidemics, the tree will be matched quite early in the season, and it will suffer accordingly. However, every tree can tolerate an occasional bad epidemic. Equally, in a few of these epidemics, the tree will be matched so late in the season that it suffers no parasitism at all. On average, it will be matched sufficiently late for the parasite to do only very minor damage in each season.

Now consider an evergreen tree which has a continuous epidemic. Its first infection must be an allo-infection but, after that, it can remain parasitised by auto-infection for the rest of its life, and all auto-infection is matching infection. Vertical resistance would protect this evergreen tree only until the first matching allo-infection occurred, probably when the tree was still a very young seedling. The vertical resistance would then be useless for nearly five hundred subsequent summers. A gene-for-gene relationship cannot function in a continuous epidemic and, consequently, its evolutionary survival advantage is negligible. For this reason, a gene-for-gene relationship never evolves in host-parasite systems that have continuous epidemics.

Most people think that deciduous trees shed their leaves in order to avoid a winter, or a tropical dry season. And so they do. But this is not the only reason. They also shed their leaves to achieve a break in their parasitism, and to resuscitate their biochemical locks. This additional function of leaf-shedding explains several conundrums that baffled botanists for years. For example, it explains why a temporary resistance should evolve in a tree that lives for centuries. It also explains why a tree such as rubber (Hevea brasiliensis) should be deciduous, and have vertical resistance to a disease called leaf blight (Microcyclus ulei), even though it occurs wild in the Amazon valley, which is continuously warm and wet. And it explains why the members of the Mendelian school could not find any single-gene resistances in various important crops derived from wild plants that have continuous epidemics, such as sugarcane, citrus, and olives.

This, then, was the bane of Mendelian breeding for resistance. If a crop is derived from a wild plant that is an evergreen perennial, it will have horizontal resistance but no vertical resistance. Conversely, if the wild progenitor of a crop is an annual herb, or a deciduous tree or shrub, that crop will have both horizontal and vertical resistances. The evolutionary survival value of a gene-for-gene relationship in a discontinuous epidemic is remarkable and, for this reason, it will often, but not necessarily, evolve in annual herbs, and against the leaf parasites of deciduous trees and shrubs. A Mendelian breeder, looking for a genetic source of qualitative, vertical resistance, will not find it in evergreen perennials. He may find it in crops with discontinuous epidemics, but he will not necessarily do so. A biometrician, on the other hand, looking for quantitative, horizontal resistance, will invariably find it, in any crop, and against any parasite of that crop.

It will be remembered that a Mendelian breeder needs a genetic source of resistance. If he cannot find it, the resistance breeding cannot even begin. A biometrician, on the other hand, does not need a genetic source of resistance. He needs merely to increase an existing level of quantitative resistance by changing gene frequencies in a mixed population. He can thus breed any crop for resistance to any parasite, and he can do so without first finding a source of resistance.

We should note also that most of the crop species in temperate countries have discontinuous epidemics, and vertical resistances, because they evolved in a region that has winters. And most of the research in crop science has been done in temperate regions, and on temperate crops, grown in the wealthy, industrial nations. Conversely, many tropical crops have continuous epidemics, and they lack vertical resistance. But relatively little research has been done on these tropical crops, grown in impoverished, non-industrial countries.

These differences of climate and research have done much to exaggerate the importance of vertical resistance, and to disguise the importance of horizontal resistance.

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CHAPTER SEVEN
Populations: Genetically Uniform and Genetically Diverse

A plant population may be genetically uniform or genetically diverse. Agricultural crops are plant populations that are typically uniform, because uniformity is essential in modern crop husbandry. It is a great advantage, for example, if all the plants in a wheat crop are the same height, mature at the same time, and have the same milling and baking characteristics. There is a further advantage when all the wheat crops on one farm, and in one region, are identical, because the harvested wheat can then be stored and transported in bulk. In the old days, wheat would be stored and transported in sacks, and each sack would have to be man-handled many times, as well as labelled to show which variety of wheat it contained.

A second, very good reason for crop uniformity in agriculture has already been mentioned. This concerns the problem of preserving the agriculturally valuable traits of a cultivar (i.e., a cultivated variety), such as its yield, its quality of crop product, its agronomic suitability, and its resistance to parasites. The natural method of reproduction by sexually produced seeds results in genetic diversity and variability. With variability, these valuable traits, which have been carefully accumulated by artificial selection, tend to be lost. This problem is normally solved in one of three ways, depending on the somewhat artificial method of propagation of the crop in question. As it happens, each of these solutions positively requires crop uniformity.

The first method of propagation is by true seeds in species that are inbreeders, and this includes important cereals, such as wheat and rice, and most of the protein producing crops, such as the many different species of peas and beans. These two categories of crop provide most of the world's food. As we saw earlier (Chapter 2), the Danish botanist Johannsen solved this problem by inventing the pure line, which breeds true to type. The best individual plant in a mixed population is selected as the parent of a new pure line. It is allowed to self-pollinate but, because it is heterozygous, its progeny are variable. The best individual in this second generation progeny is then selected, and allowed to self-pollinate. Its progeny are also variable, but less so. This process of reducing variability by self-pollination and selection is continued until no more variation is detectable. In theory, this process is complete after twelve generations of self-pollination but, in practice, 4-6 generations of selfing and selection are usually adequate. A modern cultivar of an inbreeding crop is thus homozygous, or very nearly so, in all of its genetic make-up. It is a genetically uniform pure line, it breeds true to type, and its valuable traits are preserved indefinitely. Even if some cross-pollination does occur within the cultivar, the two parents are genetically so similar that no significant variation results.

The second method of propagation is by true seeds in species that are outbreeders, such as maize, millets, sorghum, and various cultivated species of the onion and cucumber botanical families (Liliaceae and Cucurbitaceae). A cross-pollinating crop is heterozygous. It often does not breed true to type, and many of its valuable traits can be lost with seed propagation. Nor can it be self-pollinated without a totally unacceptable loss of vigour. This problem is solved by using hybrid varieties, a method which is described in Chapter 20. The details do not matter here, other than to comment that hybrid varieties also lead to genetic uniformity during cultivation.

Finally, many species of crop are so heterozygous that propagation by true seed is impossible, because the loss of valuable traits is almost total. The classic wine grapes, as well as apples, potatoes, sugarcane, figs, olives, dates, and pineapples are typical examples. In yet other species, the process of domestication has led to an almost complete loss of true seed, and seed propagation is then doubly impossible. These seedless species include crops such as bananas, garlic, ginger, horseradish, sisal, turmeric, and yams. In all these crops, valuable traits can be preserved only by vegetative propagation. Each cultivar is then a clone, characterised by the fact that all the individuals in it are genetically identical.

So, let us make no mistake about it. Population uniformity is essential in modern, commercial crop husbandry. There are very few exceptions to this rule, and they involve only a few outbreeding pasture species, such as alfalfa, otherwise known as lucerne (Medicago sativa), that are cultivated as so-called "synthetic varieties", which are genetically improved, mixed populations.

It should be added, however, that most subsistence crops in the tropics are grown as mixtures. First, there is usually a mixture of different species, such as maize, beans, and sweet potatoes in one field. Second, each of these species is genetically diverse, and is either a landrace or a mixture of several different clones. This is one of the reasons why the need for crop protection chemicals is usually less in subsistence crops. However, this kind of genetic diversity is not often practical in commercial farming, mainly because it is so labour-intensive.

In complete contrast to commercial agriculture, wild plant populations are always genetically diverse. Although all the individuals in a wild plant population may belong to the same species, they vary among themselves to such an extent that no two individuals are alike. In this respect, they are like human populations, in which no two individuals are genetically identical, apart from monozygotic (i.e., identical) twins. It is a matter of common observation that humans vary considerably in every inherited trait, and the same is true of wild plant populations. A few wild species of plant have a natural vegetative reproduction, and they can produce clones in which all the individuals are genetically identical. However, there is always a limit to this vegetative reproduction, and the total tissue of a natural clone rarely exceeds the size of a large tree. The overall population of clones then has a genetic diversity similar to that of a mature forest.

This contrast between uniformity and diversity of population brings us to the crux of the whole discussion. We saw earlier that the Mendelian method of breeding discriminates in favor of single-gene, vertical resistances that are part of a gene-for-gene relationship. We saw also that the gene-for-gene relationship operates as a system of locking with, possibly, only one allo-infection in a thousand being a matching infection. The essential feature of a system of locking is that it can work only if there is diversity. A system of locking is ruined by uniformity. Consider what happens when every door in the town has the same lock, and every house owner has the same key, which fits every lock.

This, then, is how the members of the Mendelian school went wrong. They would transfer a single tumbler from a single lock in a genetically diverse wild population to a cultivated plant. They would then multiply that cultivated plant into a genetically uniform pure line, hybrid variety, or clone, which would become a new cultivar. That cultivar might be grown on a huge area of land in a uniform plant population that totalled millions, probably billions, possibly even trillions, of individual plants, all with the same lock. We might, perhaps, refer to this extraordinary situation as monolock.

These uniform populations would remain resistant only because the parasite was often strangely slow to respond to this bizarre situation. Several years might elapse before a parasite with a matching key appeared but, when it did appear, it would respond with the population growth of an r-strategist. This parasite growth would be a population explosion and, because the system of locking had been destroyed, it would be a completely uncontrolled population explosion. Because of the genetic uniformity, every allo-infection, from one host individual to another, within that cultivar, would be a matching infection. There was nothing to stop the population explosion, except some residual horizontal resistance. But, as we shall see in a moment, the pedigree breeding method actually reduces the level of horizontal resistance, and a modern cultivar with a matched vertical resistance is usually highly susceptible.

The failure which follows the appearance of a matching strain of the parasite is known as the "breakdown" of vertical resistance. Within a single season, an apparently immune cultivar can suddenly become extremely susceptible. This cultivar must then be abandoned, and replaced with a new one which has a different vertical resistance that has not yet been matched. And the process is repeated. Again, and again. This has been called the "boom and bust" cycle of plant breeding. It need hardly be added that nothing can be more disheartening for a plant breeder than to see a wonderful cultivar, the result of years of patient and painstaking work, ruined, because its resistance has suddenly ceased to function.

During all this time, almost no one was thinking in terms of horizontal resistance. No one bothered to measure the susceptibility of a failed cultivar, or to study its remnant horizontal resistance. For much of this time, the very existence of horizontal resistance was not even recognized. And, even if the possibility of horizontal resistance was acknowledged, it was not believed to have any practical value. Furthermore, there was such an urgent need to produce replacement cultivars that no one had time to study such apparently unimportant and secondary issues. Besides, these scientists were all members of the Mendelian school. They were not interested in quantitative variation.

Some of the very few scientists who were exceptions to this rule, and who both studied and utilised horizontal resistance, are mentioned in the chapters on wheat (19), maize (20), potatoes (18), coffee (21), sugarcane (22), lupins (25), and tropical roots (27). One quite exceptional scientist, in this regard, is Luigi Chiarappa, of the Food & Agriculture Organisation of the United Nations (FAO). He had the foresight, and the intellectual courage, to initiate the International Program for Horizontal Resistance (FAO/IPHR) in 1975, at a time when hostility to the very concept of horizontal resistance was at its height. Another exceptional scientist was D.H. Lapwood, at Rothamsted, in England, who was studying the mechanisms of horizontal resistance to potato blight, even before Vanderplank published his classic book in 1963. Another was Helen Hart, who was working with horizontal resistance to wheat rust in St Paul, Minnesota, more than sixty years ago. The importance of her work, and her originality, were neither recognised nor rewarded. It should also be added that Vanderplank himself did many years of successful work in South Africa, breeding potatoes for horizontal resistance, but he published little concerning this innovative and creative research.

A few scientists have attempted to improve the efficiency of vertical resistance by cultivating crops with a diversity of vertical resistances. In Britain, they have been growing mixtures of several different barley cultivars with some success. And, in the United States, they have used so-called multilines in oats. A multiline is a population which contains several different pure lines, that are morphologically very similar, but which have different vertical resistances. However, the use of genetic diversity in commercial agriculture involves considerable technical difficulties and, in agriculture as a whole, it is not very practicable.

This, then, is the real dilemma of crop science, and of the world food problem. We must have genetic uniformity in our crops. But, if we are to employ vertical resistance effectively, as the system of locking in which it evolved to function, we must have genetic diversity in our crops. The conclusion is obvious. Genetic uniformity is essential in our crops and, consequently, we cannot expect to protect them successfully with vertical resistance. We have to consider the use of horizontal resistance, if we wish to avoid the use of chemical pesticides.

So it turns out that the early members of the Mendelian school never did have any economically important, single-gene characters after all. They thought they did, but they were wrong. They had single-gene resistances all right, but their value was entirely spurious. And the fact that the members of the Mendelian school so dominated plant breeding for most of the twentieth century stems from an unnecessary and, indeed, deplorable, scientific dispute. It was a dispute that made the members of the Mendelian school unnecessarily assertive, and needlessly competitive. It was also a dispute that was resolved, scientifically, some sixty years ago.

During the past half century, crop scientists have been gradually abandoning vertical resistance breeding because its value was so obviously limited. But, these scientists usually concluded either that vertical resistance was the only kind of resistance that occurs, or that horizontal resistance cannot be useful. They then came to the false conclusion that the only alternative to vertical resistance is to use crop protection chemicals. This is the main reason why we now use these chemicals in such depressingly large quantities.

CHAPTER EIGHT
Response to Selection Pressure: Genetic Flexibility and Inflexibility

There is another aspect of population diversity and uniformity which is of special relevance to plant breeding. This the question of genetic flexibility. A plant population can be either flexible or inflexible, genetically. In this context, geneticists speak of selection pressures, using the word "pressure" in the sense of bringing pressure to bear, persuasion, influence, or coercion.

A genetically flexible population will respond to selection pressures, and its genetic composition then changes. For example, if a host population has too little horizontal resistance to a parasite, there will be selection pressure for more resistance. The flexible population then responds to this selection pressure and, in a few generations, it becomes more resistant.

The mechanism of this response is that resistant individuals in the population produce more progeny than susceptible individuals, simply because they are less parasitised. The resistant individuals have a reproductive advantage and, consequently, in the next generation, there are more of them. The susceptible individuals have a reproductive disadvantage and, consequently, in the next generation, there are fewer of them. A similar response can occur to selection pressures for all other variables, including tolerance to environmental factors such as frost, drought, high winds, long days, or acid soils.

This genetic flexibility is totally dependent on genetic diversity. If there is population uniformity, no individual can have a reproductive advantage over any other individual, because they are all identical. Such a population cannot respond to selection pressures. It is genetically inflexible.

Obviously, modern crop populations are genetically uniform and genetically inflexible. They cannot respond to selection pressures. We positively want them that way in order to preserve their valuable agricultural characteristics that have been so carefully accumulated by artificial selection.

Wild plant populations, on the other hand, are genetically diverse and genetically flexible. They can and do respond to selection pressures. If a wild plant population has too little horizontal resistance, it will accumulate an adequate level of resistance in the course of a few generations. And this is true of any inherited character that is quantitatively variable.

Ecologists are familiar with this concept of diversity and flexibility, and they recognise it with the term ecotype. An ecotype is a sub-population of a species, and it possesses special characteristics suited to its own particular locality within the ecosystem. The selection pressures vary from one part of an ecosystem to another, and different selection pressures produce different ecotypes. Ecotypes are genetically diverse and genetically flexible. One ecotype can be changed into another simply by exposing it to the appropriate selection pressures, for a sufficient number of generations.

The rate of change of ecotypes depends on two factors. First is the frequency of generations. Annual plants have at least one, and sometimes several, generations each year. Their ecotypes can accordingly change quite quickly, within a matter of a few years. The ecotypes of long-lived trees will obviously change much more slowly.

The second factor is the strength of the selection pressures. When an ecotype is well suited to its environment, there are no selection pressures, and the ecotype can then remain unaltered for many generations. But when the selection pressures are strong, the rate of change is rapid. This is exactly what happened with the maize crops in tropical Africa, when they were exposed to a re-encounter disease, discussed in Chapter 20.

This question of genetic flexibility brings us right back to the beginning of the discussion, and the comparison between the members of the Mendelian school and the biometricians. The ability of a character to vary quantitatively, in response to selection pressures, is very valuable in a natural ecosystem. On the other hand, a single-gene character is not quantitatively variable, and it will not change in response to selection pressures. Its frequency in the population can change, but the character itself is fixed and, in an individual, it is either present or absent, with no intermediates. Single-gene characters can be extremely valuable in special circumstances, such as providing a system of biochemical locks and keys in plant parasitism. But these circumstances occur rather infrequently. This explains why polygenic inheritance is so much more common than monogenic inheritance. Single-gene characters are rather rare in plants, and the members of the Mendelian school consequently had great difficulty in finding single-gene characters of economic importance.

Crop scientists do not normally think in terms of genetically flexible ecotypes. They tend to think in terms of cultivars, which are genetically uniform, and genetically inflexible. Because they do not normally work with wild ecosystems, crop scientists are less familiar than ecologists with this concept of genetic flexibility, and they often do not appreciate the extent to which plant populations can respond quantitatively to selection pressures. In particular, they rarely appreciate just how much a genetically diverse plant population can respond to selection pressure for horizontal resistance. This type of response is the basis of the biometricians' method of plant breeding.

There can be little doubt that, for the cultivation process, crop scientists should think agriculturally, in terms of genetic uniformity, and genetic inflexibility. But, for the breeding process, crop scientists should perhaps think ecologically, in terms of populations, quantitative genetics, genetic diversity, genetic flexibility, and horizontal resistance.

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CHAPTER NINE
Damage: Frequency and Injury

At this point, it might be useful to make a distinction between the frequency of parasitism, which is the proportion of host individuals that are parasitised, and the injury from parasitism, which is the damage suffered by those parasitised host individuals. Injury is usually expressed as the average of all the parasitised individuals.

An example will illustrate the point. A pride of lions may be said to parasitise a herd of zebras. The lions may kill one zebra, which they then consume almost entirely. This represents the minimum frequency of parasitism, but the maximum injury from parasitism. In ecological terms, the parasitism has a patchy distribution, and this extreme is often called the predator-prey relationship.

At the other extreme, every zebra is parasitised with ticks, but the injury caused by these ticks is negligible. This opposite extreme represents a maximum frequency of parasitism, but a minimum injury from parasitism. In ecological terms, the parasitism now has a uniform distribution, and this extreme is often called the host-parasite relationship.

The combination of frequency and injury represents the total parasite damage to the host population. In wild plants, this total damage never exceeds a rather low, permissible level. This permissible level is governed by the fact that the parasite must not impair the ability of its host to compete, either ecologically or evolutionarily. This is axiomatic, because any parasite that impaired its host's ability to survive would also threaten its own survival. For this reason, the frequency of parasitism, and the injury from parasitism, are inversely correlated in wild plants. A high frequency always results in a low injury, while a high injury always occurs with a low frequency.

In wild plants, frequency and injury are directly related to vertical resistance and horizontal resistance respectively. Vertical resistance provides a system of locking, which obviously reduces the frequency of parasitism. Horizontal resistance provides a second line of defence which, equally obviously, reduces the injury from parasitism. In a continuous epidemic, which has horizontal resistance only, there will be a high frequency of parasitism, but a low rate of injury. In a discontinuous epidemic which has vertical resistance as well as horizontal resistance, the frequency of parasitism will be low, particularly in the early part of the epidemic. But the individual injury from parasitism will be correspondingly higher in those individuals that were matched early in the epidemic.

In modern crops, on the other hand, we often have both a high frequency of parasitism, and a high injury from parasitism. The total damage is then high. This is because the vertical subsystem no longer operates as a system of locking, and the level of horizontal resistance is low.

Because we cannot employ a system of locking in our crops, it follows that we should aim at artificially high levels of horizontal resistance. We should domesticate horizontal resistance in the same way that our ancestors domesticated other continuous variables such as the yield and quality of wheat, rice, and maize. This would result in high frequencies of parasitism which, however, would not matter because the level of injury would be negligible.

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CHAPTER TEN
Pathosystems: Wild Plants and Crops

The concept of the pathosystem is based on the general system theory. There are many different kinds of systems, such as solar systems, political systems, ecological systems (ecosystems), mechanical systems, legal systems, electrical systems, and so on. The general systems theory concerns the properties that systems have in common. It is often helpful to study a system in terms of this theory, and in terms of other systems. Recently, the general systems theory has developed remarkably in the direction of complexity theory, which concerns dynamic systems that are both complex and adaptive. The Belgian scientist, I. Prigogine, discovered that such systems have the crucially important property of self-organisation, and they include economic systems, social systems, ecosystems, evolution, and life itself. But this is another story (Chapter 29).

One of the more useful concepts to emerge from the general systems theory is the notion of systems levels. For example, a book is a simple static system which has subsystems called chapters. Each chapter has subsystems called paragraphs. Each paragraph has subsystems called sentences, and so on down through words, syllables, and letters. The book itself is a subsystem of a supersystem called a library. Each of these subsystems is a systems level, higher than the one below it, and lower than the one above it.

In biology, systems levels can often be described with the word population. Thus, epidemiologically, a forest is a population of trees, a tree is a population of leaves, and a leaf is a population of microscopic cells.

An ecosystem is a biological system. It usually occupies a well-defined area, and it involves the interactions of all living organisms within that area, both with each other, and with their environment. A pathosystem is a special kind of subsystem of an ecosystem, and it is one which involves parasitism. A pathosystem usually involves the interaction of a population of one species of parasite, with a population of one species of host, but some pathosystems are more complicated than this. A plant pathosystem is one in which the host population is a plant, and the parasite is any species in which each individual spends a major part of its life cycle inhabiting, and obtaining nutrients from, one host individual. The concept of the pathosystem thus embraces both entomology and plant pathology, but the larger herbivores, which graze large populations of plants, are normally considered to be outside the conceptual boundaries of the pathosystem, and to belong to the higher systems level of the ecosystem.

A pathosystem may exist physically, in the sense that you can walk into it and study its populations, and their interactions. Alternatively, a pathosystem may exist only conceptually, in the form of a computer model, a diagram, or a mental picture.

Plant pathosystems also have systems levels. Any pathosystem is part of a supersystem, the ecosystem. And many plant pathosystems have two subsystems called the vertical subsystem and the horizontal subsystem. As we have seen, the vertical subsystem involves a gene-for-gene relationship and its function is to control the epidemic, and the frequency of parasitism, at the systems level of the population, by controlling allo-infection with a system of locking, in a discontinuous, genetically diverse pathosystem.

The horizontal subsystem does not involve a gene-for-gene relationship, and its principle function is to control parasitism, and the amount of injury, at the systems level of the individual host, by controlling all the consequences of a matching allo-infection. The vertical subsystem is thus a first line of defence, while the horizontal subsystem is a second line of defence.

A special aspect of recognising systems levels is the concept of an emergent. This is a property that is possible at one systems level but which is impossible at any lower systems level. For example, the system of locking of the gene-for-gene relationship is an emergent. It has emerged at the population level of the system, and it cannot exist at the lower systems level of the individual plant host, or the individual parasite. It can function only if there is a mixture of many different locks and keys, and this can happen only at the population level. At the subsystem level of the individual, there can be only one lock, or only one key. And, at the subsystem level of single gene in a gene-for-gene relationship (i.e., a single tumbler in the lock, or a single notch in the key), there can be only one tumbler, or only one notch.

People who work at a lower systems level, such as studying a single host plant, or a single resistance mechanism, may fail to see these emergents which occur at higher systems levels, and this omission results in a phenomenon called sub-optimisation. This word means the analysing or managing of a system in terms of only one, or a few, of its subsystems. This is the equivalent of "not seeing the forest for the trees" and of "arguing from the particular to the general". To analyze or manage a system of locking, using only one pattern of lock, and one pattern of key, would be suboptimisation. And it is now clear that the members of the Mendelian school were suboptimising twice over when they attempted to control the crop pathosystem using only one biochemical lock, and one that was made up of only one tumbler, one vertical resistance gene.

For our purposes, there are two entirely different kinds of plant pathosystem. These are the wild pathosystem and the crop pathosystem. The differences between these two kinds of pathosystem are the foundation of this entire discussion. It was mentioned at the beginning of this book that we do not spray wild plants with crop protection chemicals, yet the world is still green. We do spray most of our crops with crop protection chemicals, at a cost of billions of dollars each year. In spite of this, crop parasites cause losses that would feed about one billion people.

The wild plant pathosystem is a self-organising, complex, adaptive system in which people have not interfered. Natural selection has ensured that it is a balanced, dynamically stable system which has survived millions of years of evolutionary and ecological competition. The wild pathosystem is also a flexible system. It has genetic diversity and its populations respond to selection pressures. The overall effect is that the parasite does not impair the ability of its host to compete, and to survive.

As we have already seen, any parasite which threatens the evolutionary survival of its host also threatens its own survival. If the host becomes extinct, the parasite becomes extinct with it. So, wild parasites do not threaten the survival of their hosts. We can conclude with absolute certainty that every wild plant pathosystem, that has survived until the present, is a dynamically stable system in which neither the host's evolutionary survival, nor its ability to compete in a wild ecosystem, is impaired by its parasites.

The crop pathosystem is very different, and all these differences are due to the activities of people. First, the host population has been changed in various ways. The species itself has been changed genetically by the process of artificial selection and domestication. Domesticated plants have been further changed by modern plant breeding and, as we have seen, these plants are now cultivated in large areas of genetically uniform populations, in the form of pure lines, hybrid varieties, and clones. These uniform populations also have population densities that are usually much higher than those of most wild pathosystems. Both genetic uniformity and a high host population density assist parasites considerably.

Second, the environment has been changed. Land that may once have been covered with mixed forest has been cleared, drained, ploughed, harrowed, seeded, weeded, manured, treated with pesticides, and, perhaps, irrigated. Third, the parasite population has been subjected to some very strange selection pressures that would never occur in a wild pathosystem. Because of the use of pesticides, the parasite has also been liberated from the constraints of many of its own enemies (Chapter 14), and its population explosions can be much greater as a result.

One of the effects of all this artificiality is that the genetic diversity, the genetic flexibility, and the discontinuity of the wild pathosystem have been replaced with uniformity, inflexibility and, because of modern monocultures, a large measure of epidemiological continuity. As a direct consequence, the crop pathosystem is now an unstable, unbalanced system. Without the use of chemical pesticides, some of our crops could not be grown at all, and many others would suffer intolerable reductions in the yield and quality of their crop product.

The positive side of this somewhat dismal picture is that our greatly expanded human population can still feed itself. Some environmentalists talk romantically of a "return to nature", and they deplore the artificiality of modern agriculture. But we must be realistic. We must remember that modern agriculture supports a human population density that is some hundreds, perhaps a thousand times, greater than the population density of our primate ancestors, who inhabited the world before the emergence of human tools and hunting. If we abolished agriculture, so that only hunter-gatherers could survive, most of the people in the world today would die of starvation. Even a return to the pre-industrial agriculture of last century, with its harvesting by hand, and its horse-drawn ploughs and wagons, would kill off more than three quarters of the world's present human population. So, however much we may deplore the artificiality of agriculture, we must appreciate that the only alternative (at present) is a really massive human mortality. (There is a third possibility which is several decades into the future, and which is discussed in the last chapter in this book). However, the main purpose of this digression is to emphasise that the crop pathosystem is very different from the wild plant pathosystem.

We should now examine the diagram on page 66 in its entirety, the ten pairs of biological contrasts. The wild pathosystem is clearly on the left-hand side of the diagram, with its genetic diversity and flexibility and, frequently, its discontinuity. It is on this side of the diagram that we also find the vertical subsystem functioning as a system of locking controlling allo-infection. In its turn, this produces many non-matching interactions, resulting in low frequencies of parasitism, but relatively high rates of injury. This is also the side of the diagram that was chosen, in effect, by the members of the Mendelian school, with their single gene inheritances and their pedigree breeding.

The crop pathosystem is on the right-hand side of the diagram, with its genetic uniformity and inflexibility, and its tendency to epidemiological continuity. Also on the right-hand side of the diagram is the horizontal subsystem, controlling auto-infection and matching interactions, and producing high frequencies of parasitism, but low rates of injury. This is also the side of the diagram that was chosen, in effect, by the biometricians, with their polygenic inheritances and their population breeding,

The conclusion seems inescapable. The vertical subsystem is the wrong subsystem for the crop pathosystem. Since 1905, crop scientists have had a choice between the two subsystems. Because of a concatenation of circumstances, which included a silly scientific dispute, and the vociferous clamour of the members of the Mendelian school, who had single-gene resistances, but nothing else of economic significance, the whole of crop science was led up a blind alley. And it is still stuck in there, apparently unable to back out.

In fact, that option still exists. We can investigate the horizontal subsystem at any time. And, if these investigations are satisfactory, we can employ horizontal resistance at any time also. This is the best hope we have and, apparently, the only hope we have, of reducing, or even eliminating, both the crop losses caused by parasites, and the use of crop protection chemicals in our crops.

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CHAPTER ELEVEN
The Disadvantages of Vertical Resistance

At this point, it may be useful to summarise the disadvantages of vertical resistance, while recognising that it does have two very considerable advantages.

Two Advantages of Vertical Resistance

It was mentioned earlier that the beauty and elegance of the Mendelian gene-transfer techniques captured the imagination of plant breeders all over the world. This is the main attraction of vertical resistance. It is so scientifically elegant, and so easy to see, and to manipulate in a breeding program. Vertical resistance also has the very considerable practical advantage that it normally confers a complete protection against the parasite in question. It confers an apparent immunity. Opposing these two advantages, however, vertical resistance has several, very serious disadvantages.

Temporary Resistance

First, as is now abundantly obvious, vertical resistance is temporary resistance when it is employed on a basis of host population uniformity. It fails to operate on the appearance of a matching strain of the parasite. And, because vertical resistance was used whenever it could be found, this disadvantage has tormented most of twentieth century crop science.

Genetic Source of Resistance Essential

The second disadvantage of vertical resistance has already been mentioned. This is the need, indeed the necessity, of first finding a genetic source of resistance. If a source of resistance cannot be found, the breeding cannot begin. There are some famous crop parasites, such as Colorado beetle (Leptinotarsa decemlineatd) of potato, and Take-All disease (Gäumannomyces graminis) of wheat, for which a source of vertical resistance has never been found, and Mendelian resistance breeding has never been attempted. If it is concluded that breeding for resistance is not possible, alternative methods of control must be used. Usually, the only alternative control involves chemical pesticides, and this is another reason we now use these substances in such horrifying quantities.

A side-effect of this need for a genetic source of resistance comes from a natural difference between pests and diseases. As it happens, gene-for-gene relationships occur much more frequently with plant diseases than with the insect parasites of our crops. There are good biological reasons for this difference, which is related to asexual reproduction in an r- strategist parasite. Sexless reproduction leads to much more rapid population explosions. It is also much more common among crop pathogens than it is among the insect pests of crops. As we have seen (Chapter 6), the evolutionary survival value of a gene-for-gene relationship is in the control of parasite population explosions.

What matters here is that the members of the Mendelian school could not often find a source of resistance to insect pests. As a direct consequence, there was much less breeding of crops for insect resistance than there has been for disease resistance. This is yet another indication of how the members of the Mendelian school have dominated, and distorted, plant breeding during the present century.

There is another, rather disturbing, side-effect of the need for a genetic source of resistance. As we have seen (Chapter 6), a gene-for-gene relationship, and vertical resistance, cannot evolve in a continuous pathosystem. Because of winters, most temperate crops are derived from discontinuous wild pathosystems, and they have many vertical resistances. Many tropical crops, on the other hand, are derived from continuous wild pathosystems, and they have no vertical resistances. Consequently, it was mainly in the poorest, non-industrial, tropical countries that resistance breeding programs were never even started, because genetic sources of resistance could not be found.

The "Red Queen" Situation

The third disadvantage of vertical resistance may be called the "Red Queen" situation, named after Lewis Carrol's Alice Through The Looking Glass. It will be remembered that the Red Queen said to Alice "Now here, you see, it takes all the running you can do to keep in the same place". If a plant breeder is under continuous pressure to produce new cultivars, in order to replace those whose vertical resistances have failed, it is difficult to make progress in other directions. It will be remembered that resistance to crop parasites is only one of the four primary objectives in plant breeding. The others are the yield and quality of the crop product, and agronomic suitability.

A breeder may be forgiven if he concludes that these other objectives are collectively more important than parasite resistance. He may also conclude that the control of crop pests and diseases is really the responsibility of the entomologists and plant pathologists. It is their duty to ensure that these pesky parasites do not damage the magnificent yield, superb quality, and agronomic excellence of his new cultivars. So, the breeder abandons resistance breeding, and dumps this problem in the lap of his colleagues. Sadly, almost the only weapons available to the entomologists and pathologists are crop protection chemicals. This "Red Queen" situation, and the consequent abandoning of the resistance objective in plant breeding, is perhaps the chief reason why we now use these chemicals in such large quantities.

The Vertifolia Effect

There is a fourth disadvantage to breeding for vertical resistance that is insidious, and largely unappreciated, but dangerous for this very reason. This is the decline in the level of horizontal resistance that slowly but inexorably occurs. This effect was first observed by Vanderplank, who called it the "vertifolia effect" after a potato variety of this name. Ironically, this potato variety had been named "Green Leaf on account of its vertical resistance to blight (Phytophthora infestans). It was only after this vertical resistance had broken down that it was discovered that the "Vertifolia" potato was quite unusually susceptible to blight, because it had a remarkably low level of horizontal resistance.

Horizontal resistance can only be observed and measured in terms of the level of parasitism. If there is no parasitism during the breeding process, because of a functioning vertical resistance, or because the breeder is protecting his screening population with insecticides and fungicides, the level of parasitism, and the level of horizontal resistance, cannot be observed. Individuals with high levels of horizontal resistance are relatively rare in a breeder's genetically mixed population. This means that individuals with only low or moderate levels of horizontal resistance are more likely to be selected, on the basis of their other attributes. In the course of many breeding generations, the level of horizontal resistance in the breeding population as a whole declines until it reaches dangerously low levels. This explains why the breakdown of vertical resistance is so very damaging in most modern cultivars. The second line of defence, the horizontal resistance, is largely lacking.

This cryptic loss of horizontal resistance also explains why many modern cultivars need such large quantities of chemical pesticides if they are to be cultivated at all. Not a few breeders, who abandoned resistance breeding years ago, have been protecting their screening populations with crop protection chemicals. This makes the breeding work incomparably easier (Chapter 18). Sadly, it also leads to this hidden decline in the level of horizontal resistance. It produces a progression of cultivars that are increasingly susceptible to a widening range of parasites, and requiring an escalating need for pesticide protection. We have actually been losing horizontal resistance to crop parasites for most of this century, and most modern cultivars have considerably less resistance than the cultivars of 1900.

(To avoid possible confusion, it should be mentioned that pedigree breeding can increase the level of quantitative variables, such as yield, although it is not necessarily the best method for doing this. This is why modern plant breeding has generally been successful in the objectives of improved yield, quality of crop product, and agronomic suitability. These characters were visible, and could be selected, even though they were quantitatively variable. The vertical resistance was used because it was so suitable for the back-crossing process, even though it later proved to be ephemeral. The horizontal resistance was valuable, but it was not selected because its effects were invisible, being concealed by either vertical resistance or crop protection chemicals. And, on the occasions when its effects were visible, they were completely obscured by parasite interference, Chapter 14).

Problems with Comprehensive Resistance

There is another disadvantage in breeding for vertical resistance. Most species of crop have dozens of pests, and dozens of diseases. Unfortunately, it is not really feasible to breed for vertical resistance to more than one species of parasite at a time. The basic idea of pedigree breeding is to produce one cultivar with vertical resistance to one species of parasite, a second cultivar with vertical resistance to a second species of parasite, and so on. This results in a series of cultivars, each with one vertical resistance to a different species of parasite. Using gene-transfer methods, these vertical resistances are then all combined in a single cultivar, a "super-cultivar" with resistance to everything. At least, that is the idea. And it is a neat idea. Unfortunately, it is almost impossible to achieve in practice. The sheer volume of breeding work is so exorbitant that one or more vertical resistances are likely to be matched before the breeding is completed. Furthermore, such a super-cultivar is like a chain, in that it is only as strong as its weakest link. And, like the chain, the super-cultivar would be ruined with the failure of only one weak link, one short-lived vertical resistance.

Loss of Genetic Diversity

Vertical resistance usually confers complete protection against a parasite, and this protection functions over a very wide climatic range. This means that a vertical resistance is relatively insensitive to climate, and a single cultivar can then be cultivated over a huge area. This was an essential aspect of the early cultivars of the green revolution. This degree of crop uniformity has certain economic advantages but it also has two drawbacks. First, a huge area of a single cultivar is very vulnerable to a new, matching strain of the parasite. And, second, the widespread use of a single cultivar leads to a loss of genetic diversity. In its turn, this threatens to destroy unexplored sources of resistance. Our preoccupation with vertical resistance is the main reason for the current concern over genetic conservation (Chapters 19 & 20).

Man-Made Problems

It is difficult to avoid the conclusion that most of our crop parasite problems are man-made. And that most of these problems stem either directly or indirectly from our misuse of vertical resistance, and our neglect of horizontal resistance.

The happy corollary of this sad situation is that all these man-made problems can be corrected. And the discerning reader may already have observed that Part Three of this book is labeled "Solutions".

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CHAPTER TWELVE
Horizontal Resistance Compared

Horizontal resistance does not have these disadvantages. It undoubtedly has some disadvantages, which I shall describe in a moment, but, in general, its advantages are striking.

Permanent Resistance

The first, and most obvious, advantage of horizontal resistance is that it is durable resistance. It cannot be matched, because it always is matched. It operates against strains of the parasite that have already matched the vertical resistance of the host, and that have already commenced the process of parasitism. Consequently, horizontal resistance cannot break down, like vertical resistance. Horizontal resistance occurs in all plants, independently of any vertical resistance genes that they may be carrying, and it operates against all strains of the parasite, independently of any vertical parasitism genes that they may be carrying. For all practical purposes, it is permanent resistance.

Complete Resistance

Second, horizontal resistance is a quantitative variable, with all degrees of difference between a minimum and a maximum. This means that the level of horizontal resistance can be changed. An inadequate level of horizontal resistance can be increased by further breeding. In theory, at least, the level of horizontal resistance can be increased until the parasite in question is controlled completely. In practice, this may require a level of horizontal resistance that approaches, or even exceeds, the maximum and, unfortunately, no one knows what the maximum levels of horizontal resistance actually are. It is a measure of the neglect of horizontal resistance, during the twentieth century, that the maximum level has not yet been determined against any species of parasite, in any species of crop. The opponents of horizontal resistance are apt to claim that the maximum levels will be inadequate, but they are just guessing, because no one knows for sure.

However, some evidence is available. The difference between the near-minimum and the near-maximum levels of horizontal resistance can be enormous. This has been clearly demonstrated, for example, in potato blight (Chapter 18), tropical rust of maize (Chapter 20), coffee rust (Chapter 21), coffee berry disease (Chapter 21), Phylloxera of grapes (Chapter 23), and various diseases of sugarcane, such as smut, rust, and mosaic virus (Chapter 22). With all these parasites, very low levels of horizontal resistance can lead to a complete loss of the crop, while very high levels provide a control of the parasite that is, in effect, complete.

This range of differences is to be expected because, in the wild, the epidemiological competence of parasites can vary to a similar extent. In a favourable climate, the parasite will have an epidemiological competence that is maximal, and wild host ecotypes in that area will accordingly accumulate the maximum levels of horizontal resistance. Conversely, in an unsuitable climate, in which the parasite can only just survive, or in the physical absence of the parasite, the need for horizontal resistance will be minimal. In these circumstances, host ecotypes will lose most of their horizontal resistance, and they will then be highly susceptible, if taken to an area where the parasite has a high epidemiological competence.

It appears, therefore, that plant breeders have a very wide range of levels of horizontal resistance available to them. Artificial selection for high levels of horizontal resistance should accordingly provide a total control of many parasites, of many crops, in many areas. Consequently, it is probable that horizontal resistance can provide protection that is complete, as well as permanent.

Genetic Source of Resistance Not Necessary

A further advantage of being quantitatively variable is that no genetic source of resistance is necessary when breeding for horizontal resistance. With horizontal resistance, we can breed for resistance to those many species of crop parasites, particularly the insect pests, for which no resistance breeding was ever attempted by the members of the Mendelian school, simply because no source of single-gene resistance could be found. This emancipation from the practical constraint of first finding a source of resistance is critically important, and it must be explained.

Let us suppose a hypothetical plant population in which every individual has only ten percent of the alleles contributing to horizontal resistance. Every individual in that population is thus highly susceptible. And this means that the population as a whole is highly susceptible. But let us suppose also that this population is genetically diverse. Each of the individuals in it has a different ten percent of resistance alleles. This means that all the alleles for horizontal resistance are present in the population, but they are spread so thinly that every individual is susceptible.

As we saw earlier, breeding for horizontal resistance by recurrent mass selection involves changing gene frequencies. In the course of each generation of plants exposed to suitable selection pressures, the percentage of those resistance alleles increases by transgressive segregation (Chapter 20). This is a process of concentration that can continue until it approaches one hundred percent, which is a very high level of resistance. This concentration of resistance alleles can be compared, somewhat loosely, with the concentration of alcohol that occurs when wine is distilled into brandy.

It follows that breeding for horizontal resistance requires a reasonably broad genetic base (i.e., a reasonably diverse parent population) to ensure that all the alleles are present. But, apart from that, all the plants in that parent population can be susceptible. If it later transpires that the genetic base was too narrow, and that it could not provide the level of horizontal resistance required, the breeding base can be broadened by adding new genetic material to it.

Comprehensive Resistance

There is yet another advantage to horizontal resistance. A little-known aspect of recurrent mass selection is that it permits screening for many different variables at the same time. This means that the breeder can exert simultaneous selection pressures for all the breeding objectives. In effect, the breeder need screen his plants for only four things: high yield, high quality of crop product, good agronomic suitability, and good health in the presence of all the locally important parasites. In systems terminology, this means working at the highest systems level. It is called the holistic approach.

In each screening generation, the criterion of good health means simply the least parasitised host individuals, taking all locally important species of parasite into account. In practice, this is usually very easy to measure. The greenest individuals, or those with the highest individual yields, are the least parasitised. Severely parasitised plants cannot be the most green, or have the highest yields. In each screening generation, the best plants are selected as parents for the next generation, regardless of how poor they may be. In each generation, the best plants are better than those of the previous generation, and a steady improvement in all the desired variables is achieved.

This means that, in addition to being permanent resistance, and complete resistance, horizontal resistance can also be comprehensive resistance, in the sense that it operates against all the locally occurring species of parasite. It need hardly be added that, if a cultivar has resistance that is permanent, complete, and comprehensive, it will not need any chemical pesticides to protect it from its parasites. And if this were true of all cultivars of all crops, the use of chemical pesticides on our crops would cease. (However, it must be remembered that the herbicides, or weed-killing chemicals, are not included in this discussion).

A full appreciation of the potential of horizontal resistance requires a comparison with the "Red Queen" situation of vertical resistance breeding. It will be remembered that this takes all the running you can do to stay in the same place, and it leads eventually to the abandonment of resistance breeding. In complete contrast, breeding for horizontal resistance is progressive and cumulative. A good cultivar need never be replaced, except with a better cultivar. Ideally, the better cultivar should be superior in all respects, including its yield, its quality of crop product, its agronomic suitability, and its horizontal resistance to all locally occurring parasites. This progress can continue, no doubt with diminishing returns, until a plateau is reached beyond which no further progress is possible.

This plateau represents the ultimate practical productivity of a pesticide-free agriculture. It is a level of productivity that is at least twenty percent higher than our current levels, because that is the present rate of loss to crop parasites. It may be a level of productivity that is considerably higher still, because of the many constraints that the Mendelian breeding methods have imposed on crop improvement as a whole. No one seriously suggests that the members of the Mendelian school have taken crop husbandry to the limits of production, even with the use of crop protection chemicals. But, if we use horizontal resistance, those limits are in full, clear view.

So, how realistic is the possibility of attaining these ultimate limits of production? The fact is that no one knows for sure, and scientific opinions differ widely. At the very least, it is reasonable to suggest that the matter merits investigation. After all, if true, these prospects would solve many problems. If only partly true, they would be worth pursuing. And, even if they were proved completely false, their investigation would still have been justified. For the present, readers of this book can form their own judgment from the examples of horizontal resistance that are given in Part Two. But, first, we must consider some of the disadvantages of horizontal resistance.

Disadvantages of Horizontal Resistance

Quantitative variables, such as horizontal resistance, require the entirely different breeding methods of the biometricians. Many crop scientists are loyal to the Mendelian outlook, and they are reluctant to switch to these alternative techniques. Furthermore, there are many pedigree breeding programs which represent decades of patient and painstaking work. These programs cannot be changed to breeding for horizontal resistance, and no one wants to abandon them. Not yet, anyway. There would have to be some very convincing demonstrations of the feasibility and value of horizontal resistance before anyone would seriously consider abandoning such old and well established programs. And these demonstrations take time. Horizontal resistance breeding programs will thus require entirely new research projects.

Another difficulty with horizontal resistance is that gene-transfers are impossible. It is just not possible to transfer a good level of horizontal resistance from a resistant plant to a susceptible cultivar. This type of hybridisation would normally lead to a halving of that good level of horizontal resistance. On average, there would be a further halving of whatever resistance remained with every generation of back-crossing to the susceptible cultivar. Horizontal resistance is not amenable to gene-transfer methods. For this reason, when breeding plants for horizontal resistance, it is necessary to select for all desirable variables simultaneously. This explains why the existing vertical resistance breeding programs could not be converted to breeding for horizontal resistance.

Another of the problems with quantitative variables is that they have a maximum. There is a genuine fear that the maximum attainable levels of horizontal resistance may not be enough to provide a complete control of all the parasites of a crop. This point can only be resolved by practical experiments. And these experiments have still to be done. Indeed, it is high time they were started. In the meanwhile, all we can say with complete confidence is that even small increases in the current levels of horizontal resistance would be an improvement, and would lead to a reduced use of chemical pesticides.

Even small increases in the level of horizontal resistance would make all other aspects of crop pest management more effective, easier, cheaper, and safer. This would happen because crop protection chemicals would need to be applied less frequently, in lower concentrations, of less hazardous chemicals. But, for all other conclusions, we have to wait and see. In the meanwhile, any opponent of horizontal resistance, who claims that these experiments are not worth doing, can only be guessing. We should also remember that, in science, hostility to something new must always be suspect.

A further disadvantage of horizontal resistance is that the weather is variable, and an occasional freak season may so favour the parasite that a normally adequate level of resistance becomes inadequate. However, we can now handle meteorological data well enough for farmers to be given sufficient warning of a freak epidemic, and they can then use crop protection chemicals. Given an appropriate level of horizontal resistance, this should not happen more than once or twice each century. If it happened more often than this, the level of horizontal resistance could probably be increased by further breeding.

A minor disadvantage has already been mentioned. The primary function of horizontal resistance in a wild plant pathosystem is to reduce the injury from parasitism, rather than the frequency of parasitism. This means that cultivars with high levels of horizontal resistance are likely to have negligible injury from their parasites, but they are likely to show a very high frequency of parasitism. In other words, every plant will be parasitised, but only to a trifling extent. Most consumers have got used to fruit and vegetables that are entirely free of pest and disease blemishes. This is part of the pesticide mentality. Perhaps we should encourage the public at large to regard a few quite minor blemishes as evidence for freedom from crop protection chemicals. We should also remember that a few parasites are necessary in order to maintain the agents of biological control. This topic is discussed in the next chapter.

A further drawback of horizontal resistance is that it is "site-sensitive". Let us consider two different sites, two different agro-ecosystems. A cultivar might be in perfect balance with the first of these sites. That is, it has exactly the right amount of horizontal resistance to control every species of parasite at that site in, perhaps, ninety-seven seasons out of every hundred. The second site, however, is climatically different, and the epidemiological competence of parasites varies with climate. A difference of temperature, or rainfall, can increase or decrease the population explosion of a parasite. Consequently, a cultivar which is perfect in one site may be unsuitable in another site, because it has too much resistance to some parasites, and too little to others.

In practice, this means that there must be a separate breeding program for each site. This is called on-site selection. However, this need for a multiplicity of breeding programs is no great hardship because most epidemiological "sites" are quite large. Much of a country the size of England, for example, would normally be a single site or, at most, two or three sites, for most species of crop.

Some environmentalists might even consider this site sensitivity to be an advantage, because it helps to maintain genetic diversity in our crops. The use of vertical resistances, which operate over a much wider climatic range, can lead to a loss of genetic diversity and, as we have seen (Chapter 7), this is one of the main causes of the current concern about genetic conservation.

Another disadvantage of quantitative variables is that they can be lost just as easily as they can be accumulated, and horizontal resistance is no exception. A loss of horizontal resistance is called the erosion of horizontal resistance, and is discussed more fully in a moment. Fortunately, the various techniques, already described, for preserving agricultural traits in seed-propagated inbreeding crops, seed-propagated outbreeding crops, and vegetatively propagated crops, will normally prevent the erosion of horizontal resistance.

Finally, many of the opponents of horizontal resistance claim that there is a fundamental conflict between this kind of resistance and the components of yield, quality, and agronomic suitability. They agree that the levels of horizontal resistance can indeed be increased, but they argue that this can be done only at the expense of these other valuable traits. This conclusion is based on the general observation that wild plants have high resistances but low yield and quality, while cultivated plants have high yields and quality, but low resistances. However, such a conclusion is not necessarily sound, because this situation in our crops could also have arisen, and probably did arise, from the use of pedigree breeding methods.

The converse argument is that one of the biggest constraints on yield and quality today is the damage caused by crop parasites, in spite of the use of chemical pesticides. If we could reduce, or even eliminate, that damage by using horizontal resistance, then this resistance would improve the yield and quality, rather than lessen them. To say nothing of reducing, or even eliminating, those crop protection chemicals. So who do we believe? For ease of discussion, only yield need be considered, while bearing in mind that the same arguments can be applied to other quantitative variables, such as the quality of crop product, and agronomic suitability.

Both horizontal resistance and yield are quantitative variables. Each has a minimum and a maximum. We want the maximum of both of them but, before the maximum of either can be reached, there is probably a point at which they come into conflict. The horizontal resistance can then be increased only at the expense of the yield, and the yield can be increased only at the expense of the horizontal resistance. The obvious questions are: Where does this point of conflict appear? And is it of practical significance?

This problem can be illustrated by the example of wheat. The world average yield of wheat is 1.4 tonnes/hectare. The average for the North American prairies is 2.2 t/ha. The average in Western Europe is 5.0 t/ha, while the best individual farms in that region produce 10.0 t/ha. The experimental maximum (but commercially uneconomic) yield is 15.0 t/ha, which is more than ten times the world average. No one knows the ultimate potential yield of wheat. It might be 20.0 t/ha. Somewhere between the minimum and the maximum yields, there is almost certainly a point at which horizontal resistance and yield come into conflict. But where?

Obviously, this point can be determined only by experiment and, unfortunately, these experiments have yet to be done. In the meanwhile, we can only guess. My own guess is that the point of serious conflict between yield and resistance is close to the maximum commercial yields now being obtained on the highest yielding farms in Western Europe. That is, at about 10 t/ha, which is approximately halfway between the theoretical minimum and the theoretical maximum. But let us be conservative, for the sake of equable discussion, and put it at half this level, at 5 t/ha.

At first sight, this would mean that the successful and universal use of horizontal resistance would increase the world average yield of wheat from 1.4 t/ha to 5 t/ha. This would more than treble the world's wheat production without any increase in the area of cultivation. But the calculation is not that simple, and not that rosy. The constraints on the world average yield are not all due to parasites. Other constraints include low rainfall, bad soils, inadequate fertilisers, storms, weeds, poor farming, and so on. So let us suppose that half of the total constraints are due to parasites. The universal use of comprehensive and complete horizontal resistance, combined with the maximum yield that can be combined with that resistance, might then increase the world average yield of wheat from 1.4 t/ha to 3.2 t/ha, which is an increase of rather more than 125%.

Which is not bad, even if it is a mere estimate, based on guesswork. Nevertheless, this level of improvement, in all our crops, could to do a lot to alleviate the world food problem, possibly right up to the time when human population growth is finally stabilised. It could also do a lot to alleviate the pesticide pollution problem. The real point, of course, is that we need to know for sure. This matter merits scientific investigation. We simply cannot afford to neglect it any longer.

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CHAPTER THIRTEEN
The Erosion of Horizontal Resistance

It must be clearly recognised that horizontal resistance can be eroded in a number of ways. But this quantitative loss of horizontal resistance is very different from the qualitative breakdown of vertical resistance, and it is important not to get the two of them confused. At first sight, the very thought of an erosion of horizontal resistance is upsetting, even alarming. Horizontal resistance is supposed to be durable, and to persist indefinitely, or at least during the foreseeable, agricultural future.

For people who understand horizontal resistance, and who are working with it, erosion is important only occasionally, and these occasions can usually be avoided, or they are easily controlled. However, for people who do not understand horizontal resistance, such as pedigree breeders working exclusively with vertical resistance, the mere possibility of an erosion of horizontal resistance often provides an adequate excuse to deride it, and to neglect it experimentally.

Host Erosion

The erosion of horizontal resistance that occurs most commonly is a result of genetic changes in the host population. This kind of erosion is called the host erosion of horizontal resistance, and it is the converse of the accumulation of horizontal resistance that occurs when there is positive selection pressure for it.

A host erosion can occur either during breeding, or during cultivation. Horizontal resistance can be lost during breeding because of the absence of a parasite. As we shall see in later chapters, such an absence can occur naturally (Chapter 20), or because of a functioning vertical resistance (Chapter 18), or because of protection with pesticides (Chapter 18), or during breeding of a crop with parasites that accumulate only slowly, such as the potato viruses (Chapter 18). In other words, horizontal resistance is eroded if there is no selection pressure for it during the breeding process. Most of the current susceptibilities in modern crops are the result of a host erosion during breeding.

A host erosion of horizontal resistance during cultivation can occur only if the crop is genetically flexible, as happened with the open-pollinated, subsistence maize crops in tropical Africa (Chapter 20). This kind of erosion occurs either when the parasite is absent from the agro-ecosystem in question, as with tropical rust of maize, or when the parasite has a very limited, patchy distribution, as with maize streak virus (Chapter 20). In practice, these days, very few commercial crops are genetically flexible during cultivation, although many subsistence crops are flexible. In those commercial crops that are genetically flexible during cultivation, such as alfalfa (Medicago sativa), the selection pressures for resistance must be continuously maintained in populations that are being used for seed production.

A host erosion of horizontal resistance can also occur in special circumstances. For example, there is a North American insect parasite of the roots of grapes, called Phylloxera (Chapter 20). In the 1860s, Phylloxera was found in France and the European wine industry was faced with total ruin. The problem was solved by grafting the very susceptible, classic wine grapes on to rootstocks of wild American grapes which have very high levels of horizontal resistance to Phylloxera. That resistance has now endured for more than a century in Europe.

In California, however, there is a different situation. Because the resistant rootstocks depress the yield of grapes somewhat, Californian vines are often grafted on to hybrid rootstocks. These hybrids are half wild American, and half European, and their use increases the yield of grapes. Unfortunately, they are also moderately susceptible to Phylloxera, and this pest has recently become a serious nuisance in some Californian vineyards that have these hybrid rootstocks. It is important not to misinterpret a situation such as this, and to attribute it to a breakdown of vertical resistance, or to a parasite erosion of horizontal resistance (see below).

Parasite Erosion

An erosion of horizontal resistance can occasionally occur as a result of population changes in the parasite. This is called the parasite erosion of horizontal resistance. It is an apparent erosion which, in fact, is not due to any change in the resistance itself. There is an increase in the level of parasitism, resulting from an increased parasitic ability in the parasite.

Most species of parasite have a strict limit to their parasitic ability and they cannot increase it beyond that limit, at least during the foreseeable agricultural future. (This argument follows logically from the fact that any parasite which endangers its host's ability to survive, also endangers its own survival). In practice, a parasite erosion of horizontal resistance is normally important only with a special category of parasite called a facultative parasite. This is a parasite that can change between the ability to extract nutrients from a living host, and the ability to extract nutrients from dead plant material. These two abilities are inversely proportional. That is, the greater the one, the less the other.

For example, there is a soil-inhabiting fungus called Fusarium oxysporum f.sp. lycopersici that causes a wilt disease of tomatoes. If tomatoes have not been grown in that soil for many years, the non-parasitic form of the fungus predominates. Under these circumstances, tomatoes can be grown with very little loss from wilt disease. However, if tomatoes continue to be grown in that soil, season after season, the parasitic ability of the fungus increases. This causes an increase in the frequency of wilt disease, and an apparent loss of resistance in the tomatoes.

A parasite that can obtain nutrients only from a living host is called an obligate parasite. There does not appear to be a single known example of a significant parasite erosion of horizontal resistance occurring with an obligate parasite.

Environment Erosion

In addition to host and parasite erosion, an environment erosion of horizontal resistance is possible. This again is an apparent erosion of resistance, and it occurs when someone takes a cultivar from an area where the parasite has a low epidemiological competence, to an area where its epidemiological competence is considerably higher. Typically, this happens when a cultivar that is suited to a dry climate is taken to an area with a humid climate. This happened when the coffees of arid Harrar were taken to the much wetter areas of south-west Ethiopia (Chapter 21). Environment erosion also accounts for many susceptibilities in ancient clones being grown in new areas (Chapter 23), and it is also the main reason for practicing on-site selection (Chapter 12).

False Erosion

Finally, there can be a false erosion of horizontal resistance. This can result from sloppy experimental work, inaccurate measurements, mixing of labels, and so on. It can then transpire that a genetic line that was believed to be resistant is, in fact, susceptible. This happened typically with some new sugarcane cultivars that had not been adequately tested for resistance to mosaic virus (Chapter 22). These cultivars were mistakenly believed to be resistant. When they later became severely diseased with mosaic, in farmers' fields, some scientists concluded, quite incorrectly, that there had been a breakdown of vertical resistance.

A false erosion of resistance can also result from psychological errors. For example, there may be a cultivar that is the standard of resistance, against which all other lines are compared. As resistance accumulates in the entire breeding population, during a number of years of breeding, the resistance of that standard cultivar appears to decrease, relative to the population as a whole. This is obviously an illusion, but it can be an alarming one, if its cause is not understood.

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CHAPTER FOURTEEN
Three Sources of Error

There are three phenomena, called parasite interference, population immunity, and biological control, which all suggest that considerably less horizontal resistance than we may think will achieve a satisfactory control of many crop parasites.

Parasite Interference

It was mentioned earlier (Chapter 1), that there are lies, damn lies, and statistics. Although statistics is a perfectly respectable branch of mathematics, it can be misused and abused. In the study of crop parasites, statistics has been misused and, as a consequence, it has caused a level of confusion and uncertainty that boggles the imagination. This is not the fault of the statistics. It is the fault of the scientists who misused these mathematical techniques.

When I was in my first job, in Africa, statistically controlled field trials were all the rage. Various "treatments", such as the amounts and kinds of fertiliser, had to be laid out in carefully measured field plots which were both replicated and ran-domised. And there had to be "local control", which involved untreated plots for purposes of comparison. The mathematics were quite complicated, and they were the bane of non-mathematical crop scientists. This was in the days before computers, when calculating machines were mechanical, would do only simple arithmetic, and had to be cranked by hand.

The mathematics had been worked out during the 1930s, mainly by the British mathematician, R.A. Fisher;, and the standard text was a book by Fisher and Yates. This statistical methodology was excellent for investigating agronomic variables, such as the spacing between the plants, or the yields of different cultivars, but it was a source of major error when it came to crop pests and diseases. This was first recognised by J.E. Vanderplank who called it the "cryptic error" in field trials. The error occurred because crop parasites, air-borne, are mobile. They can move from one field plot to another, and this phenomenon is now called inter-plot interference, or parasite interference.

This parasite interference can easily increase the levels of parasitism in test plots by a hundred-fold, and sometimes by as much as a thousand-fold. This happens because the "control" plots, included for purposes of comparison, contain plants that are highly susceptible, and highly parasitised. These parasites then move into neighbouring plots in large numbers.

Perhaps the most dramatic example of parasite interference is seen in the small plots used by wheat breeders. These plots consist of only a few plants taken from the seeds of one head of wheat. Each plot consists of a single row of wheat. Occasionally, one row of vertically resistant wheat has several very susceptible plots on each side of it. Rust spores cannot match the resistant wheat. They can only produce minute, hypersensitive flecks that result from non-matching allo-infections. But these flecks occur in their millions. There can be so many of them that the resistant wheat appears diseased, and the wheat breeders warn that this phenomenon must not be mistaken for true disease. This indicates how misleading parasite interference can be.

Parasite interference is responsible for three different kinds of error.

The first error concerns the use of crop protection chemicals. If test plots sprayed with a pesticide suffer parasite interference, they will need more pesticide than if there were no interference. Recommendations to farmers, concerning pesticide use, are often based on erroneous field trials. This error occurred so commonly during the 1950s and 1960s that no one can be quite sure how excessive our use of crop protection chemicals was during that period. Indeed, no one is quite sure how excessive our current use of crop protection chemicals may be, because of this error in field trials. It is not an error that the people who sell crop protection chemicals are keen to correct.

The second error concerns vertical resistance. It will be observed that parasites moving from one field plot to another are allo-infecting the new plot. If the receiving plot has an unmatched, and functioning, vertical resistance, the interference will have no effect at all, other than the hypersensitive flecks mentioned above. The function of vertical resistance, after all, is to control allo-infection. Consequently, under the conditions of maximum interference, which occur typically in pedigree breeders' small screening plots, vertical resistance looks perfect, in the sense that there is no parasitism. But this perfection is an illusion, because neither the temporary nature of the vertical resistance, nor a low level of horizontal resistance, are apparent. This illusion has been deceiving members of the Mendelian school of plant breeders for the whole of the twentieth century.

The third error concerns horizontal resistance. This kind of resistance can be seen and measured only after vertical resistance has been matched. If the matched plot in question has the level of its parasitism increased by, perhaps, one hundred-fold, or even one thousand-fold, because of parasite interference, the horizontal resistance will look terrible. Under these circumstances, pedigree breeders can hardly be blamed if they conclude that horizontal resistance is useless or, even, that it does not exist. Far more important is the fact that this level of horizontal resistance may be entirely adequate to control the parasite completely, when it is employed in farmers' fields that are free from interference.

No one can be blamed for not appreciating this, when gazing at those frightful looking pedigree breeders' plots, with their parasitism increased several hundred-fold because of parasite interference. But it is sad to think that countless numbers of good lines, with perfectly adequate levels of horizontal resistance, have been needlessly thrown out in the past, because of the vivid, but entirely false, appearance of extreme susceptibility produced by parasite interference.

To sum up, parasite interference has misled crop scientists in three ways. It has repeatedly produced false results in pesticide spray trials and, as a result, we probably use more crop protection chemicals than are strictly necessary. Second, inter-plot interference has glamorised vertical resistance, far beyond its merits. And, finally, interplot interference has obscured the value of horizontal resistance to such an extent that, for years, most crop scientists never realised that this kind of resistance even existed.

Population Immunity

Population immunity is a term coined by J.E. Vanderplank to describe the fact that a plant population may be effectively immune to a crop parasite, even though the individuals in that population are less than immune. At first sight, this appears to be errant nonsense but, in fact, it really happens, and it is quite important. This effect also suggests that, when breeding plants for horizontal resistance, we probably need considerably less resistance than we may think.

Population immunity is a consequence of population growth. Unlike an individual's growth, a population's growth can be positive or negative. If there are more births than deaths, the population size is increasing, and its growth is described as positive. If the births and deaths cancel each other out exactly, the population size is unchanging, its growth is zero. And if there are more deaths than births, the population size is decreasing, and its growth is negative.

Consider the population growth of a crop parasite. If the parasite population growth is positive, this means that, on average, each parasite individual spawns more than one new individual. In the case of an r-strategist parasite, each individual may spawn very many new individuals, in a very short time, and the positive population growth is then so rapid that it becomes a population explosion.

Now suppose that the crop in question has a level of horizontal resistance that severely restricts the reproductive rate of the parasite. On average, each parasite individual spawns only one new individual before it dies. The parasite population growth is then zero. Finally, suppose a slightly higher level of horizontal resistance. On average, each parasite individual now spawns less than one new individual. In practice this means that most individuals spawn one new individual, but a few spawn none at all. The parasite population is now decreasing. Its population growth is negative.

An epidemic can develop only when the parasite population growth is positive. And a damaging epidemic can develop only when the population growth is strongly positive. If the parasite population growth is zero or negative, there is no epidemic, and the host population is effectively immune, even though the individuals in it are less than immune. This is population immunity.

One of the dangers of measuring horizontal resistance in the laboratory is that population immunity cannot easily be taken into account. A level of horizontal resistance that looks like susceptibility in the laboratory may prove to be population immunity in farmers' fields. For this reason, laboratory measurements of horizontal resistance should be relative measurements. That is, the level of resistance should be described as being either higher or lower than that of other cultivars of known field performance.

While discussing population immunity, it is perhaps worth making the point that both vertical resistance and crop protection chemicals increase the death rate of the parasite, while horizontal resistance reduces the birth rate. Reduction of the birth rate is a more effective control method than increasing the death rate, because unborn parasites never take nutrients from the host. Dead parasites may have stopped taking nutrients from the host, but they had already taken a lot before they died.

Biological Control

"Little fleas have lesser fleas, upon their backs to bite 'em". Plant parasites are "little fleas" and they have their own "lesser fleas" which are hyper-parasites and predators which consume them, and keep their numbers down. Many parasites also have competitors, which are harmless on our crops, but which also help to keep the parasite numbers down. There may also be antagonistic micro-organisms which help to keep down their numbers. The efficacy of the antibiotic penicillin in killing many species of bacteria illustrates how effective these biological antagonisms can be.

The term "biological control" refers to the overall effect these biological reductions have on a crop parasite population. It is sometimes possible to vanquish a crop parasite completely by the careful manipulation of its natural enemies, its parasites, predators, competitors, and antagonists.

However, in modern crop husbandry, the opposite effect is far more common, and far more important. This opposite effect is the loss of natural biological controls because of an excessive use of crop protection chemicals, which also kill hyper-parasites, predators, competitors, and antagonists. There is apparently no recognised word or term that describes this loss of biological control, and this is an indication of how little its importance has been appreciated. We might, perhaps, call it biological anarchy.

Biological anarchy occurs most commonly with the insect pests of crops, but the effect can probably be detected, to a greater or lesser extent, with all categories of plant parasite that have been treated with chemical pesticides. There is a clearly established case, for example, with coffee berry disease (Chapter 21). This microscopic fungus is parasitic only on coffee berries. Between berry-bearing seasons, it resides harmlessly in the bark of the coffee tree, constituting about 5% of the innocuous, microscopic, bark inhabitants. When coffee trees are sprayed with a fungicide to control coffee berry disease, many of these competing bark inhabitants are killed, and the coffee berry disease fungus population then increases to occupy most of the bark. In the next season, the severity of the disease is increased accordingly.

An example of aphid reproduction might also be useful. Suppose that every aphid has ten offspring, and that all the offspring survive to produce ten more offspring in each generation. After ten generations, there will be 1010 aphids (i.e., 10,000,000,000). Now suppose that ladybirds are eating half of the aphids, so that only five of each aphid's offspring survive to reproduce in each generation. After ten generations, there will be 510 aphids (i.e., 9,765,625) which is approximately one thousandth of the earlier total. And, if only one aphid survives to reproduce in each generation, after ten generations there will be only one aphid. In practice, ladybirds really do eat a lot of aphids. But if all the ladybirds are killed by an insecticide, and all the aphids are resistant to that insecticide, there will be many more aphids than if the insecticide had never been used in the first place.

The loss of biological control is possibly at its most conspicuous in the cotton crop. Cotton is a "political" crop, in the sense that it is excessively regulated by marketing boards, growers' associations, banks, chemical corporations, and departments of agriculture. Very often, the farmer himself is given no choice in the use of crop protection chemicals. He is compelled to conform with general regulations which ensure that all the cotton crops of an entire region are treated in the same way. Because high yields and high quality are so important to the various regulating bodies, the tendency is always to use too much pesticide, rather than risk using too little. This tendency has been dubbed the "pesticide overload", or the "pesticide overkill". The immediate effect of the overload is a reduction in the cotton pests, but the long-term effect may be an increase in pests, because of the biological anarchy. This, in its turn, often leads to a further increase in the pesticide overload.

In fact, there are two biological factors to be taken into account. The first is biological anarchy, the loss of biological control, because of the destruction of natural predators, hyper-parasites, competitors, and antagonists. The second factor is that a crop parasite may develop a new strain that is less affected, or even completely unaffected, by that pesticide. This is an effect closely similar to the failure of vertical resistance. The farmers must then use a different pesticide, and there is then a "boom and bust" in pesticide effectiveness which is likely to be repeated, again and again. In the meanwhile, the population explosion of a new pesticide-resistant strain of a major pest is liable to become worse, because (i) it is unaffected by the old pesticide, (ii) a new pesticide is not immediately available, and (iii) the parasite's natural enemies have been destroyed by the pesticide overload, and there is biological anarchy.

Very minor parasites of cotton are liable to become major parasites, when there is biological anarchy, because their natural enemies have been destroyed. The classic example of this was in America, and was the tobacco bud worm, which normally never parasitises cotton. But, under the influence of the pesticide overload, it became a major pest of cotton, because it was unaffected by all the available crop protection chemicals, and its natural enemies had all been killed.

This biological anarchy is a general effect which must be assumed to occur in most crops that are treated with chemical pesticides. Consequently, in many crop pathosystems, the biological controls are no longer functioning, or they are functioning at a reduced efficiency. This is the basis of the concept of integrated pest management (IPM) which is a crop parasite control technique that depends heavily on the restoration of biological controls. Pesticide use is minimised, so as to interfere with biological controls as little as possible. IPM can be dramatically successful in crops that have been subjected to a serious pesticide overload. The very success of IPM is an indication of how important this loss of biological controls, this biological anarchy, really is.

The overall effect of biological anarchy is that many crop parasites become much more serious than they need be. This has two important consequences which must be emphasised. First, when a pesticide-resistant strain of the parasite appears, it is likely to behave with a ferocity that would be impossible if its natural enemies were keeping its numbers down. This means that a new pesticide-resistant strain of a parasite is likely to be far more damaging than if we had never used the crop protection chemicals in the first place. And, if we decide to abandon the use of crop protection chemicals in a particular crop, we shall have to endure serious, although rapidly diminishing, crop losses for several seasons until such time as the natural biological controls are fully restored.

Second, if we want to measure the level of horizontal resistance in potential new cultivars, we must do this under conditions in which there is no biological anarchy. If we measure horizontal resistance under field conditions, in which the parasite has considerably increased numbers, because of biological anarchy, that level of resistance will appear inadequate. But, once the biological controls are restored, that same level of resistance might be high enough to control the parasite completely.In practice, this means that field measurements must be made in quite a large area that is free of crop protection chemicals. It may not always be possible to find such an area. The only alternative would then be to use laboratory measurements which, once again, must be relative measurements. A closely similar problem is in trying to assess how much horizontal resistance we are likely to need in a breeding program. To do this, we must use a parasite whose biological controls are functioning to their full extent.

It is worth noting also that IPM will be successful only when there has been a serious pesticide overload. This approach cannot be expected to have dramatic results when there is no biological anarchy. Nor can it succeed when there is a serious deficiency of horizontal resistance.

Biological control can be enhanced by the culture and release of the various controlling organisms that contribute to it. With re-encounter and new encounter parasites, it may be necessary to go to the centre of origin of the parasite, in order to find biological control agents.

Once again, we may need much less horizontal resistance than we may think, in order to control crop parasites in a pesticide-free agriculture. In fact, this is a reciprocal effect. The best way to restore lost biological controls is to use horizontal resistance. And the best way to maximise the effects of horizontal resistance is to restore lost biological controls.

It was mentioned earlier (Chapter 9) that the use of horizontal resistance will lead to a very high frequency of parasitism, but a negligible injury from parasitism. It is doubtful if even artificially high levels of horizontal resistance will ever provide an absolute control of a crop parasite, in the sense that the parasite disappears completely. But this is a good thing. If we are to maintain a population of hyper-parasites and predators for the purposes of biological control, we must also maintain a small population of crop parasites for them to feed on. This small population will exist because even the maximum levels of horizontal resistance will always permit the parasite to cause minor blemishes that are economically unimportant, but ecologically crucial. These minor blemishes will maintain both the crop parasites, and the agents of their biological control.

These three factors of parasite interference, population immunity, and biological control, suggest that levels of horizontal resistance that appear to be quite inadequate at present, will achieve a control of many crop parasites that, for all practical purposes, is effectively complete.

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CHAPTER FIFTEEN
The Disadvantages of Crop Protection Chemicals

At this point, it might be appropriate to take a cool look at crop protection chemicals, quietly and objectively, and free from the rhetoric of some of the more passionately involved activists. There is one over whelming advantage, and there are seven quite serious disadvantages, to the use of crop crop protection chemicals.

The overwhelming advantage is that we still produce enough food to feed everyone in the world. This achievement depends, beyond question, on using crop protection chemicals. If we were to stop using crop protection chemicals, completely, tomorrow, hundreds of millions of people would soon die of starvation. Much as we may hope to abandon the use of of these chemicals, we cannot do it overnight. It will require at least a decade to produce a significant alleviation in pesticide use, and probably several decades to achieve the maximal replacement with horizontal resistance. This is fact. We have to recognise it and accept it.

We must recognise also that the efficiency and safety of crop protection chemicals has been improving steadily. Gone are the days when we treated our crops with the salts of lead, arsenic, and cyanide. After World War II, DDT became available and it had to be applied to crops at a rate of 2kg/ha. Later, the much less hazardous synthetic pyrethroids were developed, and these need be used at only one twentieth of the DDT rate, namely at O.1kg/ha. Now there is a new insecticide called aldicarb which need be applied at a rate of only 0.05kg/ha. In other words, it is forty times more effective than DDT, and it has less hazardous side-effects. Much as we may dislike the use of crop protection chemicals, we must recognise this general trend of improvement, which is likely to continue.

Readers who would like to know more about pesticide use are advised to study The Pesticide Question, edited by Pimental and Lehman, 1993. (See bibliography).

Let us now consider the seven disadvantages of crop protection chemicals, and compare them with the use of horizontal resistance.

Cost

Crop protection chemicals are expensive, both to buy, and to apply. But there is no question that they are economical, and that they pay for themselves, usually 4-5 times over, in increased yields, and an increased quality of crop product. When I was a child, for example, before the days of DDT, it was quite common to find a grub inside a half-eaten apple. This can be a revolting experience, particularly if most of the grub appears to be missing.

Be that as it may, the cost of these crop protection chemicals, and their application, is passed on to the consumer. In comparison, the use of resistant cultivars costs nothing and, if the same effect could be achieved with resistance, the costs of buying and applying the pesticide would be eliminated.

In practice, the use of a resistant cultivar is not necessarily cost-free. That resistant cultivar may have a lower yield, or a lower quality of crop product, even when parasite-free, than the susceptible cultivar does when it is treated with crop protection chemicals. Furthermore, in some crops (e.g., apples, and the grubs of the codling moth), it may prove impossible to achieve adequate levels of resistance. But, provided all other things are equal, crop protection chemicals are expensive, while the use of horizontal resistance costs nothing.

Repetition

Second, the effect of a pesticide application is usually lost quite quickly, and the pesticide must then be applied again. Most crop protection chemicals have to be re-applied every 10-20 days, but some have to be applied more frequently than that. This is partly because the pesticide tends to be washed off in rain, partly because it is non-persistent (i.e., it decomposes), and partly because the new parts of rapidly growing plants require additional protection. In comparison, vertical resistance usually lasts for several years, and horizontal resistance lasts for ever.

Breakdown

Third, many crop protection chemicals behave like vertical resistance, in the sense that the parasite is able to produce a new strain that is unaffected by that chemical. DDT-resistant houseflies are the classic example. The use of that pesticide must then be abandoned, and it must be replaced with a new one. This has happened so frequently with modern crop protection chemicals that many people now believe that there is no limit to the capacity for change of our crop parasites.

In practice, this accumulation of pesticide resistance in crop parasites is often quantitative. This means that the recommended rates of pesticide application become inadequate. These rates are then increased but, in their turn, these too become inadequate. This gradual increase in the use of a pesticide can continue until the rates of application are absurd. This quantitative loss in effectiveness is a prime cause of pesticide overload.

Some crop protection chemicals have remained effective for a century or more without any suggestion of resistant strains of the parasite appearing. This is true of Bordeaux mixture, for example, as well as natural insecticides such as rotenone and pyrethrins. Nevertheless, most crop protection chemicals eventually succumb to new strains of the parasite, either qualitatively or quantitatively. Vertical resistance also breaks down to new strains of the parasite, but horizontal resistance does not.

Expertise

Fourth, most crop protection chemicals require considerable expertise in their use. This expertise is required first of the person who decides which chemical should be used. All too often, this decision depends on a salesman, and pesticide use is then governed, at least in part, by irrelevant factors, such as advertising and sales skills. The same criticism applies to the rates of application, which are often too high, or too frequent, because of an over-zealous sales pitch. Expertise is also required by the farmer himself, and his employees, if the pesticide is to be fully effective, and the safety precautions are to be properly implemented. All too often, this expertise is either lacking or inadequate. It need scarcely be added that, at the farmer level, the control of parasites by the use of horizontally resistant cultivars requires no expertise whatever.

Hazards

Fifth, many crop protection chemicals are hazardous, either to people, or to the environment, or both. The hazards to the consumers of crop products are usually slight or insignificant, but they concern very large numbers of people. The dangers are more keen for the much smaller numbers of people who actually work with these chemicals. These are mainly agricultural workers, and the dangers can become serious, even acute, when safety precautions and supervision are inadequate.

The hazards to the environment are many and various. The best known dangers are the killing of non-target animals, such as birds and pollinating insects. Occasionally, there is a risk of irreversible damage, when a rare species is threatened with extinction. Some animals are particularly sensitive to the presence of crop protection chemicals. For example, there is now a serious decline in the world population of frogs, and several rare species appear to have disappeared, probably forever. Other species suffer from the side-effects of crop protection chemicals. For this reason there has been a dramatic decline in the numbers of insect-eating birds. Butterflies, which were so common, and so beautiful, when I was a child, are now rare. Usually, pesticide hazards are not discovered until considerable environmental damage has been done. There is then, quite rightly, a public outcry, and the difficult task of crop parasite control becomes even more difficult.

Sadly, many of these hazards are not due to the pesticide itself, so much as to its misuse. DDT, for example, is an excellent insecticide which is also incredibly cheap. Unfortunately, it was applied to agricultural crops with such abandon, and in such enormous quantities that there was serious environmental damage. Nevertheless, in my opinion, DDT should not have been banned. Its use should have been controlled. Had that control been present from the outset, it is likely that many DDT-resistant insects would never have appeared, and those thin-shelled eagle eggs would never have become a problem. It must also be remembered that not all crop protection chemicals are hazardous. To the best of our knowledge, a century of use of Bordeaux mixture has not harmed anyone or anything.

Once again, a comparison with horizontal resistance is illuminating. Horizontal resistance is absolutely safe, both to people and to the environment.

Destruction of Biological Controls

Sixth, the routine use of many crop protection chemicals has led to the debilitation, or even the local elimination, of biological control agents. This has made many crop parasites more serious, and more difficult to control. This biological anarchy has already been discussed (Chapter 14) and it is difficult to assess its overall importance. The best indication comes from the fact that many specialists in integrated pest management (IPM) depend very heavily on a restoration of biological controls that were lost because of pesticide use. This damage to biological control may turn out to be a much more important side-effect of pesticide use than many crop scientists currently realise. It is needless to add that the use of horizontal resistance does not damage biological controls. Indeed, it is the best means of restoring them.

Incomplete Effectiveness

Lastly, the effectiveness of crop protection chemicals is far from complete. As we saw earlier, we are still losing about 20% of pre-harvest crop production because of parasites, in spite of the massive use of crop protection chemicals costing billions of dollars each year. In food crops alone, this pre-harvest loss is enough to feed about one billion people. So what is wrong? Is it possible that our farmers are using too few crop protection chemicals, at too low a concentration, too infrequently? Is it possible that our farmers are applying these chemicals in the wrong way, at the wrong time, or too inexpertly? Is it possible that the crop protection chemicals themselves are not much good? Or is possible that crop protection chemicals are not the answer anyway? If crop protection chemicals are not the answer, there is really only one alternative. Guess what it is.

CHAPTER SIXTEEN
So How Did Things Get So Out of Hand?

When I was an undergraduate, in the late 1940s, we were actually taught that all resistance to crop parasites was temporary resistance, and that all parasite resistance in plants was bound to fail sooner or later. Our teachers seriously questioned the wisdom of breeding plants for resistance, and of constantly trying to keep "one jump ahead of the parasite". They told us about some recent and dramatic crop losses resulting from failures of resistance. They suggested that we would do better to study crop protection chemicals. They quoted some remarkable new chemicals that were being discovered. One of them was very new, and very exciting. It was di-chloro, di-phenyl, tri-chlor-ethylene, commonly called DDT. Its Swiss discoverer, Dr Paul Müller, had just been awarded the 1948 Nobel Prize for Physiology or Medicine, because DDT was so effective in killing mosquitoes for the control of both malaria and yellow fever, to say nothing of killing houseflies for the control of both typhoid and cholera, and fleas for the control of bubonic plague. These were just the human diseases. There were many insect-borne animal diseases as well, not to mention the innumerable crop pests, that could be controlled with this chemical. DDT was also safe, or relatively so, when it is remembered that commonly used insecticides in those bad old days included lead, arsenic, cyanide, and the fumes of burning sulphur. Furthermore, DDT was incredibly cheap. There was even talk, in those days, of combining it with paint, to produce insect-free houses.

Obviously, our teachers said, the future lay with chemicals, not with host resistance. There was nothing special about this teaching. It was typical of its time, and what is often called "state of the art". It also represented the "cutting edge of research", and the "received wisdom". It is perhaps worth adding that modern scientists often debate which of two chemicals has saved more human lives. Is it DDT, through the control of malaria, yellow fever, typhoid, and cholera, or is it penicillin?

It should also be mentioned that, in spite of the received wisdom, there are a few examples (a mere half dozen) of vertical resistance which has proved durable over many decades. Thus, wheat in Canada has durable vertical resistance to a disease called stem rust (Puccinia graminis tritici), and tomatoes in the United States have durable vertical resistance to a wilt disease (Fusarium oxysporum f.sp. lycopersici). The reasons for this durability are too complex to discuss here, but the durability itself merits two comments. First, if we can demonstrate that a vertical resistance is durable then, obviously, we should use it. However, we should note also that vertical resistance that is durable in one part of the world is usually temporary resistance in another.

Second, these few examples of durable vertical resistance have done much to mislead the members of the Mendelian school, and to make them hope that many other examples of single-gene resistance would also prove to be durable. It was perhaps this misplaced hope, as much as any other factor, that persuaded the members of the Mendelian school to persist so doggedly, and for so long, with the breeding of plants for temporary resistance.

Because of the "Red Queen" effect (Chapter 11), plant breeders have been abandoning vertical resistance breeding ever since World War II. What they should have done was to consider the use of horizontal resistance. But, at that time, horizontal resistance was so little understood, and its value was so doubted, that breeding for it appeared to be both a daunting task, and a futile task.

We must recognise also that the effects of Bordeaux mixture in the 1880s, and of DDT in the 1940s, were stunning. Crop scientists were completely dazzled. As more and more of them began to abandon vertical resistance breeding, they chose crop protection chemicals because they were so dazzled. In comparison, there was nothing very dazzling about horizontal resistance.

We should remember too that, during the whole of this century, crop scientists have been faced with the world food problem. With the human population doubling every thirty years, crop scientists were compelled to double agricultural production every thirty years also. Much of that increase came from putting more land under the plough. Nevertheless, it was production, per se, that was given the first priority in crop science. The manner of that production was a secondary consideration.

The corollary of this situation must also be recognised. There has been some truly remarkable progress in improving the yield, quality, and agronomic suitability of crops during the present century. The human population has increased dramatically, since the Mendelian school came into existence, yet we still produce enough food for everyone. The famines we have witnessed in recent years are due to local disasters, and to administrative incompetence, even political malice, rather than to a world shortage of food. The success of crop science in feeding the world has been impressive. The complaint of this book is not about the amount of food we produce, so much as the fact that, in the field, we lose about one fifth of our production to crop parasites, in spite of an extravagant use of crop protection chemicals.

A complete lack of public interest, combined with a largely incomprehensible, technical jargon, has made crop science a closed shop, almost a secret society, for most of this century. What it needs now is a healthy dose of public scrutiny. And that is one of my most carefully considered objectives in writing this book.

CHAPTER SEVENTEEN
Cultivar Cartels

There are some powerful vested interests that are determined to maintain the status quo concerning crop breeding and crop pesticides. These vested interests are both scientific and commercial.

Crop scientists have ignored horizontal resistance, so consistently, and for so long, that many of them are now reluctant to admit that it may constitute a superior alternative. So long as it was believed that there was only one kind of resistance to the parasites of plants, it was possible to blame nature for the failure of resistance breeding. If a new strain of the parasite appeared, and the resistance failed, that was clearly the fault of Mother Nature, not of the scientists. If a genetic source of resistance could not be found, and the breeding could not even be started, that too was the fault of Mother Nature.

In these circumstances, resistance breeding was clearly an unprofitable business. And, it seemed, the only alternative was to use crop protection chemicals, apart from a few subsidiary pest control methods such as crop rotation, and the burning of crop residues. (Modern entomologists also have some neat tricks to induce sterility, such as swamping the female part of a population of insects with sterile males, or luring all the males into traps with sex attractant chemicals. These artful dodges are occasionally very effective, but only occasionally.)

Against the apparent failure of resistance breeding, we must also recognise the success of crop protection chemicals. If you happen to believe that vertical resistance and crop protection chemicals are the only alternatives, then it is reasonable to choose success over failure, and crop protection chemicals over vertical resistance breeding. Nonetheless, crop scientists have known for decades that there was a third alternative, now called horizontal resistance. It was wrong of them to ignore it, and it is now difficult for them to admit this. Hence their vested interest in the status quo.

Various seed producing industries have commercial vested interests. The most prominent of these is the certified potato seed industry, although there are many others. Highly specialised farmers produce crops solely for seed purposes, and these crops are approved by government inspectors who certify them free of various parasites. The inspectors usually certify them in other ways also, such as trueness of variety, and purity of variety. Ordinary farmers then buy this certified seed for planting their crops. But this certified seed is expensive. With potatoes, for example, the cost of certified seed is usually the biggest single input in commercial potato cultivation.

The producers of certified seed positively resent any suggestion of new resistant varieties that can be grown from the farmer's own harvests, without any need of seed certification. These producers of certified seed actually want susceptibility to seed-borne parasites. Without it, there would be little need for their expensive seed, certified free from parasites.

There can be no doubt that resistance which was complete, comprehensive, and durable, would largely destroy these specialised seed industries. Indeed, we are forced to conclude that these seed industries would never have been born, but for the susceptibilities which make seed certification necessary. This need for certified seed of cultivars that are susceptible, is also a clear indication of the overall failure of resistance breeding. (If the need for certification for freedom from diseases were to disappear, there would still be a need for seed certified for both identity of variety, and purity of variety. But the seed industries would be very greatly diminished.)

A second source of commercial vested interests is even more important. This lies with the manufacturers of crop protection chemicals. These chemical corporations have no intention of promoting horizontal resistance, which threatens a major reduction of their market. Indeed, these chemical corporations are apparently doing the very opposite. They are buying up plant breeding institutes, presumably with a view to controlling plant breeding policy. And they are buying up seed production and marketing organisations, presumably with a view to controlling the crop varieties that are available to farmers. We may be forgiven for assuming an ulterior motive, and for suspecting that these varieties are likely to have very high yields, and a high quality of crop product, but that they are also likely to have very high susceptibilities to various parasites. They would then require large amounts of crop protection chemicals for their successful cultivation. What better way could there be of guaranteeing the market for crop protection chemicals?

Indeed this situation is occurring already. Farmers in western Europe now routinely spray their wheat crops with crop protection chemicals. This is an entirely new, and very disturbing development. It arose because the European wheat breeders largely abandoned resistance breeding. They produced new wheat cultivars that have very high yields, but that are also susceptible to various wheat parasites. The spraying process requires a tractor to be sent through the wheat, and the tractor wheels flatten some of the wheat, producing characteristic "tramlines" that can be seen from the air. However, the loss of this wheat in the tramlines, and the costs of spraying, are more than made up by the increased yields resulting from the use of crop protection chemicals.

The pesticide manufacturers often refer to their take-overs of plant breeding and seed production organisations as "diversification". But appearances are against them, and their apparent desire to control plant breeding, and the cultivars available to farmers, is highly suspect. There is not the slightest doubt that they positively need susceptible cultivars, which are essential if there is to be a large market for crop protection chemicals.

The pesticide industry is a powerful, self-interested, international group of manufacturers that has the financial resources necessary for intense political lobbying, widespread commercial advertising, and the establishment of a powerful cartel in farmers' seeds. There appears to be only one possibility of frustrating this monopolistic development. And that is what this book is all about.

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PART TWO
Examples

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Reiteration

Readers who chose to skip most of Part One may care to have a brief reiteration. There are two kinds of resistance to crop parasites, called vertical resistance and horizontal resistance. Vertical resistance operates as a system of locks and keys and, like any system of locking, it requires a diversity of many different locks and keys. Its function is to control an epidemic in a population of plants, and it does this because relatively few parasites have a key that fits the lock of the host plant they are trying to infect. Horizontal resistance is a second line of defence. It operates after a vertical resistance lock has been unlocked by a parasite, and its main function is to control the actual parasitism within an individual plant.

For most of this century, crop science has been dominated by the Mendelian school of genetics, and by the use of the locks of vertical resistance. Unfortunately, the members of the Mendelain school employed this resistance on a basis of uniformity, with every plant within a crop variety having the same lock. This is the equivalent of every door in the town having the same lock, and every house owner having the same key, that fits every lock. We call it monolock. This monolock explains why the resistance of that variety is liable to fail when a matching strain of the parasite appears. Under these circumstances, vertical resistance is temporary resistance. For many years, crop scientists believed that this was the only kind of resistance available to them.

Since World War II, the spectacular promise of chemical pesticides, combined with the repeated failures of vertical resistance, persuaded crop scientists to favour crop protection chemicals over resistance breeding. They chose this course under the extreme pressure of human population growth, which led to a doubling of our population every thirty years, and a doubling of the world food requirements every thirty years also.

During all of this time, horizontal resistance was neglected to the point of being almost totally ignored. It is still being neglected. Nevertheless, it promises to solve the problem of crop parasites which currently destroy about one fifth of all crop production in spite of an extravagant use of crop protection chemicals. Properly utilised, horizontal resistance could provide us with a largely pest-free agriculture, and a largely pesticide-free agriculture as well.

Part Two of this book attempts to substantiate this claim, by providing some examples of how horizontal resistance can do just this.

Scientific readers who require a comprehensive scientific review of breeding work on horizontal resistance are advised to see Simmonds, 1991; details are given in the bibliography.

CHAPTER EIGHTEEN
A Short History of Potato Parasites

Introduction

When the Spanish first introduced potatoes (Solanum tuberosum) to Europe, from the New World, in the sixteenth century, this crop was little more than a botanical curiosity. This was because potatoes were tropical plants that were acclimatised to the short days of equatorial regions. Consequently, they would not form tubers during the long days of a European summer, and the delayed crop would be ruined by frost before it was mature. Ireland was the first country in Europe to cultivate potatoes on a large scale because it has a very moist and mild climate, with little frost. For this reason, potatoes are often called the Irish potato, among English speaking people, to distinguish them from the very different, tropical, sweet potato (Ipomea batatas).

About two centuries of largely unconscious selection by European horticulturists, assisted no doubt by some natural selection, eventually produced new, long-day varieties of potato that were suitable for cultivation during the long summer days of temperate regions. These modified potatoes also had improved agronomic characteristics, such as larger tubers on shorter stalks. These genetic changes were completed in the eighteenth century, and potatoes quite quickly became a major food crop.

There were two reasons for this rapid rise in popularity, apart from the fact that potatoes are an excellent food. The first was the industrial revolution, and the growth of urban populations living in manufacturing towns. These people needed cheap food and, in those days, potatoes were much cheaper than bread. Bread was expensive because of protective tariffs on wheat imports, imposed by European governments to encourage their own farmers. Wheat also had to be harvested by hand, because this was before the days of mechanical reapers. Because labour was always in great demand at harvest time, it was both scarce and expensive. This set an absolute limit to the amount of wheat that a country could produce.

The second reason for the popularity of potatoes was that much of Europe has soils that are unsuitable for growing wheat, and the people who lived in these areas traditionally grew rye, and lived on rye bread. Ireland cannot easily grow wheat because its climate is too wet, and this was another factor contributing to its adoption of potatoes.

Today, rye bread is something of a luxury but, in those days, it was a sign of poverty. This was because rye has a disease caused by the fungus Claviceps purpurea, which produces poisonous granules called ergots. The ergots would be milled with the rye, to produce poisonous flour, and poisonous bread. The poison causes a disease known as ergotism, or "Saint Anthony's Fire", which results from a restriction in the circulation of the blood. Mild cases produced hallucinations and cramps, but more severe poisoning would lead to gangrene, loss of limbs, abortion in pregnant women, and death.

At that time, the cause of the poisoning, which varied greatly in severity from year to year, was not known, although its asso-ciation with rye bread was recognized. When potatoes became available as an alternative food, they quickly became popular in the rye growing districts, and the widespread outbreaks of ergot poisoning became a thing of the past. To this day, the old rye districts of eastern Germany, Poland, and western Russia still have the highest rates of potato consumption in the world. In its turn, an abundance of cheap food led to population increases. It has even been suggested that, without potatoes, neither the industrial revolution, nor World War I, could ever have happened, because there would not have been enough workers, or soldiers.

Potato Blight

In the 1840s, there was a major tragedy. A fungal parasite with the ugly name of Phytophthora (pronounced fie-TOFF-thora) infestans was accidentally introduced to Europe from Mexico, possibly via the United States. The potatoes of Europe had evolved in South America, far away from Mexico, and they had never encountered this fungal parasite before. Accordingly, it was a "new encounter" disease.

The wild potatoes of Mexico occur North of the equator, at altitudes of about 8,000 ft., and they are separated from the highlands of South America by both sea, and a belt of lowland, tropical jungle. Botanically, the two areas are entirely isolated from each other. Consequently, the South American potatoes in Europe had little resistance to this new encounter, the Mexican parasite, and a really dreadful new plant disease appeared. This was the first time in history that anyone had seen an exceptionally bad plant disease. After a few days of cool, moist weather in late summer, the green potato fields of Europe would turn into a black stinking mush, with not a speck of green to be seen anywhere. When the tubers were dug up, it was discovered that they too were rotten.

The disease was first observed in northern France in 1845, and it spread rapidly throughout Europe, quickly becoming a cause of major concern. It also became the cause of a major controversy which, indeed, represented the birth of the science of plant pathology. The Rev. M.J. Berkeley, in England, proposed the astounding view that the microscopic fungus, which was always associated with the disease was, in fact, the cause of the disease, and not one of its effects. Berkeley was anticipating Louis Pasteur's germ theory of infectious diseases by nearly a quarter of a century. Inevitably, Berkeley was widely disbelieved and his opponents offered many alternative suggestions. The newly discovered and mysterious "electricity" was widely blamed, as was the atmospheric pollution caused by that new abomination, the steam railway. Berkeley's view was not popular, but he was right.

The potato blight was soon found in Ireland, and all thinking people knew that great trouble was in store. At that time, the Catholic Irish were still being cruelly exploited by the Protestant English. Potatoes had been introduced to Ireland in the late sixteenth century, at about the time of the Desmond rebellion. Gerald Fitzgerald, 14th Earl of Desmond, was an Irish Catholic who led an army of Italians and Spanish, backed by the Pope, fighting for the defence of Catholicism, against the Protestant English. The English won this war, and they brutally suppressed the insurgents. English rule in Ireland became harsh. In 1649, Oliver Cromwell waged a ruthless campaign against the Irish, and gave much of their land to English Protestants, who became the new, land-owning aristocracy. By this time, potatoes were well established in Ireland and they became the staple food of the peasants.

Irish agricultural labourers had niggardly wages, and they paid back a considerable proportion of them as rent to their English landlords. With the appearance of potato blight, the landlords feared for their rents, and for the safety of their country mansions, should rioting begin. But the peasants feared for their lives, because they lived almost exclusively on potatoes, and they had no money to buy alternative foods.

In these days of universal social security, old age pensions, medical plans, and other expressions of government concern for the individual voter, we are apt to forget that the more ancient role of governments was to make laws and wars, and little else, other than collecting the taxes required to pay for these activities. If the poor and the starving needed help, this was the function of the church, the aristocracy, and various public charities supported largely by private benefaction. But, when a major disaster struck, such as the failure of the potato crop in Ireland, these non-governmental organizations were quite unable to cope. The very poor then starved, and died. The slightly less poor voted with their feet, and went somewhere else. This was the cause of the great migration of Irish people to the United States. Much of the residual hostility in America towards the English stems not from the Boston Tea Party, nor from the war of 1812, but from Irish resentment of English neglect during the great potato famine.

In the 1840s, Britain was already in the process of moving away from a primitive type of government towards a more concerned and caring administration. Britain had already abolished slavery, for example, decades before either Russia or the United States. Nevertheless, the prime minister, Robert Peel, made very cynical use of the Irish famine, in connection with one of the great political controversies of nineteenth century Britain. This was the issue of the corn laws. These laws imposed import duties on wheat, and they helped British farmers by maintaining the scarcity prices which had prevailed since the time of the Napoleonic wars at the turn of the century.

Peel had been elected on a mandate to maintain the corn laws, but he used the potato famine as an excuse to repeal them, and to initiate a great period of free trade. This action led to the defeat of Peel's government. It also brought down the price of bread dramatically. And, eventually, it had a considerable influence on Mid-West America because it opened up an important new market for wheat. This development came soon after Patrick Bell's invention of the mechanical reaper, in Scotland, in 1827, which he took to Canada in 1833. Later, Cyrus Hall McCormick started manufacturing his famous reapers in Chicago. The repeal of the corn laws also coincided with the building of the Erie canal, which opened up the North American prairies, via the Great Lakes, to the East Coast of the United States, and the markets of Europe.

However, all this happened too late for the poor in Ireland, who lived in turf hovels, went bare foot, and dressed in rags. They owned nothing except their potato crops, and sometimes a pig, which was also fed on potatoes. When the potatoes died, their entire supply of food was lost.

At that time, the Victorian novelist Anthony Trollope lived in Ireland. He was irritated by some of the more sensational reports of the gutter press, and he tended to play down the horrors of the famine. Nevertheless, his accounts make chilling reading. He wrote: "Early in the autumn of 1846, the disease fell on the potato gardens like a dark mantle; before the end of September, entire fields were black, and the air was infected with the unwholesome odour of the blight; before the end of October, it was known that the entire food of the country was gone." That winter was unusually severe, and it actually snowed in November. Destitute peasants, evicted from their land, and their hovels, for failing to pay their rents, could not be housed, or even fed, by the totally inadequate poor houses. They wandered the countryside, desperate, starving, freezing, and dying.

Potatoes are a very productive and nutritious food crop and, consequently, the population of Ireland had increased considerably since the use of potatoes had become widespread. In 1800, the population was estimated at four million but, by 1845, it had increased to eight million. In Europe, the 1840s were known as the "Hungry Forties" because of the shortage of potatoes. In Britain, however, this period was called "The Great Irish Famine" because the Irish were so totally dependent on potatoes. No accurate figures are available, but it is estimated that about one million Irish people died of starvation. This was twelve percent of the population. Another million and a half people emigrated, mainly to America. This was another twenty percent of the population, making a total of one third of the whole population. The remaining two thirds suffered very serious hunger and malnutrition.

It has been related how an Irish priest, one Father Matthew, travelled from Cork to Dublin in 1846, and observed that all the potato crops were luxuriant and healthy. He praised God for His Mercy and Goodness, because he believed that the potato crops would once again be productive. In those days little was known about infectious disease epidemics, and still less was known about plant diseases. Father Matthew believed that the rotting of the potatoes during the previous summer was a rare phenomenon, unlikely to be seen again. But, when he traveled back to Cork a few days later, he observed with sorrow a wide waste of black, putrefying, and stinking vegetation.

In those days, the government would provide poor relief only on a basis of "fair" exchange. The destitute were put into a workhouse, and were expected to do work for the government in exchange for board and lodging. Nothing would be given for nothing and, in practice, this meant that the destitute had to work at menial and often meaningless tasks, such as picking oakum out of old ropes. When the potato crops failed, there was no way that the government could provide workhouses for millions of starving Irish. So, believing itself to be both benevolent and enlightened, the government provided work on the roads.

Starving labourers were expected to do back-breaking work building new roads, in exchange for little more than a bowl of porridge. Many of these roads went from nowhere to nowhere. The people were weak and under-nourished, and quite unfit for manual labour. Furthermore, they often had to walk long distances to their work. One road contractor commented that he was ashamed, as an engineer, to allot so little work to each daily task, but that, as a man, he was ashamed to exact so much.

Forty Years of Blight Damage

When potato blight first appeared in Europe, it was extremely damaging, and entire crops of potato were wiped out. Nevertheless, the blight epidemics declined in severity, and they were never again so damaging. At the time, this was attributed to a mysterious (and inexplicable) decline in the virulence of the blight fungus. We now know that it was due to a fundamental change in the overall population of the potatoes themselves.

Each potato variety is a clone. It is propagated sexlessly, by vegetative propagation, from "seed" tubers and, consequently, all the plants within one clone are genetically identical. New potato clones are produced from true seeds, which develop sexually from pollinated flowers, and which differ genetically among themselves because they are the result of genetic recombination. There were very many potato clones in Europe at that time and, among other things, they differed considerably in their susceptibility to blight.

In 1845, the first full year of the blight epidemic, the most susceptible potato clones were totally destroyed. They became extinct. In the second year, the slightly less susceptible clones became extinct and, by the fourth year, only moderately resistant clones remained. The entire potato population of Europe had been fundamentally changed towards blight resistance, and the blight epidemics declined accordingly.

New varieties of potato were repeatedly being produced from true seed by breeders, seed merchants, farmers, and even amateurs. In those days, plant breeding was an art, rather than a science, and there was a powerful incentive to breed new potato varieties because, at that time, this was the only way to avoid the severe loss of vigour that was apparently caused by vegetative propagation with seed tubers.

With the benefits of modern science, we now know that this loss of vigour occurs because of an accumulation of virus diseases, which are transmitted by seed tubers, but which are not transmitted through true seed to new seedlings. Consequently, the potato breeders had to produce many new varieties from true seed, in order to solve the problem of this loss of vigour. Modern potato farmers do not need to do this because they use certified seed potatoes which have been officially inspected and shown to be free of viruses.

After the arrival of blight in Europe, resistance to this disease became the first selection priority when choosing which seedlings should become new clones. Indeed, any seedling that was not resistant to blight was soon killed, and only those which could survive the blight stood any chance at all of becoming a new clone. Like the changing of potatoes from short-day to long-day responses, this was another example of unconscious selection. Whether or not the breeders wanted blight resistance, or even knew about it, they had no choice in the matter, because it was only the resistant seedlings that could survive, let alone yield a good crop.

This process of selection for blight resistance continued for some forty years following the first appearance of blight. However, the genetic base of potatoes in Europe was a narrow one, and it was apparently derived almost entirely from the original material introduced by the Spanish. This meant that there was a limit to the level of blight resistance that could be achieved within this breeding stock. There is little doubt that the breeders of Europe achieved that limit of resistance, and that no further progress was possible without a broadening of the genetic base by the introduction of new breeding stock from South America. Nonetheless, that limit was enough resistance to allow an economic cultivation of potatoes without any fungicidal spraying against blight. Potatoes were cultivated in spite of the blight, and they yielded well enough to make them an important food crop throughout Europe. This was a very considerable increase in resistance when compared to those susceptible clones of the Hungry Forties, reduced to that black, stinking mush.

As we now know, this blight resistance in potatoes was the resistance of the biometricians. It was the continuously variable, polygenically inherited, horizontal resistance. It is safe to assume that the potatoes in Europe, at the time of the first appearance of blight, had levels of horizontal resistance that approached the minimum. And, as we have seen, this level of resistance leads to the complete destruction of the crop, with not a speck of green to be seen anywhere. In a moment, we shall discuss new potato clones in Mexico which have levels of horizontal resistance that approach the maximum. In many areas, these clones can be grown without any spraying, and without any losses from blight. The difference between the minimum and the maximum levels of horizontal resistance to blight can thus be enormous. It can be the difference between a complete loss of crop, and no loss of crop.

Bordeaux Mixture

In the 1870s the vineyards of France began to be ravaged by another foreign fungus, called Plasmopora viticola (pronounced Plaz-MOP-ora vitty-cola), which had also been introduced from the New World, and which is distantly related to the potato blight fungus. This second foreign fungus causes a disease of grapes called downy mildew. At the time of its introduction to Europe, it too was a new encounter disease, and it did nearly as much damage to the vines as the blight had first done to the potatoes. But there was one important difference. The clones of classic wine grapes are the result of many centuries, if not millennia, of selection, and they are among the most difficult of all crops to breed. There could be no question of replacing the susceptible vines with resistant ones, without a totally unacceptable loss of wine quality. There was consequently no possibility of the downy mildew epidemics declining as the potato blight epidemics had declined, because of genetic changes in the host population. The French wine industry was faced with absolute ruin.

Then, in the summer of 1882, a certain Professor Pierre Marie Alexis Millardet was examining mildewed vines in the famous Médoc area of Bordeaux, when he noticed that some of the vines at the Château Beaucaillon were green and healthy. This was so extraordinary that he made enquiries, and he discovered that it was a local custom to spatter the vines near the public road with a poisonous looking substance in order to discourage passers-by from eating the grapes. He also learned that this substance was a mixture of copper sulphate and lime, and that it was intended to resemble the verdigris of corroded copper vessels.

Millardet realised that he had found a substance that would solve the problem of downy mildew. He called it Bouillie bordelaise which, in English, is Bordeaux mixture, and it was the first fungicidal spray for crops. It was also an incredibly efficient fungicide. An explosion of research followed. The best proportions of copper sulphate and freshly slaked quicklime were worked out. The best concentration of the mixture was determined, and spraying schedules were devised. Entirely new kinds of machines, called sprayers, were invented for applying the mixture to the vines. Endless other mixtures were tested but, almost without exception, they were found to be either useless, or positively toxic to the vines. Soon, all the vineyards of Europe were being sprayed with Bordeaux mixture. And all the potato crops too, because it was quickly discovered that Bordeaux mixture would also control potato blight.

It was not long before all the paraphernalia of wooden tubs, water carts, sprayers, copper sulphate, and lime, were seen in the potato fields, as well as in the vineyards. Spraying potatoes against blight became a routine part of potato cultivation throughout Europe and, later, the world. This was over one hundred years ago, and we may note in passing that we still spray our potato crops with fungicides to control blight, although Bordeaux mixture itself has been supplanted by more convenient proprietary products.

At this distance in time, it is difficult to appreciate the impact that Bordeaux mixture made on peoples' minds. To begin with, both potato blight and the downy mildew of vines had had powerful social consequences. Throughout Europe, few people had escaped being personally effected by one or the other of them, if not both. After all, people were starving because of potato blight. These two plant diseases had also had an enormous economic impact. It has been said the the mildew of the vines cost France more than the Franco-Prussian war. And the efficiency of Bordeaux mixture was spectacular. It controlled these two diseases cheaply, efficiently, safely, and completely. Crop scientists can scarcely be blamed if they have been looking for comparable pesticide successes ever since.

There was another aspect of this story that also concerns us. When Bordeaux mixture was first introduced, there was some vociferous opposition to it. "Copper is poison" its opponents cried, quite incorrectly. And they claimed that the people of Europe would all die from eating poisoned potatoes, and drinking poisoned wine. As a matter of historical fact, not one person died in this way. Nor was human health endangered. Indeed, the very opposite was true. Had Bordeaux mixture been discovered some forty years earlier, it could have saved at least a million lives in Ireland alone, during the great potato famine, and probably as many again in the potato eating districts of continental Europe. Not all crop protection chemicals are hazardous. Bordeaux mixture is not only the oldest crop fungicide. It is also the safest.

Forty Years of Bordeaux Mixture

The effect of Bordeaux mixture on potato cultivation was dramatic and wonderful. But the effect of Bordeaux mixture on potato breeding was disastrous. For the next forty years, potato breeders were able to protect their new seedlings with this fungicide. This meant that they could then ignore blight susceptibility, and concentrate on the other main breeding objectives which, as we have seen, are tuber yield, tuber quality, and agronomic suitability. The breeders concluded, no doubt, that blight resistance was no longer important, because the crops could be protected so easily, and so effectively, with Bordeaux mixture. Suddenly, all the pressure for finding blight resistant seedlings was gone. Breeders could screen their potato seedlings under the protection of this fungicide, and this made the breeding incomparably easier. They could then ignore the problem of blight killing off the majority of their precious seedlings, the problem that had so dominated their work for the previous forty years.

Between about 1885 and 1925, some of the most famous of all potato varieties were produced. Many of them are still being cultivated, such as Russet Burbank (1890) in the United States, King Edward (1902) and Majestic (1911) in Britain, Bintje (1910) in Holland, and the old Dutch variety Alpha (1925) in many other parts of the world, and they remain some of the most popular potatoes among consumers.

But there was one great drawback to this easy breeding. The accumulation of blight resistance not only stopped. It went into reverse. This happens with any inherited character which is quantitatively variable, and which is not contributing to survival. If it has no survival advantage, whether natural or artificial, it tends to be lost from a population that is genetically flexible because of sexual reproduction.

As we have seen (Chapter 8), plant breeders talk of selection pressure, using the word pressure in the sense of "bringing pressure to bear". They also talk of positive and negative selection pressures. When blight first appeared in Europe, there was very strong, positive selection pressure for resistance, because only resistant clones could survive, and the entire potato population was quickly changed genetically towards an increased resistance. But, when Bordeaux mixture was introduced, there was negative selection pressure for resistance. This happened because spraying with Bordeaux mixture made it impossible to distinguish between resistant and susceptible seedlings, and susceptible seedlings were in the majority. There was then a greater probability that susceptible seedlings would be selected. And the trend of the previous forty years was reversed. Resistance began to be lost. As we have seen also (Chapter 13), this loss is known as the erosion of horizontal resistance, and the erosion continued for another forty years, until the potato breeders were jolted out of their complacency by World War I.

As a consequence of Bordeaux mixture, the progression of new potato varieties gradually became more and more blight susceptible. "Bintje", for example, is one of the most blight susceptible varieties known. It is still widely cultivated because of its culinary popularity, but its cultivation is difficult because of its susceptibility, and its need for fungicidal protection. Equally, "Alpha" is the most popular potato in Mexico, but it is also the standard of blight susceptibility used in the measurement of blight, in the remarkable potato breeding of that country, to be discussed in a moment.

This loss of resistance to blight first became apparent during World War I, when there were acute shortages of food in Europe. There were also acute shortages of other commodities, such as copper, which was needed by the armaments industry, for the manufacture of brass rifle cartridges, and brass shell cases for the field guns. Copper was also needed for spraying the potato crops, particularly as the potato varieties of that time were so susceptible to blight. Germany was critically short of copper, and could not spray many of her potato crops which were consequently ruined. Food shortages undoubtedly contributed to her defeat, and these shortages resulted mainly from savage blight damage to the unsprayed potato crops. Various countries (on both sides of the firing line) decided that potato blight had strategic significance, and that the time had come to breed potatoes for resistance to this disease, so that potato crops would not have to be sprayed. And they decided to use the very latest scientific knowledge and techniques. That is, they decided to use the newly discovered breeding methods of the Mendelian school of genetics.

Forty Years of Scientific Potato Breeding

From about 1925 until about 1965, potato breeders were using resistance to blight that was genetically controlled by single genes. These genes had been discovered in wild potatoes growing in Mexico and, with some difficulty, they were transferred to cultivated potatoes. Each gene conferred an apparent immunity to blight and, when a potato possessing such a gene was crossed with a susceptible potato, the seedlings would segregate according to Mendel's laws of inheritance, with a ratio of three resistant seedlings to each susceptible seedling.

Soon, potato breeders in Germany, the Netherlands, the United Kingdom, and other countries were using this approach. There was a lot of optimism, and a lot of talk of abolishing blight, and abolishing the need for Bordeaux mixture. Sadly, this optimism was premature.

In 1953, a group of British and Dutch scientists published an important discovery. They showed that, for every Mendelian resistance gene in the potato host, there was a corresponding, or matching, gene in the blight parasite. They published this discovery unaware that H.H. Flor;, working with rust of flax, in the United States, had made the same discovery in 1940 (Chapter 3). It was a measure of how compartmentalised crop science had become, that the scientists working on one crop were ignorant of such an important discovery made in another crop. As we have seen, Flor called this genetic link between the host and the parasite the gene-for-gene relationship, and this is the definitive characteristic of vertical resistance.

The potato breeders gradually discovered that the resistance they were using to control potato blight was likely to fail after only a few years of commercial cultivation. It was vertical resistance. As the breeding of a new potato cultivar requires about eight years of work, it was clearly very difficult for the breeders to keep producing new cultivars, with new and different vertical resistances, to replace those cultivars whose older resistances had failed. During the 1960s, several potato breeders, who had spent their entire careers working with vertical resistance to potato blight, reached retiring age with a sense of despair, and a tragic feeling that their careers had been a waste of time. The breeding programs for blight resistant potatoes had failed, and this is why we still spray our potato crops in order to control blight.

Sex in the Blight Fungus

When the blight fungus was first discovered in Europe, the German mycologist Heinrich Anton deBary was studying reproduction in microscopic fungi. He showed that most fungi similar to Phytophthora infestans had two entirely different methods of reproduction. One method is a sexless, or vegetative, reproduction in which the fungus buds off microscopic spores in vast numbers. This kind of r-strategist reproduction enables the fungus to multiply very quickly, and cheaply, whenever weather conditions favour it. This rapid reproduction produces a population explosion of the blight fungus, and it explains why potato blight epidemics can develop so rapidly, and cause so much damage, over such a wide area.

The other method of reproduction is sexual, and is the result of two different mating "types" fusing their cells together to produce a new genetic combination. Technically, these "types" are male and female but they are not called this because their sex cannot always be recognised. Each fusion of two cells normally produces a single spore, called an oospore, which is also a resting spore that enables the fungus to survive adverse weather during a winter or dry season. This sexual reproduction leads to genetic recombination. The spore will be genetically different from its parents. In particular, its combination of vertical parasitism genes is likely to be different from either of its parents. This provides a wild fungus population with the maximum genetic diversity at the start of the next epidemic. This is the time that it needs diversity most, in order to overcome the system of locking of the gene-for-gene relationship.

When deBary studied Phytophthora infestans, however, he could not find any sexual reproduction, or resting spores, at all. This discovery baffled him, and it continued to baffle scientists all over the world for about a century. Then a Mexican scientist, Jorge Galind, working in the centre of origin of blight, discovered that there were two mating types of Phytophthora infestans in Mexico, and that resting spores were common in that country. It then became clear that the blight fungus had originally been taken to North America and Europe, and from Europe to all the rest of the world, as one mating type only. And a single mating type cannot have sexual recombination with itself. It also became obvious that this single mating type had probably been taken from Mexico only once, because it is most unlikely that it could have been taken out more than once, as only one mating type, and the same mating type.

The accident which meant that Europe and North America had only one mating type of Phytophthora infestans was to have profound effects on the development of crop science and, more specifically, on the breeding of plants for resistance to their parasites. With only one mating type, the blight fungus could overcome vertical resistances only by producing new strains through genetic mutation. The rate of mutation is rather slow, compared with the rate of production of new strains by sexual recombination. This meant that a vertical resistance to blight would remain unmatched, and effective, for several years of commercial cultivation, and this encouraged the breeders to use this kind of breeding, and this kind of resistance.

In Mexico, where the resting spores of blight are common, vertical resistance fails quite rapidly, because the many resting spores produce many different strains of the fungus, and the vertical resistance is matched quickly. Had both mating types of blight been present in Europe, and the rest of the world, breeding potatoes for vertical resistance to blight would never have started, because the resistance would usually have failed within one screening season. The breeders would have been compelled to use an entirely different breeding technique designed to accumulate polygenically inherited, horizontal resistance, and the influence of this potato work on the breeding of all other crops would have been profound.

Quite recently, the second mating type of blight was accidentally, but very carelessly, introduced to Europe and, from there, it has spread on seed potatoes to most parts of the world. This means that vertical resistance to blight is even more futile than it was before. And, if blight is to be controlled by horizontal resistance, rather higher levels of this kind of resistance will now be required. This is because the resting spores increase the initial inoculum, which is the amount of the blight fungus at the start of each epidemic. The very name of this potato disease may also have to be changed. In temperate countries, potato blight is usually called "late blight" to distinguish it from another disease (called early blight) that occurs rather earlier in the season. With a wealth of oospores in the soil, late blight will start much sooner each season, and it will no longer be "late".

Tuber-Borne Diseases of Potato

It was mentioned earlier that virus diseases of potatoes are transmitted by the vegetative propagation of seed tubers, but not by the sexual propagation of true seeds. This meant that virus diseases would gradually accumulate within a clone of potatoes until the yield of that clone was severely depressed. So far, the only method of solving this problem has been to produce seed tubers that have been inspected in the field, and which have been certified free from viruses. However, these tubers are expensive.

The key point about this problem is that these virus diseases spread quite slowly within potato crops. Without any artificial control of the viruses, a potato clone can usually be cultivated for about a decade before the accumulation of viruses renders it unprofitable. In fact, the viruses spread so slowly that a potato breeder rarely sees any evidence of them in his screening populations. This is a situation that is comparable with the breeding of potatoes after the discovery of Bordeaux mixture. It is impossible to select for disease resistance if that resistance cannot be seen. And it is impossible to measure disease resistance if the disease is absent. Furthermore, the disease must be universally present. That is, every individual in the screening population must carry the disease, otherwise individuals which have escaped infection will be disease-free, and will be mistakenly identified as resistant.

These potato viruses are European in origin. In theory, the exotic potato should have accumulated resistance to them after centuries of breeding in Europe. But that resistance can accumulate only during the breeding process, with its sexual reproduction, and genetic flexibility. Resistance cannot accumulate during the cultivation process, with its sexless reproduction, and genetic inflexibility. Because the viruses spread so slowly, they appear only rarely in a breeder's screening population. Consequently, there is no selection pressure for horizontal resistance to them. Like the loss of blight resistance under the shield of

Bordeaux mixture, we have been losing rather than gaining resistance to these viruses. In modern potato cultivars, the level of horizontal resistance to these viruses is probably close to the minimum.

This problem has been with us for centuries. Potato breeders have been ignoring resistance to virus diseases since before the appearance of blight in Europe, during the forty blight years before the discovery of Bordeaux mixture, during the forty years of breeding under the protection of Bordeaux mixture, during the forty years of breeding for vertical resistance, and ever since. The problem is still with us.

Modern potato breeders would have been happy to incorporate virus resistance in their varieties but, unfortunately, they could not find a "genetic source" of resistance. What they should have been doing was to breed for horizontal resistance by inoculating every single seedling in their screening population with these viruses. But suggest such a thing to a modern potato breeder, and he would be horrified. The viruses would make a frightful mess of his beautiful seedlings. Most of the seedlings would probably be killed, and only a few of them would survive as hideously distorted cripples. However, these cripples would have some resistance. And they could produce true seed. And the next generation would be a little more resistant. About a dozen generations of this degree of selection pressure is all that would be required to reduce these viruses to unimportance. And the same is true for other potato diseases which are spread by seed tubers. There are quite a few of them, and they have picturesque names like wart, scab, root knot, scurf, black leg, ring rot, gangrene, jelly end rot, soft rot, and dry rot.

Why is this important? Certified seed tubers are expensive. In fact, the cost of this certified seed is the largest single input in the commercial production of potatoes. This cost is passed on to the consumer, and it should not be necessary.

This point is well illustrated by potato cultivation in non-industrial countries. Subsistence farmers cannot afford certified seed. Nor can they obtain it, because few of the non-industrial countries have an effective seed certification scheme. As a result of my own horizontal resistance breeding work in Nairobi, Kenya now has a couple of potato varieties, called Kenya Akiba and Kenya Baraka (Akiba is Swahili for a store of food, and Baraka is Swahili for blessings), which can be grown without any spraying against blight, and without the use of expensive certified seed. Because there are two potato crops each year in Kenya, these cultivars have now been cultivated for more than forty vegetative generations without any detectable loss of vigour.

Unfortunately, my breeding work came to an abrupt halt when an irresponsible, visiting, foreign scientist, speaking out of turn, expressed the view that there was no such thing as horizontal resistance, and that my work was a waste of time. I was invited to leave the country. However, my two varieties now occupy about sixty percent of a greatly expanded potato acreage in Kenya. The staple diet in the highlands of that beautiful country is now changing away from traditional maize and beans, towards a much more nutritious diet of potatoes and milk.

It should be mentioned also that the Mexican variety "Sangema", described in a moment, is being cultivated in Rwanda, in central Africa, without any spraying, and without any use of certified seed. But, sadly, these two, small, African countries are the exceptions that prove the rule when it comes to this inexpensive and unpolluted potato cultivation. They do suggest, however, that a comparable freedom from certified seed and spraying is a realistic research target in every potato growing country, in spite of the fact that countries in Europe and North America have a considerably more complex pattern of potato parasites.

In this connection, it is perhaps worth commenting that a few "organic" farmers in the industrial world manage to grow potato crops without any spraying against blight or potato beetles. They can do this only because all their neighbours are spraying their potato crops, and these potato parasites are consequently quite rare. If no farmer was spraying his potatoes, the epidemics of blight and potato beetle (see below) would quickly build up to the point that potato cultivation was no longer economic. Indeed very few tubers would be harvested from such ravaged crops.

Potato Breeding in Mexico

In the Toluca Valley, in Mexico, which is the home of the blight fungus, John S. Niederhauser, of the Rockefeller Foundation, was the first modern crop scientist who deliberately chose to work with the biometricians' many-gene resistance, which we now call horizontal resistance. Niederhauser started working on potatoes, as a mere sideline of his work on cereals and beans, in 1947. Initially, he used the vertical resistance which, at that time, was the foundation of every potato breeding program in the world. However, the Toluca Valley differed from the rest of the world in having an abundance of blight oospores. This had two important effects on the blight epidemics in Mexico.

First, the epidemics are much more severe, because large numbers of oospores ensure that the initial inoculum of the blight is very high. This means that there is plenty of the blight fungus around, particularly at the beginning of the epidemic. Second, the great diversity of oospores produces an equally great diversity of blight races. This means that vertical resistance breaks down very quickly in Mexico, and this is especially true of foreign cultivars which usually have only one or two genes for vertical resistance. Niederhauser showed that vertical resistance was useless in Toluca, because it was usually matched almost as quickly as the potato sprouts appeared above the ground.

Edible potatoes from the Andes were introduced to Mexico by the Spanish in the eighteenth century, but it was immediately discovered that they were very difficult to grow. As we now appreciate, this was because of blight, which was not even known, scientifically, at that time. Mexican farmers discovered that they could grow potatoes in the high sierras, where it is too cold for the blight fungus. They could also grow them at low altitude, under irrigation, during the dry season, when blight epidemics could not develop. But this kind of cultivation is limited, and the country was deprived of a valuable food crop over much of its agricultural area.

John Niederhauser showed that it was possible to grow potatoes in the blight areas if they were sprayed with a fungicide. But there was a difference. In Europe, a potato crop must be sprayed about five times in order to control blight. But, in the Toluca Valley of Mexico, where John Niederhauser was working, the same potatoes must be sprayed up to twenty-five times, if the blight is to be controlled.

Niederhauser discovered that potato cultivars differed very considerably in the amount of blight that developed after the vertical resistance had failed. This was because of the second line of defence, which Niederhauser called "partial" (i.e., incomplete) resistance to blight, and which Vanderplank later re-named horizontal resistance, when he recognised that the concept applies to all plant diseases. As we now know, it was the same kind of resistance that led to the decline in the severity of those first blight epidemics in Europe, during the Hungry Forties. It was also the resistance that accumulated during forty years of potato breeding in the absence of Bordeaux mixture, and was largely lost again, during forty years of breeding in the presence of this fungicide. It is also the resistance that invariably remains in any plant after vertical resistance has been matched, even if it is at a very low level in many modern crop varieties.

Niederhauser was the first scientist who both recognised and used horizontal resistance. He deliberately abandoned vertical resistance, and he bred potatoes for higher and higher levels of the quantitatively variable, many-gene resistance. Perhaps his best known cultivar is Atzimba, which needs little spraying. When Niederhauser left Mexico, in 1972, he had produced many new resistant varieties, and his breeding work was continued by Mexican scientists who now have even better cultivars, including Sangema*, mentioned above.

The most popular potato in Mexico is still the old Dutch cultivar Alpha which, as we have seen, was bred during the forty year period when potato breeders were using the protection of Bordeaux mixture. And its level of horizontal resistance to blight is low. As already mentioned, when grown at Toluca, it has to be sprayed with a fungicide up to twenty-five times each season in order to control the very severe blight of that area. By way of comparison, a modern Mexican cultivar, such as Rosita, Tollocan, or Sangema, has so much horizontal resistance to blight that it needs to be sprayed with a fungicide only once or twice each season, in Mexico. By way of further comparison, the wild potatoes of Toluca are never sprayed at all, yet they get so little blight that scientists often have difficulty in finding it for research purposes. (Unfortunately, these Mexican potato cultivars cannot be utilised in temperate countries because they are short-day, tropical plants).

This indicates the importance of Niederhauser's work, and it is a very real indication of what can be achieved with horizontal resistance. In most of the plant breeding during the present century, horizontal resistance was ignored because it was unknown and unrecognized. Consequently, instead of being

*This name is derived from the first names of the three Mexican scientists who bred this cultivar, thus Santiago Delgado Sanchez, Gelasio Perez Ugelde, and Mateo A. Candena Hinojosa

increased, it tended to be lost, and most modern cultivars, of most species of crop, now have levels of horizontal resistance similar to Alpha's horizontal resistance to blight. It is probable that, when we start breeding other species of crop for horizontal resistance, we will achieve levels of horizontal resistance similar to Tollocan and Sangema, or even higher.

During the past thirty years, other scientists, working both with potatoes, and with other crops, have gradually concluded that breeding for vertical resistance was unsatisfactory. What they should have done was to imitate the work of John Nieder-hauser, and worked with horizontal resistance. But they did no such thing. Many of them failed even to recognise the existence of horizontal resistance. Others refused to believe that horizontal resistance could provide an adequate control of crop parasites. Yet others declined to use it on the grounds that working with it was too difficult. They wanted to breed for horizontal resistance using their Mendelian breeding methods and, under these circumstances, this kind of resistance is indeed difficult to accumulate.

When the Rockefeller Foundation sent John Niederhauser to Mexico, it also sent Norman Borlaug to the same area to work on wheat. These two brilliant scientists had closely parallel careers. Norman Borlaug produced the "miracle" wheats of the Green Revolution (Chapter 19) but, because breeding for horizontal resistance was so novel, and so difficult, he failed to produce wheats with horizontal resistance. The miracle wheats have vertical resistances, and they are still vulnerable to new strains of various parasites. On the other hand, John Niederhauser did produce horizontal resistance but, because this kind of breeding was so novel, and so difficult, he failed to produce a green revolution in potatoes, comparable to the green revolutions in wheat and rice.

Norman Borlaug has saved millions of people from death by starvation, and hundreds of millions more from malnutrition. It could be said that he has redressed the horror of the potato blight famine, many times over. For this achievement, among the greatest this century, he was awarded the richly deserved Nobel Peace Prize.

But, in the long run, John Niederhauser's achievement is likely to be deemed even more valuable, because he discovered a crucially important scientific principle that can now be applied to all crops. And, when it is, we may well see a completely new green revolution in every one of them. It is John S. Niederhauser who is likely to earn that final accolade, "a paragraph in the history books".

Potato Breeding in Scotland

More than thirty years ago, N.W. Simmonds, in Scotland, attempted a highly original experiment. He wanted to prove that modern potatoes (Solanum tuberosum) really were derived from the Solanum andigena potatoes of South America. He also wanted to show that horizontal resistance to blight could be accumulated in these very susceptible potatoes. Using recurrent mass selection, and selecting for both the agronomic characteristics of modern potatoes, and quantitatively variable resistance, he was able to report very considerable progress after only four generations of breeding. This progress occurred in yield, long-day tolerance, tuber qualities, and blight resistance. Many of his selections compared quite favourably with commercial cultivars, and Simmonds called this material "neo-tuberosum".

Quite apart from making him one of the early pioneers of horizontal resistance, Simmonds' work is of relevance to Part Three of this book. It provides an interesting illustration of what the members of a plant breeding club might accomplish.

Colorado Beetle

The Colorado beetle, named after the state of Colorado in the U.S.A., where it was first found, is a beetle that looks like a large ladybird. It is about half an inch in length and has characteristic orange and black stripes on its wing cases. As American pioneer farmers moved West, their potato crops eventually came into contact with this new encounter parasite, and it then transpired that these cultivated potatoes had very little resistance to it. The greatly expanding beetle populations began to spread eastwards. During the 1860s, they reached Illinois and Iowa, and they were so numerous that they were a nuisance in the streets and houses. By the 1870s, they had reached Canada, Vermont, and New York.

Farmers in North America began to protect their potato crops with an insecticide called Paris Green. This was a powder consisting of copper aceto-arsenite. Its insecticidal ingredient was arsenic, and the substance was extremely poisonous, and very dangerous to both people and the environment. However, it was the best available insecticide at that time.

In 1877, the beetles were found, but exterminated, in Germany and various European governments became very alarmed. For the first time in history, they enacted legislation to prevent the arrival of a new crop parasite, and the Colorado beetle became a topic of major concern. During World War I, there was even a suggestion that the Allies should drop live Colorado beetles from airplanes over the potato crops of Germany. This was possibly the first recorded example of an attempt at biological warfare. Fortunately, wiser councils prevailed and this outrageous idea was abandoned.

In 1922, Colorado beetles were found in the South of France, potato, established beyond any hope of eradication. The beetles spread northwards, and the farmers of Europe began to spray their crops with lead arsenate, which was doubly poisonous, and doubly dangerous.

The beetles have not yet reached Britain. E.C. Large, in his book The Advance of the Fungi (1940), stated that anyone who found a Colorado beetle in Britain should send it to the Ministry of Agriculture, stating where he had found it, and giving his name and address. He should then "stand by, and watch what happened, as one who has pulled a fire alarm near a gunpowder dump".

The tiny island of Jersey, only fourteen miles from the Normandy coast, grows early potatoes for the British market. It too is still free of Colorado beetles, and it must remain that way if it is not to lose that British market. For this reason, the French Government ensures that the potato crops of nearby Normandy are given extra insecticidal sprays in order to protect the Jersey potato crops from flying beetles, which can easily cross fourteen miles of sea. This is a wonderful example of international goodwill which, sadly, has remained virtually unknown to the world at large.

With the discovery of DDT (Chapter 16), the protection of potato crops became much easier, and much safer. By the time that DDT was banned, there were other new insecticides to replace it. Nevertheless, in those areas where it is a pest, we still spray our potato crops against Colorado beetle. It seems that no one has ever attempted to breed potatoes for resistance to this insect. The reason, of course, is obvious. No one could find a source of resistance. Vertical resistances to this parasite do not exist, at least in its known hosts that have been studied in this regard. And, it appears, no one has ever attempted to breed potatoes for horizontal resistance to Colorado beetle.

No one knows whether horizontal resistance could be accumulated in potatoes to a sufficient level to control Colorado beetle. Anyone who expresses an opinion on this matter can only be guessing, because it has never been scientifically investigated. Needless to say, it should have been investigated, decades ago. And it deserves to be investigated now, pretty damn quick, as they say in the military.

CHAPTER NINETEEN
Why Did the Green Revolution Run Out of Steam?

Farmers often distinguish between intensive and extensive crops. Apples, for example, are an intensive crop because the fruit is valuable, and the crop justifies considerable work and investment. Cereals, on the other hand, are extensive crops which must be grown on large acreages, with relatively little work and investment devoted to each acre, because there is relatively little profit to be obtained from each acre.

Before the days of artificial fertilisers, farmers manured their crops exclusively with "muck", the rotting excrement of their cattle, pigs, and horses, otherwise known as farmyard manure, or F.Y.M. The work of spreading this manure over the fields was known as "mucking out" and, as people who live in the country know well, it is a smelly business. However, this method of manuring crops has two drawbacks, quite apart from the stink. First, there is always a strict limit to the amount of farmyard manure that one mixed farm can produce, and it is never enough. Second, it is a labour intensive, and expensive, method of manuring crops. For these reasons, in the old days, farmers only manured their intensive crops, and their extensive crops had to get by with manure residues left in the soil from an earlier crop. One of the several reasons for crop rotation was to ensure that each field received a dose of manure every few years.

The purpose of manuring was to provide crops with the three main nutrients of plants, which are known as N, P, and K, these being the chemical symbols for nitrogen, phosphate, and potash. Some of the more important discoveries of nineteenth century chemistry revealed that these are the main nutrients of plants. Unlike animals, which have to eat organic plant or animal tissues, plants absorb their nutrients as inorganic chemicals. For this reason, it is possible to manure plants with inorganic, or "artificial" fertilisers.

One of the first of the so-called artificial fertilisers was, in fact, a natural product, and it was called guano. This material was quarried from tropical shorelines, mainly in Peru, and it consisted of the accumulated droppings of millions of sea birds that fed on fish. Guano is rich in phosphate and nitrogen. Another natural product is rock phosphate. Later, artificial fertilisers began to be produced in factories, and it was the demand for these that first led to the growth of some of the bigger chemical corporations. The manufacture of nitrogenous fertilisers is closely similar to the manufacture of explosives. Various nitrogen compounds are the basis of explosives such as nitro-glycerine, and ammonium nitrate. The big chemical corporations grew really big, and really rich, from the demand for explosives during two world wars.

One of the few good things to come out of World War II was an enormous surplus of factory space, in all the industrial nations, for the manufacture of explosives. When the war ended, the demand for explosives disappeared, and these factories were suddenly superfluous. The only thing they could be used for, without being entirely rebuilt, was the manufacture of nitrogenous fertilisers for agriculture. This manufacture requires large amounts of energy, in order to combine atmospheric nitrogen with hydrogen to form ammonia, which is the starting point of the industrial process.

In addition to the surplus explosives factories, for nearly thirty years following World War II, there was also a period of cheap oil, and cheap energy. As a consequence, the production of nitrogenous fertilisers increased to a state of glut, and prices fell dramatically. For the first time in history, it became economic to apply nitrogenous fertilisers to extensive crops, such as wheat.

Dwarf Varieties

Traditionally, wheat had long straw. Pieter Bruegel the Elder (1525-1569) painted a scene of wheat reapers, called August or Wheat Harvest, in which the wheat is nearly as tall as some of the men who are cutting it. In those days, this meant that the straw would be about four or five feet long. Long straw was desirable for several reasons. It was easier to cut by hand, and to tie the wheat into sheaves which were then propped together in stooks to dry. The straw also had a value of its own and, indeed, was almost as desirable as the grain itself. This was because of the many farm animals, particularly cattle and horses, which needed straw for bedding.

Wheat with long straw has a serious disadvantage, however. It is liable to be blown over when it gets wet, and heavy, with the wind and rain of a storm. This flattening of a wheat crop is known as "lodging", and it makes the harvesting difficult and, occasionally, impossible. Applying farmyard manure to wheat was dangerous, quite apart from the adverse economics of this practice, because rich nutrients increase the straw length, the ear weight, and the likelihood of lodging.

Now that horses have been replaced with machines, the need for long straw has largely disappeared, and the dangers of lodging have also disappeared. This is because the modern trend has been towards the exact opposite of long straw. The so-called dwarf and semi-dwarf wheats have very short straw, measuring as little as two feet in length. These dwarf wheats have the advantage that they can be given heavy doses of fertilizer without danger of lodging. As a result, their yields can be increased considerably.

This was the basis of the Green Revolution. In the 1940s, the Rockefeller Foundation decided to undertake agricultural research in non-industrial countries and, with the cooperation of the Mexican Government, they started in Mexico. One of their scientists was Norman Borlaug who was breeding improved varieties of wheat. He became aware of the falling prices of fertiliser, of the yield increases that could be obtained from this fertiliser, if there were no lodging, and of the possibility of developing dwarf wheats that were resistant to lodging. This became the basis of his research.

The dwarf character in wheat originated in Japan, and it was incorporated into American wheats by O. A. Vogel. Borlaug took Vogel's dwarf wheats to Mexico in 1954. He bred new dwarf wheat varieties from them, and they yielded so well that it was economic to grow them with artificial fertilisers, on irrigated land, in northwest Mexico. The increase in wheat production was dramatic. Within a few years, Mexico became self-supporting in wheat. The next development was that scientists in India heard about these new varieties and, after a few experiments, they imported bulk quantities of seed from Mexico. Very soon, India changed from being a wheat importing nation to being a wheat exporting nation. Similar increases in production occurred in Pakistan, China, and various countries of the Middle East and North Africa.

In the meanwhile, other scientists of the Rockefeller and Ford Foundations were copying Borlaug's work in the Philippines, except that they were working with rice. They too produced new dwarf varieties that could be grown with cheap fertiliser, and which then had greatly increased yields. Quite quickly, countries such as the Philippines, India, Indonesia, and Thailand, increased their rice yields as much as the wheat growers had increased their wheat production.

The public relations people of these two Foundations coined the terms "miracle wheat", "miracle rice", and "green revolution". We can forgive them for their euphoria, and their Madison Avenue terminology. The effects of the green revolution really were stunning. Here, at last, was technical aid, from the Industrial World to the Non-Industrial, that really meant something. Millions of people were saved from starvation, and at least one billion people were saved from serious malnutrition. And, as we saw in the last chapter, Norman Borlaug was given the Nobel Peace Prize. It was possibly the most richly deserved Peace Prize ever awarded.

International Research Centres

It was at this point that various governments and charitable organisations decided that the world needed more green revolutions, in more crops, and more countries. The governments of industrial nations had already agreed that they should each aim at spending 0.7% of their annual budgets on assistance to non-industrial countries. To this end, many of them set up their own overseas aid organisations. None of these governments reached their 0.7% targets but, even so, most of the aid organisations failed to spend all the money that was allotted to them. They all seemed to end their financial years with budget surpluses. Obviously, the best way to utilise these surpluses was to finance new green revolutions.

A body called the Consultative Group for International Agricultural Research (CGIAR) was set up, with headquarters in New York. Its function was to allocate these surplus funds to agricultural research in the non-industrial world, and it funded various international research centres. A list of the more important centres includes CIMMYT, with headquarters in Mexico, which now looks after wheat and maize, and IRRI, in the Philippines, which looks after rice. CIAT in Colombia is responsible for cassava, beans, and other tropical crops. IITA in Nigeria has a similar mandate for the wet tropics. CIP in Peru is responsible for potato research. ICRISAT in India does research on crops in the semi-arid tropics, and ICARDA specialises in dry area agriculture, particularly in the Mediterranean region. IBPGR has a general responsibility for genetic conservation. In total, there are now eighteen centres, and their collective budget is in the region of $400, 000, 000 a year.

The CGIAR made two mistakes when setting up these large and expensive international research centres. First, they deliberately created scientific monopolies in the non-industrial world. All the money available for research on a particular crop would go to a single research centre. If two centres, such as CIAT and IITA, had over-lapping areas of research, they were carefully controlled to ensure that they did not compete with each other. The justification for this was to avoid unnecessary duplication. Research is expensive and, it was argued, duplication makes it doubly expensive.

But, in fact, duplication in research is essential, because it provides the competition which is so necessary for good science. Nothing stimulates a scientist more than the thought that a rival scientist may anticipate him, and publish first. And nothing dulls a scientist more than the knowledge that he has no rivals. The scientists at the International Research Centres have few rivals, and those they do have are critically short of research funds.

The second mistake was fundamental. It was the hope that these International Centres would produce new green revolutions. They did not. Indeed, they are a classic illustration of the completely false idea that you have only to throw enough money at enough scientists, in order to get new scientific breakthroughs. The original inspiration for good science comes from the scientists themselves, and usually from an individual scientist who, as often as not, is grossly under-funded, and probably working in an ancient laboratory that has been due for demolition for decades. Scientific inspiration does not come from money. Nor does it come from politicians, administrators, or bankers.

The CGIAR produced no new green revolutions for one very simple reason. All the plant scientists employed by the international centres had been trained in the traditions of the Mendelian school of genetics. They believed that, when breeding plants for parasite resistance, you must first find a genetic source of resistance. If no source of resistance could be found, the resistance breeding could not even begin. You then had no choice. You had to use crop protection chemicals. For these members of the Mendelian school, there were really no other possibilities.

The International Potato Centre (CIP) was possibly the worst in this respect. For years its scientists were telling the world that there was no such thing as horizontal resistance. Vanderplank's writings were ignored. John Niederhauser's work in Mexico was ignored. My own later, and much less important, potato work in Kenya was also ignored. John Niederhauser who, by rights, should have been in charge of CIP research, was rigorously excluded from its affairs. On the one occasion that I visited the place, I was shouted down during a scientific meeting. In fairness, however, I must comment that this was many years ago, and that CIP is now greatly improved. Nevertheless, the CGIAR International Centres, as a group, have a really dreadful record of ignoring horizontal resistance.

Secondary Problems in the Green Revolution

The miracle wheats and rices both ran into what the members of the Mendelian school called "secondary problems". This is because there are vertical resistances in the miracle wheats and rices, and these resistances fail periodically. On one occasion in Mexico, many tons of special fungicide had to be airlifted from Europe, at huge expense, as an emergency measure, to save a large area of wheat whose vertical resistance had broken down. A large proportion of the research budgets of CIMMYT and IRRI are spent on "maintenance research" which is their euphemism for the production of new cultivars to replace those whose vertical resistances have failed.

Rice has vertical resistance to one of its diseases, called "blast" (Piricularia oryzae), and one of its insect pests, called the brown plant hopper (Nilaparvata lugens). Blast disease has proved an intractable and recurring problem because of endless failures of vertical resistance. The brown plant hopper has proved even worse because, when the vertical resistance to it failed, the miracle rices were so susceptible that there were unheard of population explosions of this pest. There were so many hoppers around that they invaded neighbouring, resistant rice crops in huge numbers. These crops were often old, local landraces which had an adequate level of horizontal resistance to control normal infestations of brown plant hoppers. But their resistance was entirely inadequate to control this parasite interference, and the abnormal, and artificially induced, levels of infestation.

It was at this sad and sorry point that subsistence farmers in the non-industrial world were advised, for the first time ever, to start spraying their rice crops with crop protection chemicals. However, there is a happy ending to this story. Peter Kenmore, an American entomologist working in the Philippines, introduced IPM methods (Chapter 14) to the rice farmers of this country. He has been dramatically successful in reducing the use, and the cost, of insecticides while, at the same time, increasing the yields of these rice crops. His success is an example to the rest of the world. And his success will be even greater when the rice breeders finally produce new varieties with high levels of horizontal resistance.

Another problem is that the miracle wheats and rices have proved to be abnormally susceptible to a few diseases which were previously quite unimportant. The miracle wheats have little resistance to Septoria diseases, for example, apparently because these fungi have a low epidemiological competence in Mexico, where the wheats were bred and selected. These diseases are now of major importance in other parts of the world where the fungi have a considerably higher epidemiological competence.

The miracle rices were selected in the Philippines and they too had abnormal susceptibility to parasites which either do not occur, or which have a low epidemiological competence, in that area. For example, some of the miracle rices could not be grown in India because of a bacterial blight, and a virus disease called "tungro".

No New Green Revolutions

Interestingly, the entire green revolution was based on two characters, the short straw of wheat, and the short straw of rice, which are both inherited in a Mendelian fashion. This, of course, was a tremendous boost for the Mendelian school of plant breeding. Suddenly, for the first time in half a century, the members of the Mendelian school had found characters whose inheritance was controlled by only a few genes, apart from resistance to parasites, that were of major economic and agricultural significance. In spite of the secondary problems, the dwarf wheats and rices were undoubtedly the two most important agricultural achievements of the second half of the twentieth century, and they were the result of Mendelian inheritance. More than ever, the Mendelian pedigree breeding methods became "mainstream" science.

This simple fact has had two very profound consequences. First, it confirmed and prolonged the domination of plant breeding by the Mendelian school of genetics. The green revolution was claimed as a new triumph of this school. It should, perhaps, be regarded as the last gasp of the Mendelian school.

It is possible, although rather unlikely, that a Mendelian character of major agricultural importance has still to be discovered. Crop science has had nearly a century in which to find such characters and, bearing in mind that virtually every crop geneticist was a member of the Mendelian school, they have not found many. Just short straw in wheat and rice, and vertical resistances. All other single-gene characters, such as seed and flower colour, are of quite minor economic significance.

Secondly, no new green revolutions were produced by the expensive, monopolistic, international research centres because no one could find even one new Mendelian character that could make such a revolution. If we are to have new green revolutions as, indeed, we probably can, and will, they are more likely to emerge from quantitative genetics, and from breeding plants for quantitative resistance which is durable, complete, and comprehensive.

There have been other green revolutions, in the past, although they were never called this. The development of sugar beet from fodder beet (Chapter 2) in the nineteenth century created an entirely new crop, and entirely new beet sugar industries, in many temperate countries. The breeding of sugarcane, which started in the late nineteenth century, had just as dramatic an effect on sugarcane production as did short straw on wheat and rice cultivation. The development of hybrid maize in the United States, and later most of the world, was even more important. Similarly, the breeding of new soybean varieties transformed an insignificant crop into the largest crop of all in the United States. These developments all depended on quantitative genetics. On the few occasions when Mendelian characters were employed, they were a positive nuisance, because they provided vertical resistance to parasites, and nothing else.

Genetic Conservation

Finally, the green revolution, and the Mendelian school of genetics on which it was based, has led to another misconception. The centre of origin of wheat is in the Middle East, in the area that archaeologists call the "Fertile Crescent". This area used to be full of small farmers cultivating an incredible diversity of different wheat lines. When the wheat breeders needed new vertical resistance genes, to replace those that had failed, they usually searched for them in the wheats of these small farms.

The miracle wheats, however, yielded so much more than these old wheats that they quickly began to replace them. The small farmers of the Fertile Crescent, and elsewhere, discarded their old wheats in favour of the new. A side effect of this improvement was that genetic diversity began to be lost. There was a great outcry about this loss of diversity, because there was a fear that valuable genes would be lost for ever. A new scientific discipline, called genetic conservation, was born of this fear, and wheat "gene banks" were established to ensure the survival of this diversity. Soon, gene banks were being set up for many other crops also, and a lot of research was undertaken to discover how best to store seeds of large numbers of cultivars in a viable condition for long periods of time.

Genetic conservation has now become part of the received wisdom of both crop scientists and green activists all over the world. But no one seems to have questioned just what we are trying to conserve. Mendelian genes? Vertical resistance genes? This is what the original wheat conservation was all about. It concerned vertical resistance genes and nothing else, other than some vague and ill-defined unknowns. And it was copied uncritically in most other crops, irrespective of whether they possessed vertical resistances or not, and regardless of whether we need vertical resistances or not.

If the world eventually abandons pedigree plant breeding methods, and moves to quantitative genetics, we will not need these huge gene banks. Quantitative genetics does not depend on single genes. It depends on numerous polygenes which can vary in their frequency but which are almost always present. Obviously, we must have some genetic conservation, even with quantitative genetics. But we will need far less than the members of the Mendelian school suppose. Each quantitative breeding program will need a reasonably wide genetic base, but no more. In practice, that genetic base will normally consist of a range of modern cultivars, and the farming system itself will often maintain an adequate genetic diversity. If greater genetic changes are required, a gene bank could be useful. But even if the old cultivars and primitive archetypes have been lost, the breeders can usually go back to the wild progenitors to find genetic diversity, if absolutely necessary. So, it can be argued that our current gene banks are something of a white elephant, and a rather expensive white elephant at that. (This topic is discussed further in Chapter 21).

CHAPTER TWENTY
Maize in Tropical Africa

On his return from the New World, Columbus took maize to Spain. From the Iberian Peninsula, the Portuguese then took maize to West Africa and, soon after, to the countries of the Indian Ocean. Maize has thus been in Africa since the early sixteenth century, and it has been the staple food of much of Africa ever since.

There is a disease of maize called "tropical rust" caused by a microscopic fungus named Puccinia polysora. It is called "rust" because the fungus produces spots of rust-coloured spores on the leaves, in a manner closely similar to coffee rust (Chapters 4 & 21). And it is called "tropical" rust because it has (i.e., it is able to cause) epidemics only in the tropics. For this reason, the disease could not survive in the Iberian Peninsula (assuming it ever got there, which is doubtful) and, consequently, it did not reach Africa, which remained free of this maize parasite for about four centuries.

Tropical rust apparently arrived in Africa as a result of the development of trans-Atlantic air transport. It is thought that the rust was accidentally introduced, in the 1940s, on green corn cobs flown from tropical America to West Africa. The rust then became a "re-encounter parasite", so called because it had been separated from its host for some four centuries, and then re-encountered it again, in a new area.

The disease in tropical Africa was devastating, and it damaged the maize crops, in much the same way that blight damaged the potato crops of Europe, during the hungry forties. This was another example of crop vulnerability, resulting from an extreme susceptibility in the absence of a foreign parasite. When the parasite was inadvertently introduced, the susceptibility was revealed, and the vulnerability was manifested. Potential damage became actual damage.

Tropical rust reached East Africa, in Kenya, in 1952. As a young plant pathologist, straight out of university, I arrived in Kenya in 1953, and this disease proved to be one of the formative experiences of my career. On my arrival, I found government officials in a state of considerable alarm, because there were real fears of a very serious famine.

Based on earlier experience gained in West Africa, a team of scientists in Kenya had launched a breeding program for resistance to tropical rust. They used the accepted procedures of the day, and they first looked for a genetic source of resistance. They could not find a source of resistance in Africa, and they had to use resistant lines imported from Central America. Inevitably, this was vertical resistance and, as it happens, vertical resistance to tropical rust of maize breaks down extremely quickly. In Kenya, the tropical rust fungus produced new strains so rapidly that each new vertical resistance failed while the breeding work was still in progress.

I myself was too junior to be involved in this work, but no one could prevent me from observing it with a lively curiosity. On my first visit to Coast Province, I was shown the disease. The maize crops resembled scrap metal in junk yards, with many leaves showing little but the colour of rust, with scarcely a speck of green to be seen anywhere. Many of the plants were killed before they could even form flowers, let alone produce seed. It was a depressing sight. However, matters soon began to improve.

The first good news was that the disease lived up to its name. It really was confined to a hot, tropical climate. The equator runs right through the middle of Kenya and, at the equator, the disease loses epidemiological competence at altitudes above 4000 feet. At sea level, it lost epidemiological competence at the latitudes of the Tropics of Cancer and Capricorn. The fears of a major famine receded rapidly when it was realised that the Highland maize crops were safe. Most of the people of Kenya, and the bulk of the agriculture, are located in the Highlands, which are all above 4000 feet in altitude.

The next good news was that the severity of the disease appeared to be slowly declining, and it continued to decline until, about six or seven years after its first appearance, it ceased to have any importance whatever. The problem solved itself. It did so without any help from either plant breeders, or plant pathologists. As we now know, the problem solved itself naturally, by the operation of normal biological processes. These processes led to a steady accumulation of horizontal resistance until the disease was no longer important. If we analyse just what happened during these processes, we can learn some important lessons on how to breed plants for horizontal resistance.

These are those lessons.

Lesson 1: The bankruptcy of the pedigree breeders' resistance

Perhaps the first lesson was that the traditional approach to breeding crops for resistance to their parasites was useless. The pedigree breeders used genetic sources of single-gene resistance in maize imported from Central America, and they employed gene-transfer techniques to incorporate them into the local maizes. Obviously, these were vertical resistances. Unfortunately, the parasite was able to match them so quickly that the use of this kind of resistance was futile. Being in Kenya at that time, I shared the dismay of the breeders when their first resistance gene, named Rppl, was matched in field trials, well before any seed was available for farmers. The second resistance gene, named Rpp2, was matched in the research greenhouse, at an even earlier stage than the first gene. Then the combination of both genes was matched. By this time, it was apparent that tropical rust was no longer a serious disease, and the breeding program was abandoned.

With this disease, the resistance produced by the Mendelian breeding method proved to be unusually short-lived, because the rust is able to produce new strains so quickly. I know of only one disease in which vertical resistances are matched more quickly. This is potato blight in Mexico, where foreign commercial cultivars with vertical resistances are matched in their first season, almost as soon as the sprouts appear above the ground (Chapter 18).

When we look at all the vertical resistances of crops, there is thus great variation in the durability of those resistances. With tropical rust of maize, it fails so quickly that it has no agricultural value at all. At the other extreme, a few examples of vertical resistance have endured for most of this century, and they are very valuable (Chapter 16). The majority of vertical resistances fall between these two extremes, and are of limited value. For example, after nearly half a century of wheat breeding in Kenya, it was calculated that the average commercial life of a new wheat cultivar was four and a half years. It takes about eight years to produce a new wheat cultivar, using pedigree breeding methods.

Lesson 2: The vindication of the biometricians

The accumulation of polygenic resistance in the African maize landraces was a total vindication for the biometricians. However, no one recognised this at the time. In the late 1950s, we in Kenya knew that the tropical rust of maize had declined to insignificance, but we had no explanation for this. We knew that the official resistance breeding program had failed, but we were having similar experiences in our breeding of other crops, such as wheat and potatoes, in which valuable resistances were repeatedly being lost. The difference was that the tropical rust of maize was no longer a serious disease, while the various parasites of wheat and potatoes continued to be very serious indeed.

This accumulation of resistance in maize went largely unnoticed in the world at large, and many crop scientists are still unaware of it. This is understandable because things that happen in Africa tend to remain unknown, unless, sadly, large numbers of people happen to die. Nevertheless, this accumulation of resistance in maize was among the most important crop science events of the twentieth century. It was important because it both demonstrated the value of horizontal resistance, and it taught us exactly how to breed other crops for horizontal resistance. However, this importance did not become fully apparent until Vanderplank started publishing his highly original and innovative books on plant diseases (see bibliography). Only then did it become possible to extract a slew of lessons from the maizes of tropical Africa.

It was soon after the arrival of Vanderplank's first book, in 1963, that it dawned on me that the best way to breed our crops for parasite resistance was to imitate the behaviour of the African maize landraces, following the appearance of tropical rust. I have been trying to persuade others of this ever since, with very little success. The pedigree breeding tradition dies hard.

Lesson 3: The erosion of horizontal resistance

When maize was being cultivated in Africa, in the absence of tropical rust, it had no need for resistance to this parasite. There was negative selection pressure for resistance, and the resistance was gradually lost. This was an excellent example of the erosion of horizontal resistance. Because this erosion resulted from genetic changes in the host population, it was a host erosion of horizontal resistance (Chapter 13). It was also a massive erosion. There was very little resistance left. This is why the African maize was so susceptible to the rust when it first appeared. Furthermore, because subsistence maize crops are genetically diverse, and genetically flexible, this erosion occurred during the cultivation process.

A comparable erosion has occurred in many modern cultivars, which are genetically inflexible. But, here, the erosion occurred during the breeding process. These cultivars have been subjected to about a century of breeding with negative selection pressures for horizontal resistance. These negative selection pressures usually occurred because of a functioning vertical resistance, or because of the use of crop protection chemicals, during the breeding process. When cultivated in the absence of crop protection chemicals, many modern cultivars are now as susceptible to some of their parasites as the African maizes were when tropical rust first appeared. These maizes in tropical Africa can tell us a lot about our own crops, and the way we have been breeding them. And just how susceptible our modern crops are right now. And precisely how we can now reduce this susceptibility, by accumulating horizontal resistance.

Lesson 4: Genetic flexibility

Next, we must enlarge on the concepts of genetic flexibility, and selection pressure. As we have just seen, the African maize crops could respond to selection pressures during the cultivation process, because they were open-pollinated, genetically diverse, and genetically flexible. They could not only lose horizontal resistance, because of negative selection pressure in the absence of tropical rust. They could also gain horizontal resistance, because of positive selection pressure in the presence of the rust. And both of these processes occurred during cultivation.

Most modern cultivars are genetically uniform, and genetically inflexible. As we saw in Chapter 7, we positively want them that way in order to preserve valuable agricultural characteristics. For precisely this reason, these cultivars do not gain or lose horizontal resistance during the cultivation process. They can gain or lose it only during the breeding process.

The fact that modern cultivars need so much protection from crop protection chemicals suggests that they have lost a lot of horizontal resistance in this breeding process. And this further suggests that there is something very wrong with the breeding methods that we have been using for most of this century. Equally, these modern cultivars cannot gain horizontal resistance during the cultivation process. They can gain it only during the breeding process. If they are to do this, we must change our breeding methods in order to ensure that they include positive selection pressures for horizontal resistance.

Lesson 5: Population breeding

It will be recalled from Chapter 2, that the biometricians had developed their own method of plant breeding, known as population breeding. This method involves recurrent mass selection, in which only the best individuals of each generation are allowed to become the parents of the next generation. This is exactly what happened with the maize landraces that were exposed to tropical rust. When the rust first appeared, many of the maize plants were killed by it. Relatively few plants survived long enough to produce flowers. And only some of those were resistant enough to produce a few seeds. It was this minority of very susceptible but relatively resistant plants that became the parents of the next generation.

Had this disaster happened to modern commercial farmers, they would have rejected the cultivar, and replaced it with a different one. This, after all, is precisely what happens with the twentieth century boom-and-bust cycle of breeding vertically resistant cultivars. But the farmers in Africa were much closer to nature. They had confidence in their treasured landraces, and they resolutely refused to abandon them. They kept the few seeds produced by their devastated crops, and they cherished them. There was no question of eating them. Subsistence farmers eat their seed only the very worst of famines and, in this famine, the Government had provided food relief. So the farmers could keep their precious seeds, and these seeds became the parents of the next generation.

These farmers were all peasants. They had small, subsistence farms, and they were poor. Most of them were uneducated and illiterate. But they displayed great wisdom. This wisdom involved hope, patience, and, above all, a complete trust in nature. And their trust was magnificently vindicated as, crop after crop, their prized landraces slowly accumulated more and more resistance, and yielded more and more food, of the quality they liked best. This is the magnificent example we must follow when we set out to breed other crops for horizontal resistance.

Lesson 6: The nature of the resistance

The resistance that accumulated was undoubtedly horizontal resistance. Some scientists have attributed the decline in the tropical rust to the official breeding program for vertical resistance. One author (who, in charity, need not be named), discussing maize breeding programs, and the use of single-gene resistance to maize diseases, used the phrase "...the most spectacular was obtaining resistance to Puccinia polysora in Africa". He really believed the tropical rust problem has been solved by the vertical resistance breeding program, and he was totally ignorant of what a fiasco that program had been.

Other scientists suggested that the resistance was really vertical resistance, because they secretly believed that this is the only kind of resistance that exists. However, the resistance has now endured for nearly forty years without any suggestion of a failure. It must be remembered that, in the pedigree breeding program, three vertical resistances failed so quickly that the breeding process could not even be completed.

Other scientists have suggested that the resistance might result from a mixture of many different vertical resistances. But, were this so, the Mendelian breeders would have had no difficulty in finding resistance genes in the African maizes when the rust first appeared. In fact, they found none at all.

All the evidence is clearly in favour of this being horizontal resistance. But this evidence is circumstantial only. No one has done any research on this matter for a very simple reason. These countries in Africa are poor countries. They cannot afford academic research. They can afford research only for the most pressing of problems. And tropical rust is no longer a problem.

Lesson 7: Transgressive segregation

When the rust first appeared in the tropical maizes, there was an immediate, and very strong, positive selection pressure for resistance. As we have seen, the mechanism of this selection pressure was that the most susceptible individuals were killed. Less susceptible individuals managed to survive but failed to produce pollen or seed. The least susceptible individuals managed to produce pollen, and a few seeds, and they became the parents of the next generation.

The next generation was changed genetically because all the individuals in it were the progeny of a very small minority of relatively resistant parents. The new generation had more resistant individuals in it than did the previous generation. Even more important, the most resistant individuals in the new generation had a higher level of resistance than any of their parents. As we have seen (Chapter 12), this phenomenon is called transgressive segregation.

This fact of transgressive segregation is essential to the accumulation of horizontal resistance and, indeed, to the accumulation of any quantitative variable. Accordingly, an explanation of how it works is appropriate. Suppose that the two parents, which are highly susceptible, each has only 10% of the alleles contributing to horizontal resistance. But, if they each have a different 10% of alleles, some of their progeny will have more than 10% of the total available alleles. These individuals will be more resistant than either of their parents. Under a strong positive selection pressure for resistance, these more resistant individuals will have a reproductive advantage, and will become the parents of most of the next generation. In the next generation, the process of transgressive segregation is repeated. And the accumulation of resistance continues until all the individuals in the population possess most of those resistance alleles, and no more resistance is needed.

Lesson 8: On-site selection

On subsistence farms, each farmer keeps some of his own harvest for seed. He maintains a local landrace which is genetically flexible and has responded to the selection pressures in the local environment, just like an ecotype in a wild ecosystem (Chapter 8). Indeed, an open-pollinated landrace can be called an agro-ecotype. Consequently, a landrace is normally in a state of excellent balance with its own, local agro-ecosystem. In systems terminology, this is called local optimisation.

However, if a landrace is taken to a different agro-ecosystem, it will perform less well. This is because various environmental factors will be different. These factors include the components of climate, such as temperature and rainfall, and various aspects of the soil, such as structure, nutrients, and microbiological activity. Many of the subsystems called pathosystems will also differ, because the epidemiological competence of the many different species of parasite will also be different. In the new environment, the foreign landrace will have too much resistance to some parasites, and too little to others. For these reasons, when breeding plants for horizontal resistance, the screening must be done locally. As we have seen (Chapter 12), this is called on-site selection, which means that the screening is conducted in the area of future cultivation, in the time of year of future cultivation, and according to the farming system of future cultivation.

When the vertical resistance breeding was in progress in Kenya, the scientists concerned received a report that maize in Malawi was highly resistant to tropical rust. So they imported some of this maize for testing in Kenya. It proved to be just as susceptible as the Kenya landraces, and the scientists concluded (wrongly) that the strains of the fungus in Kenya were different from those in Malawi. Malawi is about 1000 miles south of Kenya, and it is much closer to the Tropic of Capricorn. Consequently, tropical rust has a greatly reduced epidemiological competence in Malawi. A level of horizontal resistance that was adequate in Malawi, was quite inadequate in Kenya, where the rust has a very high epidemiological competence. As we have seen (Chapter 13), this is called the environment erosion of horizontal resistance. It indicates why on-site selection is essential when breeding for horizontal resistance.

Lesson 9: No source of resistance

The maize crops that were exposed to tropical rust were landraces. This is the technical term for the genetically mixed crop varieties that were cultivated before the discovery of pure lines, and genetic uniformity. Subsistence maize crops in the tropics are some of the very few crops still being cultivated as landraces. Even though all the plants within a landrace are very similar in appearance, they differ genetically among themselves. This genetic diversity is not very great, but it is enough to embrace all the alleles necessary for the accumulation of a very high level of horizontal resistance (Chapter 12). Far more important is the fact that the African maizes accumulated high levels of horizontal resistance without the genetic source of resistance that is essential in the Mendelian breeding methods.

It follows that, when we breed for horizontal resistance, we must have genetic diversity, but we do not need very great diversity. And, above all, we do not need a single-gene source of resistance.

Lesson 10: Selection pressures

When the rust first appeared in a given locality of tropical Africa, it was extremely damaging. The selection pressure for resistance was thus very strong. But, as resistance accumulated, the selection pressure declined. This happened because the least resistant plants were no longer being killed, or even prevented from flowering. They were merely suffering a reduced rate of reproduction. Eventually, all the maize was highly resistant, and the selection pressure for resistance was reduced to a mere maintenance level. That is, if a rare, susceptible individual happened to appear within a local landrace, it would be so severely parasitised that it would have few progeny.

This steady reduction in selection pressure has two warnings for plant breeders. First, the initial selection pressure may be so high that the entire screening population is liable to be killed off entirely. If this total destruction appears likely, it is entirely reasonable to use crop protection chemicals towards the end of the screening process. This will enable the least susceptible individuals to form a few seeds.

The second warning is that, as resistance accumulates, and the selection pressure for resistance declines, the rate of breeding progress, or genetic advance, will also decline. This can be prevented by artificially intensifying the epidemics with spreader rows or surrounds. Spreader rows intersect the screening population at regular intervals, while surrounds are planted all around it. The spreader rows or surrounds are planted with susceptible plants in order to generate large numbers of parasites that then move into the screening population. However, great care must be taken to ensure that these susceptible spreader plants are not allowed to introduce any undesirable pollen into the screening population. There are various techniques for ensuring this (Chapter 25).

Lesson 11: The number of screening generations

Initially, the maize in Africa had a very low level of horizontal resistance to tropical rust, but it accumulated enough resistance to control the disease in 10-15 generations. This indicates the probable duration of a horizontal resistance breeding program. There are two generations of maize each year in most of tropical Africa, and adequate resistance thus accumulated in 5-7 years. In temperate climates, with only one growing season each year, this period would be doubled. However, the duration of the program can be reduced by beginning with plants that have a rather higher level of horizontal resistance than the African maizes started with, and by increasing selection pressures with artificial inoculation. Conversely, more time may be required if the breeding involves resistance to several different species of parasite, as will usually be the case. In general, therefore, a horizontal resistance breeding program is likely to require about ten breeding cycles to produce worthwhile results. But it can probably continue with profit for another decade or two, producing diminishing returns, but cumulative improvements, all the time.

Lesson 12: The holistic approach

Many crop scientists like to study the mechanisms of resistance, which are many and varied, but two examples of resistance mechanisms will be sufficient for our purposes. A common resistance mechanism is called hypersensitivity, and it is a form of extreme sensitivity to the presence of a parasite. When a parasite penetrates host tissue, all the host cells surrounding it die very quickly, and the parasite dies with them. This happens mainly on a microscopic scale, and the dead tissue shows as a minute, necrotic fleck which is just visible to the naked eye. This mechanism is often, but by no means exclusively, the mechanism of vertical resistance.

Another mechanism is hairiness. Very hairy plants are resistant to a range of small insect parasites, such as aphids, white flies, and leaf hoppers. This mechanism confers horizontal resistance.

It is a feature of pedigree plant breeding that the breeder usually prefers a single, prominent resistance mechanism, and preferably one whose inheritance is controlled by a single gene. Tropical rust of maize has taught us otherwise. This lesson comes mainly from a comparison with another rust disease of maize caused by the fungus Puccinia sorghi, and known as the common rust of maize. Unlike tropical rust, the common rust is not confined to the tropics, and it occurs wherever maize is grown. It has probably been in Africa for as long as maize itself. Apart from this, the two parasites are physically so similar that it takes a specialist to recognise which is which.

The maize landraces that were so susceptible to tropical rust were, at the same time, highly resistant to common rust. Some 10-15 generations later, these maize landraces were highly resistant to tropical rust as well.

It is obvious, first of all, that the horizontal resistance to one rust is entirely different from, and independent of, the horizontal resistance to the other rust. Furthermore, there are no visible differences between the resistant and susceptible maizes. The plants look the same, and the seeds look, cook, and taste the same. And there are no obvious resistance mechanisms. It is thought that the resistance to each rust is the result of many different mechanisms and that, very probably, each mechanism is quantitatively inherited, and quantitatively variable. We have no idea what these mechanisms are. Nor do we need to know. It is entirely feasible to breed for horizontal resistance without knowing anything about the resistance mechanisms involved.

In addition to the multiplicity of mechanisms to each parasite, there was also a multiplicity of parasites. Maize has many different leaf blights, stalk, cob, and root rots, and other diseases, as well as a wealth of insects that eat, suck, or tunnel through its tissues. With the one special exception of streak virus (see below), maize landraces have high levels of resistance to all of these local parasites. It could be said that the many species of the parasites of maize are so numerous that we do not normally attempt to catalogue all of them. And, it could be said also that they normally cause so little damage, that we do not even notice them. And the resistances to all of them result from so many different mechanisms that we cannot even begin to explain how they work. Nor do we need to do so.

This is the holistic approach, operating at the highest systems level. It is the converse of "reductionism", which focuses entirely on details in the lower systems levels. To breed for one single resistance mechanism, which operates against one single species of parasite, is to operate at too low a systems level. This, it will be recalled (Chapter 10), is called sub-optimisation. In systems analysis, sub-optimisation leads to false conclusions and, in systems management, it leads to material damage to the system.

Nature knows better. In wild ecosystems, in wild pathosystems, and in genetically flexible crop pathosystems, the selection pressures operate at the highest systems levels, and there is no sub-optimisation. When we breed crops for horizontal resistance, we should do the same. We too must have the holistic approach.

Lesson 13: Parasite interference

The effects of parasite interference must be taken into account when screening plants for resistance in a horizontal resistance breeding program. This interference operates between individual plants within the screening population. The most resistant plants will normally be surrounded by plants that are less resistant. Allo-infection from the susceptible to the resistant plants will ensure that the most resistant plants will have a level of parasitism that is considerably higher than if there were no interference. Even though they are the most resistant plants in the entire screening population, they may well look awful.

When screening, therefore, it is important to select the least parasitised plants, regardless of how severely parasitised they may be, or how terrible they may appear. In other words, all measurements of resistance must be relative measurements. Only the best plants are kept, however dreadful they may look. In the early stages of the program, even these best plants are likely to look frightful. In fact, a Mendelian breeder would probably abandon the entire program, on the grounds that it was futile. But those best plants represent the first stages of a gradually changing, and very important, process of quantitative improvement.

Lesson 14: Size of the screening population

In the 1950s, the average size of a subsistence farm in Kenya was about eight acres, which is roughly the area that can be hand-cultivated by one family. The whole farm would normally be planted to the same mixture of crop species, which usually included maize, sweet potatoes, cassava, various species of peas and beans, bananas, papaya, and so on, all jumbled up together. One farm thus constituted a single maize screening population which probably contained several thousand maize plants. However, when farms were within about 100 metres of each other, there would be a significant degree of pollen exchange between farms.

This indicates the size of screening population necessary for effective recurrent mass selection for horizontal resistance. The population should be numbered in thousands rather than hundreds and, depending on the size of plant, the land available, and the number of people cooperating, may be as high as some hundreds of thousands. The exact size is not critical, but a general rule is that the larger the population, the smaller is the proportion of that population that need be selected as parents of the next generation, the greater is the selection pressure, and the more rapid is the genetic advance.

Lesson 15: The range of levels of horizontal resistance

There is a very large difference between the lowest and the highest levels of horizontal resistance to tropical rust. With a very low level of resistance in Africa, the maize crops were largely destroyed. With a very high level of resistance, the tropical rust is controlled to the point of causing no significant loss of crop. This difference is far greater than most members of the Mendelian school are prepared to credit. However, we have reason to believe that the total range of differences is even greater.

The low level of horizontal resistance to tropical rust, at the time of the first re-encounter, was considerably more than the minimum level. For a variety of reasons too complex to discuss here, negative selection pressures fade away well before the minimum level of horizontal resistance is reached. The only way to discover the minimum attainable level of horizontal resistance is to conduct an experiment in which there is powerful selection pressure for susceptibility. Obviously, the test plants would have to be protected with a fungicide, once their susceptibility had been determined, but, apart from that, this would not be a difficult experiment to conduct.

The same is true of the upper levels of horizontal resistance. The African maize populations which are no longer susceptible to tropical rust probably have considerably less than the maximum level of horizontal resistance. This is because the selection pressures for resistance faded away, once the reproductive ability of the maize was no longer affected by the rust. A fairly simple experiment would determine just how much more resistance could be accumulated before the maximum attainable level was reached.

To the best of my knowledge, these experiments have never been done, and we can only speculate as to what the results might be. But we can be confident that a level of horizontal resistance that is somewhat above the minimum level, is a very high susceptibility. The destruction of the African maize crops was not complete. Even in the worst years, the farmers got at least enough seed to sow their next crop. But in terms of practical farming, the damage to their harvest was total.

From this we can conclude with complete assurance that most modern cultivars have rather more than the minimum levels of horizontal resistance. We can think of a few cultivars that would be a total loss if they were not treated with protective chemicals. Any European potato cultivar, when grown in a Mexican blight epidemic (Chapter 18), is a case in point. But even these cultivars have more than the minimum level of horizontal resistance. This should encourage anyone planning to breed for this kind of resistance, because even the most susceptible cultivars still have enough horizontal resistance to initiate a breeding program.

Equally important, the African maizes indicate that a level of horizontal resistance that is somewhat less than the maximum will provide a complete control of a parasite, without any use of crop protection chemicals. This should encourage breeders who may believe, perhaps incorrectly, that they might reach the limits of horizontal resistance breeding without actually controlling the parasites in question.

Lesson 16: Comprehensive horizontal resistance

Subsistence farmers in tropical Africa cultivate their maize without any use of crop protection chemicals, and without any serious pest problems. This means two things. First, their maize landraces have comprehensive horizontal resistance to all the local maize parasites. We can be confident of this because, if resistance to only one species of parasite was inadequate, that parasite would cause significant damage. No parasite causes significant damage, therefore none of the many horizontal resistances is inadequate (but see maize streak virus, below).

Second, we can turn this argument the other way round, and consider any parasite, of any crop, that does cause significant damage. That damage occurs because that crop has an inadequate level of horizontal resistance to that species of parasite. In other words, we can argue that any serious parasite of any crop is serious only because there is an inadequate horizontal resistance. If we can increase that horizontal resistance sufficiently by breeding, we can control all serious crop parasites with horizontal resistance. However, only time will tell how universally this argument is valid.

Lesson 17: Selection pressures for other qualities

Apart from their resistance, the new maizes that emerged from the devastation of tropical rust were indistinguishable from their susceptible progenitors. Obviously, enough horizontal resistance had been accumulated to control the disease, without any sacrifice of yield, quality of crop product, or agronomic suitability. This indicates that, when breeding for horizontal resistance, we should use the best available cultivars as parents. That is, we should use the best available cultivars in terms of yield, quality of crop product, and agronomic suitability. These cultivars will have major susceptibilities to a number of parasites, and our task is to accumulate horizontal resistance to these parasites without sacrificing those other qualities. That means we must exert selection pressures for all of those other qualities throughout the entire duration of the breeding program. In practice, this should not be difficult as we are merely preserving existing qualities. We have to ensure that they do not become eroded in the course of our breeding for resistance. In practice, some erosion is likely to occur but it will easily be restored in the later stages of the breeding program.

Lesson 18: Seed screening

We saw in the introduction that crop losses can occur both before and after harvest. Post-harvest losses can be caused by various storage insects and rotting agents, and some cultivars are more susceptible than others to these parasites. This means that it is possible to screen the harvested product for horizontal resistance to storage parasites. With some crops, such as fresh fruit and vegetables, the prospects of such work are obviously poor. With many cereals and grain legumes, the prospects of accumulating useful levels of resistance are somewhat better. In general, however, storage pests are better controlled with environmental controls. For example, storage rots will not occur if the grain is dry. And storage pests cannot survive if the grain is stored without oxygen.

A more important aspect of seed screening concerns the laboratory screening of cereals and grain legumes, in which the harvestable product is the seed itself. Yield is measured by the total weight of all the seeds coming from one plant, but it is important that these seeds have the optimum size. For example, several hundred grains that are small and shriveled are less valuable than a few tens of grains that are large and fat. For this reason, it is necessary to both weigh and count the seeds harvested from each plant. It is then possible to calculate the "hundred seed weight" or the "thousand seed weight", depending on the species of crop. The plants that have the highest yield of the optimum seed size are the one to keep as parents for the next generation. However, there is an important caveat to this rule. In the early stages of the breeding program, all the seed is likely to be shrivelled and small, simply because even the best plants were severely parasitised. Once again, all measurements must be relative measurements.

Seed can also be screened for other qualities, such as colour, hardness, and specific gravity. The specific gravity can be measured by putting the seeds into a salt solution of known concentration, and separating the "floaters" from the "sinkers". Alternatively, a machine called a gravity separator can be used. It is clear, however, that destructive tests (e.g., cooking) can be employed only after a certain bulk of pure line seed has been accumulated.

Lesson 19: Demonstration of horizontal resistance

As we saw in Lesson 6 (above) the horizontal nature of the resistance to the maize in tropical Africa has not been conclusively demonstrated, and our evidence is circumstantial only. Breeders working with horizontal resistance, however, will want a more definite indication of the nature of the resistance.

The best proof of the horizontal nature of resistance is to demonstrate the polygenic control of its inheritance. This is done by making an experimental cross with a susceptible plant, and measuring the resistance of each individual in a progeny of about one hundred plants. When the frequency of each category of resistance is plotted on a graph (see Appendix A), there should be a bell-shaped curve, which indicates a normal distribution, and is clear evidence of a polygenic inheritance. Conversely, if there is a Mendelian ratio of resistant to susceptible individuals in the progeny, the inheritance of the resistance is controlled by a single gene, and the resistance is almost certainly vertical.

It is important to measure the resistance reasonably accurately, on a scale of 0-100. In the past, many experimenters have simplified their work by classifying each of the resistant plants into one of only a few groups. When analysed statistically, these results give an impression of a Mendelian inheritance of the resistance, controlled by only a few genes. But this impression is false. It is due to the grouping, not the genetics.

Lesson 20: Measurement of horizontal resistance

The African maizes also showed us the best measurement of horizontal resistance. If there is no significant parasitism in farmers' crops, there is enough resistance. If significant levels of parasitism occur (i.e., enough to have an economic effect on either the yield or the quality of the crop product), then the resistance breeding should continue. However, these field measurements should be made under conditions in which there is no parasite interference, and in which all biological controls (Chapter 14) are functioning fully. Both of these criteria have now applied in the subsistence maize crops of tropical Africa for some forty years.

Lesson 21: Maize streak virus

There is a virus disease of maize called "Streak" which is interesting because, at first glance, it appears to contradict some of these lessons. The maize host evolved in the Americas, but the virus has an African origin. This is consequently a new encounter disease. The virus is transmitted by insects called leaf hoppers (Cicadulina spp.). This discovery was one of the first demonstrations of an insect transmission of a virus disease, and it was made by my old friend and mentor, H.H. Storey, whom I met on my first arrival in Kenya. The virus normally kills an infected maize plant and, incredibly, the maize populations do not develop resistance to it.

The explanation lies in the leaf hoppers. These insects are gregarious, and they like to congregate in colonies. In ecological terms, they have a patchy distribution. And not all the insects are carrying the virus. In practice, only about three percent of maize plants both carry the insects, and become infected with the virus. This is a classic example of a low frequency of parasitism (Chapter 9). This is not a high enough frequency to exert selection pressure for resistance and, as a direct consequence, the maize landraces are highly susceptible to the virus. Infected plants die. This is also a classic example of a high injury from parasitism. This is in sharp contrast to the tropical rust situation in which every plant in the population is exposed to approximately equal levels of infection, and the frequency of parasitism is maximal. The lesson of this is that we must achieve a uniform distribution of parasites, and a maximum frequency of parasitism, within our screening populations. Patchy distributions lead to escapes from parasitism, and a false appearance of resistance.

My friend and colleague Ivan Buddenhagen (who developed the very useful concepts of old encounter, new encounter, and re-encounter parasites, and who has a profound knowledge of crop parasite problems) showed that the plant hoppers can easily be disturbed, and they are then likely to settle on a different maize host. By disturbing the plant hoppers every day, with two men lightly dragging a bamboo pole across the tops of the plants, he soon obtained a 100% occurrence of streak in his screening population. And he showed that, in a few generations of screening, it was possible to obtain high levels of resistance to the streak virus. However, this resistance cannot be maintained in open-pollinated maize crops. With only a three percent natural infestation of leaf hoppers, there is negative selection pressure for resistance, and the maize soon becomes susceptible again because there is a host erosion of horizontal resistance. A genetically diverse, and genetically flexible crop clearly has disadvantages, as well as advantages.

High levels of resistance to maize streak can be maintained in a hybrid maize seed production program, simply by ensuring that all the parents are infested with leaf hoppers. Any parent plants that show severe symptoms of streak are then removed.

It is perhaps worth commenting that the very high susceptibility of the African maizes to streak virus is, in fact, an adequate level of horizontal resistance. So long as only 3% of the plants are infected, and killed, by the virus, the disease is quite unimportant, because a 3% loss of plants is usually not significant, and it is usually made good by an increased growth in the surviving plants. This is a classic example of a low frequency but high injury from parasitism. The total damage is low, and a higher level of horizontal resistance is not necessary. Occasionally, a freak season can so favour the leaf-hoppers that the loss of plants can be as high as 30%. However this normally happens too infrequently to justify a resistance breeding program.

In terms of wild plant pathosystems, this very patchy distribution is a survival advantage for the parasite. By confining its parasitism to a small minority of host individuals, it exerts no selection pressure for resistance. It then has a host with a very low level of resistance. If necessary, it can even consume those few host individuals entirely, without threatening the host's ecological and evolutionary competitive ability.

Maize streak has another lesson for us. I once met a maize breeder in Africa who had recently arrived from Europe. He was breeding maize for resistance to streak virus, which he did not fully understand. In his screening population, he relied on natural infection. He then removed the 3% of plants that showed symptoms of streak, on the grounds that they were susceptible. And he kept the 97% of plants that showed no symptoms, on the grounds that they were resistant. But he made no progress because, obviously, he was not screening for resistance at all. He was keeping susceptible plants which had merely escaped the disease, and which only appeared to be resistant. When we breed plants for horizontal resistance, therefore, we must be quite sure that our selections really are parasitised. They must be truly resistant, and not just apparently resistant.

Lesson 22: Hybrid Maize

There is a very important lesson about maize breeding that was not illustrated by the maize in tropical Africa, and this is a suitable moment to discuss it. American plant breeders first tackled the problem of breeding seed-propagated crops that are open-pollinated. Self-pollinated crops, such as wheat, rice, and beans, can be genetically manipulated into "pure lines" (Chapter 7) which breed true. But cross-pollinated crops cannot be treated in this way, because the process of self-pollination, which is essential for the production of pure lines, is detrimental to them. When maize is self-pollinated, it exhibits "inbreeding depression" in which the vigour and yield are severely reduced. This phenomenon in plants was first observed in England, in 1876, by Charles Darwin, famous for his theory of evolution.

Dawin also observed the converse of inbreeding depression, which is called "hybrid vigour" or, technically, heterosis. If two strongly inbred, and severely depressed, maize lines are crossed, the progeny exhibits hybrid vigour, and it yields about twenty percent more than the best open-pollinated maize crop. Such a progeny is called a "hybrid variety" and the crop is known as "hybrid corn" or "hybrid maize".

William James Beal;, in Michigan, was the first person to attempt maize improvement by exploiting heterosis. In 1908, George Harrison Shull, at Cold Spring Harbor, New York, showed that the progeny of two inbred lines of maize would produce a uniform crop, with yields superior to any open-pollinated variety. However, it proved impossible to produce adequate quantities of seed of such hybrids for commercial purposes.

In 1918, Donald F. Jones solved this problem with his research at the illustrious Connecticut Agricultural Experiment Station, in New Haven, where vitamins were first discovered. Jones used a "double hybrid" method. He produced a cross of two single crosses, using a total of four inbred lines. His double hybrid is usually represented as (A × B) × (C × D). It produced a hybrid variety that was uniform, and which yielded twenty percent more than the best open-pollinated maize.

Jones' double hybrid method solved the problem of commercial seed supply, and it became the basis of one of the most productive advances in the entire history of agriculture in the United States. The first hybrid corn seed was sold by the Connecticut Experiment Station in 1921.

A second hybrid was developed by Henry Agard Wallace, who launched his own hybrid seed production firm, and later entered politics to become Secretary of Agriculture and then Vice President of the United States. Within fifteen years of Jones' discovery, double hybrid maize was economically important and, by 1950, virtually all the corn of the corn belt was planted to double hybrids. By 1970, most commercial maize crops throughout the industrial world were double hybrids.

The double hybrid maize had a secondary effect on plant breeding that was both profound and important. The progeny of a hybrid variety does not possess any hybrid vigour, and it reverts to the lower yields of open-pollinated maize. This means that new hybrid seed must be purchased for each new crop. But farmers are happy to do this because the additional cost of hybrid seed is such an excellent investment. This rapid loss of hybrid vigour also means that a plant breeder, who produces a new and superior hybrid variety, is protected from unlawful commercial competition. No unauthorized person can produce seed of that hybrid, because only the breeder possesses the original inbred lines that produce the double hybrid.

The production of hybrid corn seed led to a surge of private enterprise in maize breeding in the United States. Many companies, which grew wealthy on the proceeds of hybrid corn seed, re-invested much of this wealth in research designed to produce even better hybrids. This private enterprise prompted an entirely new idea called "plant breeders' rights" that is highly relevant to this book, as Part Three will reveal.

Many countries now have legislation designed to protect a new crop variety, in the same way that an author's copyright protects his writing. A registered crop variety can then earn royalties, just as a book earns royalties. And a plant breeder can hope to produce a "best seller", just as an author can hope to write a best selling book.

Plant breeders' rights are not necessary in hybrid varieties of open-pollinated crops, such as maize, cucumbers, watermelons, and onions, because the hybrid vigour is lost in the next generation. But they are very necessary in all other crops, where they are as essential to private enterprise in plant breeding, as copyrights are to private enterprise in writing, painting, sculpting, photography, and music. The same is true of patents for private enterprise in inventing.

Lesson 23: Other things we did not learn from the maize in Africa

There were two other aspects of modern population breeding that were not emphasised by a study of the African maizes. These were the technique of family selection, otherwise known as "head-to-row" screening, and the technique of late selection. The details are given in Chapter 25.

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CHAPTER TWENTY-ONE
The Loss of Resistance in Coffee

The Origins of Coffee

In spite of the fact that coffee is an old world crop, there are no early historical references to it. There is no mention of coffee in ancient Egyptian, coffee in ancient Egyptian, Sumerian, Greek, or Roman records. Nor is coffee mentioned in the Bible or the Koran. It seems that the first historical reference to coffee is an Arabian one, dating from the ninth century A.D.

The Swedish taxonomist Carolus Linnaeus (1707-1778) believed coffee to have originated in Arabia and, more specifically, in Arabia Felix (Southern Yemen). He accordingly gave it the latin name Coffea arabica. This area is the source of the world's finest coffee, known as the "Mocha" variety which, sadly, is now virtually unobtainable.

In fact, Linnaeus was mistaken. We now know that coffee originated in Africa, in the eastern, equatorial highlands. Coffea arabica was probably an accidental hybrid between two wild species and, somewhat tentatively, we can both date the time of this accident, and locate where it occurred.

Arabica coffee must have appeared at least a century before its first historical record in 850 A.D., and the earliest possible date can be determined by the spice trade of the ancient Romans. In his book The Spice Trade of the Roman Empire (1969), J.I. Miller has described how the Romans obtained cinnamon (Cinnamomum zeylanicum). At that time, this spice was being produced only in the general area of lowland, tropical S.E. Asia, and its source was a well kept trade secret.

The Romans believed cinnamon to come from Africa but, in fact, it was taken to Madagascar by ancient Austronesian people, who used to sail from Indonesia, straight across the Indian Ocean, as early as the second millennium before Christ. These people also brought the banana, rice, turmeric, and an Asian species of yam, from Asia to Madagascar. The present day inhabitants of Madagascar, the Malagasy, are descended from them. Their language is not one of the African languages, and it belongs to the Austronesian family of languages. Unlike any other people in Africa, the Malagasy have also cultivated paddy rice, in the Asian manner, since antiquity.

It seems that these Austronesian sailors relied entirely on the monsoon winds to make this 6,500 kilometre journey across open ocean, and that, for this reason, their journeys were strictly seasonal. It is probable also that they relied on the coconut to provide them with both fresh water to drink, and vitamin C to prevent scurvy. One of their items of trade was the scented bark of the cinnamon tree, and the principal market for this bark was the city of Rome.

From Madagascar, the cinnamon was taken by canoe to the east coast of Africa, to an area near the modern border of Kenya and Tanzania which, in ancient times, was called Rhapta. From there, the trade route went overland. This was possibly because the sea journey round the Horn of Africa, to the Red Sea, was too hazardous. The land caravan would also be greatly enriched in the course of its travels. By the time it reached the Mediterranean, the caravan would have gained wild animals for the Roman circus, Nubian slaves, ebony, ivory, frankincense, gold, and other rare African luxuries.

The overland route went through the area of modern Kenya to southern Ethiopia where it forked. One fork went northwest to the Blue Nile, then by river boat to Alexandria and then, by sea, to Rome. The other fork went northeast to Assab, on the Red Sea coast, where the remains of an ancient Roman port still exist, and then by sea, with a short overland journey at Suez, to the Mediterranean and Rome.

The point about this trade route is that it went right through the heart of the traditional coffee growing areas of Ethiopia, and yet the Romans never knew about coffee. It is inconceivable that the Roman spice trade, which was so sophisticated that it included Indonesian cinnamon, would have remained unaware of such a stimulating and important beverage as coffee, had it been present in Ethiopia at that time. We must conclude, therefore, that coffee was not present in Ethiopia during the period of the Roman spice caravans. The Roman spice trade collapsed with the fall of Rome, and we can accordingly date the appearance of coffee at not earlier than 450 A.D., and not later than its first historical mention in 850 A.D. For convenience we can set a tentative date of about 650 A.D.

The origins of arabica coffee can be determined from botanical data with a fair degree of confidence. There are some sixty species of wild coffee in Africa and India. These wild species are all diploids. That is, they have two sets of matching chromosomes, with one set coming from the male parent and the other from the female parent. Each set has eleven chromosomes and diploid coffees thus have twenty two chromosomes. (A chromosome is a microscopic bundle of the DNA genetic code that controls all things inherited).

Arabica coffee differs in that it is a tetraploid. That is, it has four sets of chromosomes. It is thought, but not finally confirmed, that this is a new species that arose when a rare hybrid was formed between two different wild diploids. Such a hybrid would normally be sterile, because the two sets of chromosomes would not match each other. However, a spontaneous doubling of the number of chromosomes can sometimes occur, and a sterile hybrid then becomes fully fertile, because it now has two double sets of chromosomes, and the two doubled sets match each other. It is highly probable that arabica coffee was formed in this way.

An immediate question is where did this accidental hybridization occur? One of the more notable botanical features of Ethiopia is that no wild, diploid coffees occur in that country. We can be confident of this because many botanists, myself included, have searched for them without success. A second question, related to the first, concerns the identity of the wild parents of arabica coffee.

The late and little known scientist, I.R. Doughty, is reputed to have hybridized two wild diploids, Coffea eugenioides and Coffea canephora, at the Lyamungu Research Station, on the lower slopes of Mount Kilimanjaro, in Tanzania. He did this in the late 1930s, and he obtained a sterile hybrid. However, one lateral branch underwent a spontaneous doubling of its chromosomes, and it became tetraploid and fertile. Apparently this fertile branch was indistinguishable from Coffea arabica.

Unfortunately, World war II interrupted his research and, when Doughty returned to Lyamungu after the war, the hybrid tree had disappeared. Unfortunately also, Doughty, who was in many ways a brilliant scientist, disliked writing, and he published little. Doughty died many years ago, and his experimental records are lost. I met him on several occasions but, alas, it never occurred to me to discuss his coffee work. His colleague, who remembered this work, and told me of it, has also died. This evidence is consequently hearsay evidence only, and Doughty's work on identifying the wild progenitors of arabica coffee must obviously be repeated.

A few of the wild diploid coffees are cultivated, but they all produce coffee that is inferior to arabica, and they all occur wild in Western Africa. This natural distribution would explain why these coffees also remained unknown to the Romans. One of these cultivated diploids is Coffea canephora which produces the "robusta" coffee of commerce, and was used by Doughty in his hybridization experiment.

The eastern limits of the natural distribution of this species are in Uganda or, possibly, western Kenya, but well to the west of the cinnamon trade route. Doughty's other species, Coffea eugenioides, is an East African species, of no culinary value, that also extends into Uganda. If these two species are indeed the progenitors of arabica coffee, the centre of origin must be in the area where their natural distributions coincide. That is, in the general area of modern Uganda.

The hypothesis, then, is that arabica coffee is a tetraploid species, derived by hybridization between Coffea eugenioides and Coffea canephora, in Uganda, in about 650 A.D. New tetraploids often have characteristics that are considerably different from either of their parent species. Quite frequently, they have different climatic requirements from either parent and, for this reason, they often flourish in a new area, called the centre of diversification, which may be quite distant, and considerably different, from the centre of origin. Apparently, this happened with arabica coffee. Uganda is too warm and moist for arabica coffee, which probably died out there soon after it was formed. In the meanwhile, however, it was taken to Ethiopia, which became its centre of diversification.

The relatively cool highlands of Ethiopia are separated from the more lush and humid, tropical environment of Uganda by an arid and forbidding arm of the Sahara Desert, that extends from southern Sudan to the Horn of Africa. We must presume that seed of arabica coffee was taken from Uganda to Ethiopia by travelers, possibly as a gift from one king to another. We have good reason to believe this because, it seems, a disease of the wild coffees was left behind. I shall return to this point in a moment.

Coffee obviously became popular in Ethiopia, and its cultivation spread widely. By the ninth century it had become an important item of trade with the Arabs living across the Red Sea in the Arabian peninsula. It will be remembered that the Prophet had forbidden his followers to drink alcohol, and Muslims consequently had only water, fruit juices, and milk to drink. Coffee became a very important beverage for them but, after a war had interrupted the supply of Ethiopian coffee, the Arabs decided to produce their own. They started cultivating coffee in the Yemen, in southern Arabia. As we have seen, Linnaeus believed that coffee originated in this area and, following centuries of selection and improvement by Arab farmers, these crops became famous as Mocha coffee, the finest of them all.

During the seventeenth century, coffee became popular in Europe. The first coffee house in London was established in the early part of that century, and coffee houses soon became important meeting places for social, political, literary, and business activities, in both Europe and America. Samuel Pepys mentions coffee houses frequently in his London diary (1660-1669) where they were usually known by the name of the owner. Lloyd's coffee house became famous as an insurance exchange, and Boodle's and White's became famous London clubs. In France, coffee houses became so important that they gave their name, café, to most of the languages of the world.

The World Distribution of Coffee

Arabian production was inadequate for these rapidly expanding markets of Europe, and coffee became increasingly expensive. In its turn, this stimulated production in other parts of the world. The Arabs were probably the first to take coffee seeds from Arabia to India and Sri Lanka. The Dutch took coffee seed to the island of Java, in modern Indonesia. In 1706, they took one coffee tree from Java to Amsterdam and, as a gift, sent one of its progeny to the Jardin des Plantes in Paris. The French sent seed taken from their single tree to Martinique in the West Indies. Attempts to maintain a French monopoly failed, and the crop was soon being cultivated in various parts of central and south America. Four points about this world distribution of coffee are of interest.

The first concerns the narrowing of the genetic base. Coffee is most unusual, among tree crops, in being self-pollinated. This means that all the seeds coming from one tree tend to be the same. They "breed true to type". As we saw in Chapter 1, the technical term for this is homozygous. Every time coffee was moved from one country to another, transported usually as a single tree, or as only a few seeds taken from one tree, there was an increase in homozygosity, a narrowing of the genetic base. This meant that the coffee that finally reached the New World was a pure line. It was genetically uniform, and all the trees were effectively identical.

This uniformity has considerable agricultural and commercial advantages, but it makes coffee breeding very difficult, because genetic improvement depends on crossing differing types to produce variation. Coffee breeding was impossible in the New World until other coffee lines were introduced, and this happened only to a very limited extent, and only during the present century.

The second point of interest is that, when coffee was moved from one country to another, its pests and diseases tended to be left behind. By the time coffee reached the Americas, it was virtually free of parasites. This freedom from parasites gave the New World an enormous commercial advantage over the Old World, where coffee parasites were common. Until quite recently, the control of coffee parasites was extremely difficult, because modern insecticides and fungicides did not exist. The New World advantage was thus a crucial one, and it led to a commercial domination, in which the Americas now produce about eighty percent of the world's coffee. This happened in spite of the fact that, for about 250 years, the entire coffee crop of Latin America consisted of only one pure line. This degree of monoculture, and genetic uniformity, positively invites ruinous epidemics.

This brings us to the third point, and an even less attractive aspect of this situation. All this coffee in the Americas is free from parasites, but it is also very susceptible to those parasites, should they ever reach the New World. As we have seen, this situation is called crop vulnerability, which means that the crop is susceptible to an absent, epidemiologically competent species of parasite. When the parasite arrives in the area of cultivation, the susceptibility is revealed, and the vulnerability is manifested. Potential damage then becomes actual damage.

A major coffee vulnerability in the New World was due to coffee leaf rust, caused by the fungus Hemileia vastatrix, which has already been described (Chapter 4) in the discussion on auto-infection and allo-infection. This parasite was blamed, perhaps incorrectly, for the failure of several old world coffee cultures. When it reached Brazil in 1970, it caused something of a panic in the world coffee trade. It has since spread to all the coffee producing nations of South and Central America. Fortunately, it proved to be seriously damaging only on coffee grown in hot, humid climates and, because most of the New World coffee areas are relatively cool and dry, the rust is easily controlled. But we shall return to this problem in a moment.

The fourth point of interest arising from the international movement of coffee concerns the resistance of the coffee itself to its pests and diseases. When the new hybrid of arabica coffee was first formed in Uganda, about fourteen centuries ago, it had as much resistance to coffee parasites as its wild progenitors. This natural level of resistance is a very high level, because all wild plants must have adequate levels of resistance to all their parasites. This is axiomatic, because any individual plant, or species of plant, that had poor resistance would be unable to survive ecological and evolutionary competition, and would have either accumulated enough resistance, or become extinct, long ago.

As we saw earlier, the new coffee hybrid was taken to Ethiopia in about 650 A.D., and, apparently, one of its parasites was left behind in Uganda. This was the microscopic fungus Colletotrichum coffeanum (pronounced "colley-tot-tree-coum, and koff-ee-ay-noum"), which causes a disease called coffee berry disease (see below). The new coffee hybrid was then cultivated in Ethiopia for some fourteen centuries in the absence of this fungus. Plants which grow in the absence of a parasite tend to lose resistance to it. They become highly susceptible and, possibly, highly vulnerable as well. However, all the other coffee parasites were present in Ethiopia and the coffees of the Ethiopian highlands have remained resistant to all of them.

There is one exception to this rule of resistance in Ethiopia. In eastern Ethiopia, there is a relatively dry province called Harrar. The coffee of Harrar has been grown for centuries in an area where most coffee parasites have a greatly reduced epidemiological competence, due to the dry atmosphere, and the relatively dry soils. The Harrar coffee has consequently lost resistance and, when it is cultivated in wetter environments, such as southwest Ethiopia, it is highly susceptible to many coffee parasites, including both coffee rust, and coffee berry disease.

The susceptible Harrar coffee was almost certainly the coffee taken in the thirteenth century to the Southern Yemen by the Arabs, where it was grown for several centuries in a climate that is even drier than Harrar. The coffee of this area probably lost even more resistance. This was the coffee that was taken to Indonesia and, later, to Europe, and the New World. There seems to be little doubt that the coffee of the Americas is both a narrow gene base coffee, and is a very susceptible coffee. Indeed, all the arabica coffee of the world, outside of Ethiopia, has suffered a major erosion of horizontal resistance to many of its parasites.

This is a ludicrous situation. If the Dutch had taken coffee from southwest Ethiopia to Java, instead of from Yemen, there would be no serious pest or disease problems of arabica coffee anywhere in the world, apart from coffee berry disease (see below). In other words, all the serious parasite problems of arabica coffee are due to an erosion of horizontal resistance. Three points about this erosion merit discussion.

First, this comment is not a criticism of those early, and very courageous, Dutch explorers, because there was no way they could have understood this complex situation. Equally, there was no way they could have reached southwest Ethiopia which, in those days, was a completely inaccessible part of the entirely unknown, and very dangerous area known as darkest Africa. South Yemen was close to the sea and, for all that these Dutch explorers knew, it was the only place in the world where coffee was cultivated, or even existed. As we have seen, Linnaeus believed it was the home of arabica coffee.

Second, this situation indicates just how important an erosion of horizontal resistance can be. Eighty percent of the world's coffee production is in the New World because this area is free of so many coffee parasites that were left behind in the Old World. This indicates how serious these parasites really are, because coffee is so much more difficult to produce, and it has such a competitive disadvantage, when it is cultivated in the Old World.

Third, the extent of this erosion indicates the potential of horizontal resistance in a crop such as arabica coffee. Eventually, it should be possible to breed arabica coffee with enough horizontal resistance to permit its cultivation anywhere in the cool tropics, without any crop protection chemicals, and without any loss of either yield or quality. Indeed, such coffee varieties already exist, as we shall see in a moment.

Because the coffee in the New World is so susceptible, it is clearly also vulnerable to many Old World, re-encounter parasites. This is a dangerous situation, but there is one clear advantage. There is obviously tremendous scope for breeders who are working with resistance to coffee pests and diseases, provided that they are willing to work with horizontal resistance.

Coffee Berry Disease

At the end of the last century, the British started coffee cultivation in Kenya, using the narrow gene base of susceptible coffee. After World War I, they initiated a large coffee expansion project in western Kenya, near to the Uganda border. For the first time in about thirteen centuries, arabica coffee came into physical contact with its wild progenitors in its centre of origin, and the inevitable happened. Colletotrichum coffeanum moved into the cultivated coffee, and it caused a devastating disease, now known as coffee berry disease. This disease was new to science, but it was not new to nature. As we have seen, it occurred on the wild coffees all the time, and it had been inadvertently left behind when the new hybrid was taken to Ethiopia, in about 650 A.D.

As its name implies, coffee berry disease is a disease of the green, unripe, coffee berries. Although the parasite can survive non-parasitically in the bark of the coffee tree, it can only parasitise the berries, and it does not harm any other part of the tree. The berries, of course, contain the coffee beans, and they are the harvestable product. In a very susceptible tree, all the berries are destroyed by the disease several months before harvest time. Obviously, the disease can be a very damaging one.

As we have seen also, this kind of parasite is a "re-encounter" parasite. The crop was taken by people to another part of the world, and the parasite was left behind. The crop then lost resistance to the parasite. Eventually, when this susceptible crop and the parasite re-encountered each other, the parasitism was very damaging because of the loss of resistance. Coffee berry disease is a typical example of a re-encounter parasite. And it is a very damaging disease. Indeed, the coffee expansion project in western Kenya was a complete failure, and many farmers, who were mostly World War I veterans, were financially ruined.

Coffee berry disease was first described in Kenya by J. MacDonald in 1926 and, observing that some trees were more resistant than others, he recommended the use of resistance as the best means of control. But MacDonald was not believed, mainly because coffee breeding was a long-term project. It was also thought that the resistance would be temporary, and would fail when a new strain of the parasite appeared. Even in those days, it was already beginning to be believed that all resistance to crop parasites was bound to break down sooner or later. The resistance was also quantitative and this too was considered a bad sign at that time. There was no good source of resistance, and the breeding was believed to be difficult, if not impossible. The work on resistance breeding was stopped, and the research in Kenya turned to fungicidal chemicals.

Ironically, MacDonald's best coffee selections, which have useful levels of horizontal resistance to coffee berry disease, were used successfully in other parts of Africa, where the disease had a lower epidemiological competence. And, although susceptible, most of the coffee in Kenya is now considerably more resistant than the most susceptible coffees from Harrar.

I met MacDonald, when I first went to Kenya, in 1953, and when he was an old man. Sadly, his percipience concerning resistance to coffee berry disease was recognised only long after his death.

Coffee berry disease soon started to spread inexorably through the cultivated coffees of Africa. In 1970, the disease reached Ethiopia, where coffee provided 60% of the country's exports. It was apparently taken there by people trying to improve Ethiopian coffee production with seed from Kenya. Coffee berry disease is not normally carried in coffee seed, but it seems that this batch of seed was dirty, and it contained many dried remains of diseased fruit tissues. Unfortunately, these foolish people distributed this dirty seed among many friends throughout the country, and the disease erupted all over the coffee areas of southern and western Ethiopia. The disease was soon threatening to destroy up to forty percent of the already low coffee yields.

In those days, coffee in Ethiopia was being cultivated according to centuries-old traditions. It was not planted in neat rows, to permit mechanical cultivation, nor was it manured, or pruned. The crops were a genetic mixture, with most of the trees being different from each other. And the only cultivation involved the weeding of the dense tropical vegetation, once a year, so that the pickers could reach the trees. The average yield was only 10% of the best commercial yields in neighbouring Kenya. Under these circumstances, coffee berry disease was ruinous, and there could be no question of fungicidal spraying being either a practical, or an economic, proposition.

At that point, the good people of the Food and Agriculture Organization of the United Nations (FAO) were asked to help, and they invited me to go Ethiopia to direct the research on what appeared to be an insoluble problem. In fact, they had considerable difficulty in persuading me to undertake such a difficult task. However, once in Ethiopia, my colleagues and I soon discovered that, although the coffee crops as a whole were highly susceptible to coffee berry disease, there was great variation among the individual trees. The most susceptible trees lost all their berries several months before harvest time, while the most resistant trees had lost none of their berries at the time of harvest.

As already mentioned (Chapter 20), this range of susceptibilities indicates just how great the difference can be between the minimal and maximal levels of horizontal resistance. Some of the more conservative pedigree plant breeders argue that the total range of variation of horizontal resistance is so slight, that breeding for it is a waste of time. But this argument is clearly refuted by coffee berry disease.

Approximately one coffee tree in a thousand had a very high level of resistance. By travelling all over the country, and looking at about half a million coffee trees, my team of FAO and Ethiopian scientists eventually identified 640 resistant trees.

Coffee in Ethiopia normally ripens in November. In January of 1974, my Ethiopian counterpart, Dr. Teklu Andrebahn, and I, were taking a shortcut across a coffee plantation at Agaro, near Jimma, when we found one tree that was loaded with ripe cherries. This was a serendipitous discovery as exciting as Donald Johanson's discovery* of the hominid fossil "Lucy" in the Afar Desert. Indeed, Johanson's equally serendipitous discovery was quite close, in both space and time.

This single coffee tree was obviously an abnormal type which ripened some 8-10 weeks later than usual. For this reason, the pickers had ignored it, because all the berries were unripe when they were harvesting the crop. Equally obviously, this tree was highly resistant because it was carrying a huge yield of healthy berries, in spite of the fact that it was surrounded by susceptible trees, and was growing in an area where coffee berry disease was particularly severe.

Every resistant tree that we found was numbered in chronological order of discovery, with the first two digits indicating the year of discovery. This tree thus became 741, being the first resistant tree to be identified in 1974. It was unusual in another respect also. Instead of being bright red, when ripe, the berries were yellow. Tree 741 turned out to be the best of all the resistant selections. It has now become the principle coffee variety of Ethiopia and it has been planted on many thousands of hectares.

*Johanson's discovery was made on November 30, 1974, in the Afar Desert, only afew hundred miles away.

However, we did not know this at that time and, in the meanwhile, we had many other selections to evaluate. The first harvest of newly identified, resistant trees was kept for seed, and about a thousand seedlings were produced from each resistant tree. Coffee seedlings usually take three years to produce their first berries. During this period, the parent trees were repeatedly visited, and tested for resistance, yield, and cup quality. The progenies from the worst trees were discarded while those from the best trees were retained for further development. These progenies were also tested for homozygosity, and only those that were breeding true to type (i.e., those that were already pure lines from natural self-pollination) were kept. And, when the seedlings came into fruit, their resistances to coffee berry disease, and other parasites, were tested, and the horizontal nature of those resistances was demonstrated.

I left Ethiopia, to take up other FAO work, at the end of 1974, and my assistant took charge of the project. As a result of his efforts, about a dozen, highly resistant, high yielding, and high quality, new varieties were released to farmers only eight years after the disease had appeared. This was an unprecedented achievement in tree breeding, in which it takes many decades to produce useful results, using pedigree breeding methods.

Replacing the old, susceptible coffee crops with new ones in Ethiopia was a huge task. Nevertheless, by replanting with these new varieties, the country was able to modernise its coffee production with new coffee crops. These were planted in rows to permit mechanical cultivation and to provide easy access to the trees, which were properly pruned to produce high yields. These trees are also so resistant to all the locally important pests and diseases that no chemical pesticides are necessary. The new varieties have not only solved the problem of coffee berry disease. They have led to the modernization of Ethiopian coffee production as well, and the national average yield has been greatly increased as a result. The new varieties were first issued to farmers in 1978 and an estimated 50,000 hectares of the new coffee varieties have now been planted, mostly with Cultivar 741.

These new cultivars have done something else. They have provided a clear demonstration of what horizontal resistance can achieve. They have produced a control of coffee parasites that is permanent, complete, and comprehensive. They have also shown that these high levels of horizontal resistance are not in conflict with high yields, a high quality of crop product, and good agronomic suitability.

All the coffee in other parts of the world is susceptible to many different parasites, because horizontal resistance was lost during centuries of cultivation in the dry climates of Harrar and Southern Yemen. All that susceptible coffee can eventually be replaced, in the course of normal replanting, with new cultivars that are as resistant as the new Ethiopian coffees. The widespread use of crop protection chemicals, that now occurs throughout the coffee growing areas of the world, will then cease. This change-over will doubtless require many decades to complete and, before it can even be started, a lot of tests will have to be done. But, in principle, there is no reason why all coffee crops should not eventually have maximum yields, a very high cup quality, and be entirely free of both pests and pesticides.

These new Ethiopian cultivars are likely to be extremely valuable to other countries in Africa, where coffee berry disease prevents the cultivation of coffee. This is specially true of the smallest and poorest farmers, who generally lack both the expertise, and the money, to spray their crops. Unfortunately, the very nasty military dictator, Haile Miriam Mengistu, who deposed and murdered the Emperor of Ethiopia, and killed many thousands of other Ethiopians, forbade the export of any seed of these new coffee cultivars. His government adopted the attitude that the cultivars were a "trade secret" which must not be given to their "competitors". However, during the chaos of the recent civil war, and the deposing of this tyrant, someone quietly took seeds of 741 to Kenya. From there it is bound to spread to other countries in Africa. Governments that want to maintain a crop monopoly can no more succeed than the French were able to maintain their New World coffee monopoly in the West Indies, some two and a half centuries ago.

Coffee is close to being economically synthesised by chemists in factories. The coffee crop would then be ruined, just as the linseed oil crop was destroyed by plastic paints, and various fibre crops, such as Manila hemp and sisal, were destroyed by the manufacture of nylon. The coffee producing nations should not regard each other as competitors. Their real competitors are the big food and chemical corporations, which are close to producing a synthetic coffee at an economic price. Coffee producing nations should help each other as much as they can, and keep the world price of coffee as low as they economically can, for as long as they can.

Genetic Conservation

Eventually, all the cultivated coffee of Ethiopia will be replaced with new, disease-resistant varieties. In the process, the genetic variability that exists in these old coffee crops will be lost, just as much of the variability in wheat crops has been lost (Chapter 19). This raises the issue of genetic conservation which is a major concern among some crop scientists. If genetic variability is lost, plant breeding will become more difficult. In theory, if there is no genetic variability at all, plant breeding is impossible. For this reason, it is argued that we must conserve existing variation in "gene banks" which are either carefully stored collections of seeds of annual crop species, or botanic gardens of tree crop species.

When the prospect of replacing all the old Ethiopian coffee crops first arose, genetic conservationists were concerned that the variability should not be lost. A controversy developed, and it emphasized that the issue of genetic conservation is much more complex than may appear at first sight. Several arguments suggest that genetic conservation is often an expensive and, perhaps, an unnecessary luxury.

The first and very obvious argument is that farmers cannot be expected to carry the burden of genetic conservation. If superior new varieties threaten the loss of genetic variability, no farmer should be expected to cultivate the old, inferior varieties, merely to conserve that variability.

Second, there is often some doubt whether the old varieties are worth conserving anyway. In the case of the Ethiopian coffees, the old landraces are susceptible to coffee berry disease. This material is of very doubtful value in a breeding program. Only members of the Mendelian school would argue that this material may carry valuable resistance genes which must be con-served. But single gene resistances are vertical resistances, and they are liable to fail. Resistance failures can be disastrous in a tree crop that is normally replanted, somewhat expensively, only two or three times a century. As far as we know, there are no other single gene characters in coffee that are worth preserving.

A third argument against conserving the Ethiopian coffees is that very considerable variation will remain in the semi-wild coffee that occurs in the uninhabited forests of Kaffa, which is the main coffee-producing province, located in S.W. Ethiopia. This coffee consists of the self-sown remnants of abandoned cultivation. However, this coffee population will slowly change as it responds to selection pressure from coffee berry disease, because the susceptible trees will produce so much less seed than the resistant trees. The susceptibility will gradually be reduced, and it will eventually be eliminated from the population, just as the susceptibility to tropical rust was eliminated from the maizes of tropical Africa (Chapter 20). But, as one coffee generation requires three years, and most coffee trees live for about fifty years, this process will require several centuries.

But perhaps the most important argument arises from L.R. Doughty's work, already described. Possibly the best way to produce new coffee varieties is by re-synthesising Coffea arabica from its wild, diploid progenitors. It is here that the real variability exists, and these populations of diploid wild coffees are not threatened. Furthermore, new tetraploids will be both genetically stable, and highly resistant to all coffee parasites. So, it seems, genetic conservation is not necessary, at least in arabica coffee.

Vertical Resistance in an Evergreen Perennial

We must now discuss an apparent contradiction. It was stated earlier (Chapter 7) that vertical resistance required both genetic diversity, and a discontinuous pathosystem, in order to function as a system of locking. For this reason, a gene-for-gene relationship can evolve only in an annual species, or against the leaf parasites of a deciduous tree or shrub. Coffee is an evergreen perennial, and the rust pathosystem is apparently continuous. But, in spite of this, there is vertical resistance to leaf rust.

The explanation lies in a neat biological trick which suggests that the deciduous habit in trees has as much to do with parasitism as it does with the onset of an adverse season, such as a temperate winter or a tropical dry season. Rust spores must have free water on the coffee leaf in order to infect it. This means that the rust can only infect its host during the tropical rainy season. During the tropical dry season, infection cannot occur. During the dry season also, the coffee host sheds every leaf that has any rust fungus in it. These fallen leaves die, and the rust dies with them. This makes coffee functionally deciduous with respect to rusted leaves only, and the pathosystem is discontinuous. With the start of the new rains, the tree is entirely free of rust, and it can only be allo-infected. The effectiveness of its vertical resistance is renewed each dry season.

This loss of leaf during the dry season explains why leaf rust can be such a damaging disease on cultivated coffee. We cultivate our arabica coffee as genetically uniform pure lines, in a clear example of monolock, and this intensifies the rust epidemics very considerably. Furthermore, as we saw earlier, all the arabica coffee cultivated outside Ethiopia originated in the Yemen, and it is abnormally susceptible to rust. During the dry season, in other coffee growing areas, these cultivated trees are liable to lose so many leaves that their very survival is jeopardised. They have to be regularly sprayed with a fungicide if they are to retain their leaves, and to survive, quite apart from yielding well.

Indeed, in the old days, in Kenya, coffee used to be sprayed with a copper fungicide solely for its "tonic effect". It was thought, incorrectly, that the copper had nutritional value, and that this helped the tree to retain its leaves. It now appears that the fungicide was controlling invisible rust infections that would otherwise have caused the trees to shed leaves during the dry season.

From our experience in Ethiopia, it is now quite clear that arabica coffee can easily possess enough horizontal resistance to control all its parasites. And this resistance need not conflict with either the yield or the quality of the coffee beans. Furthermore, the coffee in southwest Ethiopia has so much horizontal resistance to rust that the disease is extremely rare. And this level of horizontal resistance is possible even when there is a vertical subsystem superimposed on the horizontal subsystem.

Incredible though it may seem, coffee scientists the world over are still working with vertical resistance to coffee rust. With the notable exceptions of A.B. Eskes; in Brazil, and the FAO team in Ethiopia, they are apparently all members of the Mendelian school, and they have continued to ignore horizontal resistance to this disease. Most coffee breeding in the world is now based on a series of back-crossing programs, using the apparently immune Hibrido de Timor as a source of resistance. This source of resistance is a natural hybrid between arabica and robusta coffees, and it has both vertical resistance, and a very high level of horizontal resistance, to rust. Unfortunately, its yield, cup quality, and agronomic suitability are poor, and this is why the back-crossing is necessary. However, back-crossing both reduces horizontal resistance and separates vertical resistance genes. When the vertical resistances of these new coffee cultivars fail, there may be little horizontal resistance left.

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CHAPTER TWENTY-TWO
Sugarcane

A Very Ancient Crop

There are four reasons for thinking that sugarcane (Saccharum officinarum) is of very ancient domestication. First, the cultivated canes are very different from their wild progenitor, particularly in their domestication characteristics of juiciness and sweetness. Second, sugarcane, like modern wheat and maize, cannot survive on its own in the wild. It is dependent on cultivation by people for its survival. Third, sugarcane has lost the natural ability to propagate itself by seed. Except on research stations, it can be propagated only vegetatively, by cuttings. Lastly, there is an astonishingly wide range of varieties of cane in the centre of origin, which is in the general area of Papua New Guinea.

It is thought that an accidental hybridization occurred in northern India between a sugarcane and a wild relative called Saccharum spontaneum. This produced a new species called Saccharum barberi with thinner, harder stems. These hybrid canes were better suited to the subtropics, and to high altitudes, where the original, or noble, canes do poorly. Although they produce less sugar, they are more hardy, and more resistant to pests and diseases, than the noble canes. It was one of these hybrid canes that was taken to China in ancient times and, later, another was taken to Persia (modern Iran) and, from there, to Europe.

Many of these events can be dated from historical records. Alexander the Great saw sugarcane, and sugar, during his conquest of northern India in 326 B.C. The Greeks called sugar "honey from reeds". In the first century A.D., Dioscorides wrote of "a honey called sakkharon, collected from reeds in India and Arabia felix (modern Yemen), with the consistency of salt, and which could be crunched between the teeth". The Greeks, of course, knew only about brown, or honey-coloured, sugar. This "sakkharon" was traded in Alexandria at that time, but the sugarcane plant itself did not reach the Mediterranean until the Arabs conquered Egypt, and introduced it in 641 A.D. This was the period of the lightning Arab conquests, and they took sugarcane with them all the way across north Africa, into Spain where, five hundred years later, some 75,000 acres of sugarcane were still being cultivated.

The Portuguese took sugarcane to Madeira, the Canary Islands, the Azores, and to West Africa. On his second voyage, in 1493, Columbus took sugarcane to Hispaniola (now the Dominican Republic and Haiti) where, however, both the cane and the Spanish colonizers that he left behind, were exterminated by native Caribs. West Indian sugar was first produced in Hispaniola in 1506 and, by 1550, it had been taken to most of the tropical New World.

Re-Encounter Parasites

In the course of this transfer of sugarcane from India, across Eurasia, Africa, and the Atlantic, to the New World, two quite typical things happened. The first was that virtually all the pests and diseases of sugarcane were left behind. The sugarcane industry of the New World then had an enormous commercial advantage over the Old World because, being parasite-free, it was much more productive.

The second typical happening was an extreme narrowing of the genetic base. In the centre of origin of sugarcane, there is a bewildering variety of different sugarcane clones. But, it seems, only one clone was taken to the New World. It still exists and, in India, is called "Puri". It is also known as "Yellow Egyptian" and, in Spain, it is called "Algarobena". In the New World, it is called "Creole", or "Cana Criolla", which is Spanish for "native cane". The extraordinary thing about "Creole" was that it was the only clone of sugarcane present in the New World for more than 250 years.

"Creole" is a variety of Saccharum barberi, and it is a very tough cane, which can be grown almost anywhere in the tropics and subtropics. In this sense, the New World was fortunate in its very narrow base of genetic material of sugarcane because, largely by chance, it received some of the best genetic material available, and it received it free of parasites.

Towards the end of the eighteenth century, a noble cane (i.e., pure Saccharum officinarum) was taken from the Far East to the New World, and it was found to have a higher yield of sugar than the old Creole cane. This new cane is believed to have been collected by the French admiral Bougainville, after whom one of the Melanesian group of Pacific islands, and the ornamental plant Bougainvillea, are named. He collected this new cane when he circumnavigated the world in 1766-68.

Bougainville took this cane to the French island of Bourbon (now called Réunion), in the Indian Ocean, and the cane became known as "Bourbon" when it was taken to Martinique, Guadeloupe, and Haiti. The original name of this cane was "Otaheite" and Captain Bligh, famous because of the mutiny on his ship, the "Bounty", also carried it to the West Indies in 1793.

Because of its superior yield, Otaheite (or Bourbon) rapidly replaced Creole and, once again, the entire cane industry of the New World became dependent on a single clone. This was a dangerous situation, because more and more pests and diseases began to be spread to places where they had never been seen before, as more and more transfers of crop varieties were made around the world.

Otaheite turned out to be very susceptible to what is believed to have been a new encounter parasite. This created a grave crop vulnerability. In the French islands of the Indian Ocean, Otaheite failed suddenly in the 1840s and had to be replaced with other, inferior varieties. This was about the time of the great Irish potato famine (Chapter 18) and no one in those days knew anything about plant diseases. Crop failures were attributed to such ill-defined things as evil fumes and miasmas. For this reason, we do not know what parasite of cane destroyed "Bourbon" in the French Indian Ocean islands. In 1860, Otaheite failed in Brazil. Equally suddenly, it failed in Puerto Rico in 1872 and, one by one, in all the other West Indian islands between 1890 and 1895.

Sugarcane Breeding

In 1888, a singular discovery was made simultaneously in Barbados and in Java. The British scientists Harrison and Bovell, in Barbados, and the Dutch scientist Soltwedel, in Java, discovered that it was possible, after all, to grow sugarcane from true seeds. This meant that sugarcane breeding became a practical possibility for the first time. A wave of cane breeding followed, and this had such a dramatic effect on cane production that it has even been suggested that this was the first "green revolution".

Cane breeding stations were set up in all the major cane growing countries. A convention developed in which a new cane variety was named with the initials of its breeding station followed by figures. Thus, all "Co" varieties come from Coimbatore in India, all "B" varieties from Barbados, all "H" varieties from Hawaii, and all "POJ" from the Dutch Proefstation Oost Java. Without the slightest doubt, the most famous new cane variety of all was POJ.2878. This variety was so successful that it was eventually grown in just about every cane producing country of the world, and it became an ancestor of every modern cane variety.

In spite of the magnificent example of sugar beet breeding, every one of the new sugarcane breeding stations adopted the Mendelian breeding approach. It turns out that there is not a single Mendelian character in sugarcane that is of any agricultural significance, and the biometrical, or quantitative, breeding approach would have been more suitable.

The pedigree breeders working with sugarcane believed very firmly in the importance of pedigrees. They were convinced that the only way to obtain new cultivars was to cross a high quality, high yielding "father" with a high quality, high yielding "mother". They even spoke of good and bad "blood" in sugarcane, and they believed it was imperative to know the pedigree of a cane for as many generations back as possible. Their research records resembled the stud books and pedigrees of race-horse breeders.

The chief characteristic of this procedure in plant breeding is that the breeder keeps looking backwards, to the parents, grand parents, great grand parents, and so on. This is the precise opposite of natural evolution. In the process of evolution, the past is quite literally dead and gone forever. Evolution looks forwards, not backwards. It is the fittest of the present generation that are going to have the most offspring in the next generation. The population breeding of the biometricians imitates natural evolution in that it looks forwards to the progenies, not backwards to the parents. Population breeders are not interested in pedigrees.

However, plant breeding is a continuing process and it is not easy to switch methods in the middle of that process. Once all cane breeding stations had adopted pedigree breeding methods, they stayed that way to this day. This is not to say that pedigree breeding is useless in sugarcane. It has produced some outstanding results. But population breeding can be expected to produce even better results, and more of them, in a shorter time.

The one exception to this rule of pedigree breeding in sugarcane is in Hawaii, where the cane breeders decided to launch an entirely new breeding program, using a population breeding methodology that they called the "melting pot" technique. They took pollen from about twenty good male parents, and used it to randomly pollinate millions of flowers of some twenty good female parents. They produced enough true seed to grow three million seedlings. These were screened by eye and reduced to about 600,000 selections that had the purely visual appearances of sugarcane. These selections were screened for sucrose content, and only those with very high sucrose contents were kept for further growth and screening. With each screening, there was a drastic reduction in the number of survivors, and a corresponding increase in the complexity of the screening tests became feasible.

The best selections of one screening generation became new cultivars. They also became the parents of the next screening generation, with another three million seedlings. This approach, of course, is recurrent mass selection, and it is the basic method of population breeding. It emphasizes the transgressive segregation of continuously variable characters that are polygenically inherited, such as sucrose content, total yield of cane at the time of harvest, horizontal resistance to pests and diseases, and so on.

As a result of some decades of this kind of breeding, Hawaii now has a wealth of outstanding cane cultivars which, however, are not often useful in other parts of the world because of differing environments, and differing patterns of pests and diseases.

Apart from protecting the cut surfaces of cane "setts", or pieces of stem, used for planting a new crop, Hawaiian cane farmers do not use insecticides or fungicides, and they have no important pest or disease problems. They also have the highest sugar yields in the world, with double the yield of any other country. No doubt, these high yields are due, at least in part, to the magnificent climate of these beautiful islands. But the best climate in the world will not produce high yields unless there is magnificent plant breeding as well.

Because sugarcane is derived from a continuous pathosystem (Chapter 6), all of its resistance to pests and diseases is horizontal resistance. The vertical resistances, that have caused so much trouble in crops derived from discontinuous wild pathosystems, such as potatoes, tomatoes, wheat, rice, peas, and beans, do not occur in sugarcane.

The durability of resistance in sugarcane is well established. For example, in the early part of the present century, a new encounter virus disease, called mosaic, appeared in the sugarcane of South Africa. All the existing varieties were highly susceptible, except one called "Uba", which was of such poor quality that it was described as being more like a bamboo than a sugarcane. The South African sugar industry faced ruin. It was eventually saved by POJ.2878, and varieties bred from it, which are resistant to mosaic. This disease has never again been serious in that area.

A similar story can be told of every cane producing area when the mosaic virus first appeared. There have been occasional subsequent outbreaks of mosaic virus, but only because the disease was controlled so totally by the use of resistant varieties, that breeders sometimes forgot to test new varieties for resistance to it, and they occasionally, and inadvertently, released a susceptible variety to farmers. This error has often been wrongly interpreted as a breakdown of vertical resistance, resulting from the appearance of a new, matching strain of the parasite.

In 1936, in his presidential address to the American Phytopathological Society, G. H. Coons spoke about controlling plant diseases by breeding crops for disease resistance. In those days, it was considered essential to first find a source of resistance, to use single gene resistances, and gene-transfer breeding methods, leading, as we now know, to vertical resistance which is usually temporary in its effects. In those days also, scientists working in crops such as wheat, potatoes and beans, were already beginning to think that all disease resistance was bound to fail sooner or later, because of new strains of the parasite. Coons believed otherwise. In his presidential address, he described how the historic sugarcane industry of Louisiana was ruined during the years 1923-1927 due to three recently introduced diseases. These diseases were mosaic, red rot, and root rot, to which all the old cane varieties were highly susceptible.

When the old varieties were replaced with new, resistant ones, these diseases virtually disappeared, and the state average yield of sugar increased by fifty percent. Coons believed that this resistance was durable. And he was right. It has now endured for some sixty years, and no one seriously suggests that the resistance is going to break down to new strains of these parasites. Perhaps the pedigree breeders of wheat, rice, potatoes, peas, and beans, should take a more careful look at sugarcane. Perhaps all the vertical resistance breeders of the world should visit Hawaii, to see how the sugarcane breeders of that island did it.

It is safe to assume that all resistance to sugarcane pests and diseases is horizontal resistance. In theory, this means that a sugarcane cultivar should last forever. It should never have to be replaced because of a failed resistance. In practice, however, there are two situations in which the horizontal resistance of a cane cultivar can apparently fail, or can become inadequate.

As already mentioned, a new cane cultivar may not be tested carefully enough before being released to farmers. It might be very susceptible to, say, mosaic virus, but this susceptibility has not become apparent because of faulty or inadequate testing. Because all the cane of the area is resistant, this virus is rare. It is only later, when the new cultivar is established as a crop, that there is a flare-up of the disease, and the susceptibility of the new cultivar becomes obvious. It is then very easy, and very tempting, for the crop scientists to blame nature, and to claim that the resistance was vertical, and had broken down, rather than to admit to their own carelessness. As we have seen (Chapter 13) this apparent loss of resistance is called a false erosion of horizontal resistance.

The second apparent failure of resistance occurs when a crop vulnerability is manifested, as also happened repeatedly, with mosaic virus. In the 1970s, two additional re-encounter sugarcane diseases finally reached the Caribbean, nearly five hundred years after the crop itself had been introduced there. The first of these diseases was "smut" caused by a microscopic fungus called Ustilago scitaminea. This is a spectacular disease in which the entire shoot of the cane is transformed into a smut "whip", up to six feet long and covered in black microscopic spores which are like a very fine soot. It has been estimated that one smut whip may produce as many as one hundred trillion spores. This is r-strategy reproduction at its most extreme.

The second disease was sugarcane rust, Puccinia erianthi, which is a close relative of the rusts that have caused so much trouble on wheat (Chapter 19)and maize (Chapter 20). It produces rust red pustules on the leaves and, in a susceptible cultivar, the plant is killed because of the loss of leaf.

Throughout the Caribbean, there were sugarcane cultivars that were susceptible to one or the other of these re-encounter diseases. They were susceptible only because they had been bred and selected in the absence of the diseases. These susceptible cultivars had to be replaced with resistant ones, and then the problem was not only solved, it was permanently solved.

However, the susceptibility was occasionally a very serious one, at least for a time. Cuba, for example, is the second largest sugarcane producer in the world (after Brazil), with an annual production of up to eleven million tons of extracted sugar. When rust appeared in this island, one third of the entire cane crop was planted to a rust-susceptible cultivar and, until it could be replaced several years later, Cuba suffered crippling losses in production.

At about this time, the sugarcane scientists in Barbados were anticipating the arrival of both smut and rust, because these diseases were already present in mainland South America. Barbados has its own cane breeding station, and it has a wealth of cultivars to choose from. The Barbados scientists decided to test as many of them as possible in South America, so that they would know in advance which cultivars were susceptible. The idea was to remove any susceptible cultivars from cultivation, as part of the routine replanting process, and to do this before the diseases appeared in the island. This is quite the best way of solving problems of crop vulnerability.

Barbados sent 1,600 cane cultivars to Guyana for testing. Each cultivar had to be tested twice because there were two diseases. Guyana is a very small, and a very poor, country. It has few scientists, and those it does have are over-worked. The task that these scientists undertook was a heavy one, but they knew that their results would be of immense benefit, both to Barbados, and to the whole of the Caribbean. This was a magnificent example of international goodwill, and of the assistance that non-industrial countries can give to each other.

The results were gratifying. When smut and rust finally arrived in Barbados, all the susceptible canes had been replaced, and these diseases caused no damage whatever.

CHAPTER TWENTY-THREE
Ancient Clones

There are a number of crop plants that can be propagated by vegetative methods only, using cuttings, grafts, tubers, setts, bulbs, corms, or rhizomes. This method of propagation means that these crops exist as clones. Except for the rather rare mutation, or "sport", all the individuals within a clone are genetically identical. A clonal population is thus genetically uniform, and genetically inflexible (Chapter 8). Because propagation by true seed is impossible, usually because of a complete loss of crop quality, it follows that most of these clones have been carefully preserved and nurtured by generations of farmers since ancient times. In the study of horizontal resistance, these ancient clones are of interest in a number of ways.

The first point is that these ancient clones are highly resistant to all their old encounter parasites. Any clone that was susceptible to even one of its parasites would have been abandoned centuries ago or, possibly, millennia ago.

A second point is that all this resistance must be horizontal resistance. Had the resistance to even one parasite been vertical resistance, it would have broken down hundreds, if not thousands, of years ago, and that clone would then have been abandoned. The mere survival of these clones until the present demonstrates both the durability, and the horizontal nature, of their resistance.

A third point is that the levels of resistance were high enough to permit an economic cultivation without any use of crop protection chemicals. The first highly effective fungicide, Bordeaux mixture, is little more than a century old. And DDT, the first highly effective insecticide, is only half a century old. Any clone that was susceptible, before the discovery of these chemicals, would have been abandoned. For all practical purposes, their resistance was complete.

A fourth point is that the horizontal resistance was comprehensive in the sense that all the old encounter parasites were controlled. Even one major susceptibility would have doomed a clone to rejection and extinction.

A fifth point is that many of these ancient crops are very difficult to breed, and modern plant breeders are usually unable to improve on the work of the unknown, ancient, cultivators. In spite of this, many of these crops have hundreds, sometimes thousands, of surviving clones. The production of so many clones must have required a long period of history. This is a further indication of both the antiquity of the clones, and the durability of their resistance.

A sixth point is that many of these clones have very high yields and quality. With modern plant breeding, it has proved impossible to improve on either the yield or the quality of, say, the classic wine grapes, olives, dates, hops, bananas, or pineapples. This indicates that high levels of horizontal resistance are not incompatible with high yields, and high quality of crop product.

A last point is that some of these crops have a few pathosystems that are discontinuous, and they have evolved gene-for-gene relationships, and vertical resistances. These vertical resistances must have ceased functioning at the time of the first clonal cultivation. This indicates that high levels of horizontal resistance are possible in species that were normally protected by a system of locking, based on vertical resistance, and genetic diversity.

We should also consider the antiquity of these clones. There are four categories of evidence for antiquity.

The first category of evidence involves written records which, in the case of some Egyptian, Sumerian, Indian, and Chinese records, go back as much as five thousand years.

The second category concerns the wild progenitors of crop plants. Every cultivated species of plant was derived from one or more wild species and, usually, we can identify these wild progenitors with complete confidence. Occasionally, however, there seems to be no wild progenitor, and it has apparently disappeared. The most likely explanation is that hunter-gathering people exploited it to extinction, but that they did so only after domesticated forms had been developed. This would have happened in the very early days of farming, when farmers and hunter-gatherers were still living side by side. The domesticated forms survived because farmers are always careful to preserve propagating material of their crops. But food gatherers are often careless about wild plants and, in the course of a few human generations, they would never notice the decline in plant numbers that was occurring because of their activities. Eventually, the decline would continue to the point of extinction.

Among ancient clones, this disappearance of wild progenitors has occurred with black pepper, garlic, ginger, olive, saffron, and turmeric (see below). Among other crops, which can still be propagated by seed, a loss of wild progenitors also occurred with apple, broad bean, cassava, chillies, peanuts, soybean, sweet potato, and tea.

In many crops, the changes that domestication have made are so profound that the modern crop plant bears little resemblance to its wild progenitors. Crops such as wheat, maize, sugarcane, and tobacco, have been changed so much that their wild progenitors are difficult to identify. In the hands of primitive cultivators, who did not understand plant genetics, these changes could only have occurred slowly, over long periods of historical time. Profound alteration is another indication of the antiquity of domestication.

The next category of evidence concerns the loss of seed production, or even flower production, so that vegetative propagation becomes essential. Ancient cultivators would have known that you can increase the yield of the vegetative parts of a plant if you remove the flowers. This is because the flowers and, to an even greater extent, the seeds, constitute a physiological "sink", which takes the lion's share of nutrients away from other parts of the plant. If those cultivators came across a clone which did not form seeds or, even better, did not form flowers, they would preserve that clone very carefully. Other things being equal, such a clone would save a lot of labour. If the cultivators had many of these seedless clones to choose from, they would discard the seeded forms which would then become extinct. Among ancient clones, a loss of flower and/or seed formation occurred with banana, garlic, ginger, horseradish, pineapple, and yams.

Ancient cultivators could not always find lines that did not flower, or did not set seed. Nor did they always want to, because, in some crops, it was the seed itself that was the harvestable product. This was true of all the cereals, and the grain legumes, for example. Here, the prime consideration was to find plants that did not disperse their seeds at maturity, as is natural for all wild plants. Obviously, any plant that retained its seeds until after harvest was highly valued by farmers, and was preserved. And any plant that scattered its seeds on the ground was difficult to harvest, and it would be discarded as soon as seed-retaining plants became available. The loss of seed shedding is also a sign of an ancient domestication. So is the loss of protective husks on the seed, as has occurred with maize and wheat.

A final category of evidence concerns the diversity of ancient clones. If there is a great diversity of clones, in spite of the loss of seed production, the production of that diversity must have required a long period of historical time. This is because of the sheer difficulty of producing new clones, let alone of producing good new clones, when the crop in question does not normally produce true seed. This is particularly true of crops such as banana, sugarcane, yams, ginger, and turmeric.

For ease of reference, the following list of ancient clones is in alphabetical order, rather than in any order of importance or interest.

Aroids

Aroids are tropical root crops that are largely unknown in temperate countries, because they have never become an item of international trade. There are several genera of edible aroids, of which Colocasia, originating in the Old World, and Xanthosoma, originating in the New World, are the most commonly cultivated. Aroids have a number of vernacular names, such as taro, tannia, eddo, dasheen, and coco-yam. All the cultivated aroids are ancient clones whose ages should probably be measured in millennia rather than centuries. Some modern plant breeding has been attempted, but it has not produced any new cultivars.

The cultivation of aroids requires considerably more labour than either maize or sweet potato. For this reason, aroids do not compete well, and the production of aroids has not increased very much during the past century. Nevertheless, the world production of aroids is estimated at about four million tons annually. The only serious parasite is a new encounter disease of Colocasia in the South Pacific, caused by the fungus Phytophthora colocasiae. However, the importance of this disease declines within a few decades of its first appearance, as the more susceptible clones are eliminated, and replaced with more resistant clones.

Banana

Many people speak of the banana "tree" (Musa spp.) but this is botanically incorrect. The banana plant has no woody tissues and, for this reason, it must be described, botanically, as a herb. However, it happens to be the largest known herb, and some plants grow to a height of twenty five feet. Like garlic (see below), the cultivated clones of banana do not set seed, and they are of ancient origin. They also have excellent levels of horizontal resistance to all their old encounter parasites.

The banana originated in lowland, tropical S.E. Asia, and it was taken to Madagascar and East Africa by ancient Austronesian peoples who sailed directly across the Indian Ocean in the second millennium before Christ (Chapter 21). From Madagascar, the banana was taken to East Africa, and it gradually spread overland to West Africa, where the Portuguese were the first Europeans to encounter it.

One clone, called Pisang ambon in Malaysia and Indonesia, was taken to Martinique in the early 1800s, where it was renamed Gros Michel. This name means "Big Michael" and it probably has an obscene origin. Gros Michel is now regarded as the finest eating banana in the world, and it was cultivated in many tens of thousands of acres, for many decades, by the United Fruit Company, in the so-called "Banana Republics" of the Caribbean.

This was an unprecedented monoculture. A monoculture means that a single species of crop, often a single clone of that crop, is cultivated continuously, without rotation, and without any mixing with other crops. Monocultures provide the best conditions for really damaging epidemics. Being an herb, with soft and succulent tissues, we might expect the banana to have many parasites, and the danger of damaging epidemics in this monoculture was aggravated in several ways. The banana is a perennial, evergreen plant, and its epidemics are continuous. Being a tropical plant, growing in an environment that is continuously warm and wet, it invites population explosions of parasites, with little chance of population extinctions. And Gros Michel was cultivated for decades, producing some five million tons of fruit annually, from about 250,000 acres, as a single clone, with every plant genetically identical to every other plant. And yet, there were no epidemics. At least, there were no epidemics of old encounter parasites.

It is interesting to compare this ancient clone of banana with the modern clones of potatoes, in Europe. These modern clones cannot be cultivated at all without the use of expensive seed certified free from viruses and other tuber-borne parasites, and without routine spraying with insecticides and fungicides to control leaf parasites. There is something very seriously wrong with these potatoes.

Eventually, a serious epidemic did develop in the New World bananas, but this was the result of a new encounter parasite, a fungus called Fusarium oxysporum f.sp. cubense, which causes a wilt called Panama disease. This new encounter parasite is native to the New World, and it came from wild botanical relatives of the banana. However, other banana clones were found to be resistant to them, and bananas are still cultivated in huge areas of complete genetic uniformity, in a climate that is very conducive to disease. Subsequently, other new encounter diseases, such as moko disease, and sigatoka, have become serious. It should also be noted that it has not proved possible to develop large banana plantations in tropical Africa or S.E. Asia, probably because of old encounter parasites which become serious only under conditions of extensive monoculture.

Gros Michel is still being cultivated, under its original name of Pisang ambon, in its centre of origin, where these new encounter diseases do not occur. And hundreds of other ancient clones are being cultivated by subsistence farmers throughout the tropics, without any use of crop protection chemicals, and producing an estimated fifteen million tons of highly nutritious food each year.

Black Pepper

Black pepper (Piper nigrum) originated in India, and seed propagation is not normally feasible, both because true seedlings lack the desired agricultural and culinary qualities, and because the seeds remain viable for only a few days. The crop was taken to various parts of Southeast Asia as clones. There are not many of these clones, and they are all ancient.

Pepper was in great demand in medieval Europe for preserving meat. In those days, farmers had no means of feeding their farm animals during winter, because this was before the days of fodder crops, such as turnips and fodder beet. Consequently, farmers had to slaughter all but their breeding stock in the Fall. Unfortunately, they had no really effective method of preserving meat, because this was before the days of refrigeration. The meat would be either smoked or salted, then it would be laced liberally with garlic and black pepper to disguise its poor taste. The many varieties of traditional sausage, that are typical of various countries in Europe, date from those bad old days.

The pepper trade was a monopoly. In fact, it was a double monopoly. The Arabs controlled both the sea and the land routes from India to Europe, and the Venetian navy controlled the sea routes within the Mediterranean. In those days, pepper was so valuable that it became the main incentive for both Vasco da Gama and Christopher Columbus to find alternative routes to India. Once the Portuguese had discovered the route around Africa, they sent a naval task force to grab the pepper monopoly. The wealth of both Venice and Arabia then began to decline. Since that time, black pepper clones have been taken to all parts of the wet tropics, but they have been largely supplanted by the red peppers (Capsicum spp.) of the New World. The development of new fodder crops in Europe, to feed farm animals in winter, further reduced the demand for this spice.

The world production of black pepper is now based on a very small number of clones, and they are all ancient. Parasites are occasionally damaging in modern black pepper crops but this is usually because of inappropriate cultivation methods (i.e. the crop likes a soil that is rich in humus, with plenty of organic mulch), or because of a new encounter parasite, such as a Fusarium or Verticillium fungal wilt.

Citrus

Some citrus clones are modern, and ancient clones were often propagated by nucellar seed (Chapter 28). Nevertheless, the ancient clones of citrus were cultivated for many centuries without crop protection chemicals, and even an individual tree produced from a nucellar seed lives for many decades. Such cultivation would have been impossible if these clones had been susceptible to even one species of parasite. These days, new encounter parasites, and commercial considerations, such as freedom from blemishes, increased yields of perfect fruit, an environmental erosion of horizontal resistance, dense stands, monoculture, inappropriate stock-scion grafts (graft incompatibilities), etc., have all led to an increasing use of crop protection chemicals in this crop.

Dates

The date palm (Phoenix dactyliferd) is unusual among plants in that an individual palm is either male or female, but never both. The technical term for this is dioecious (Greek = two houses). Being dioecious means that self-pollination is impossible. The only possible pollination is cross-pollination. In its turn, this means that pure lines are also impossible. Indeed, date palms are extremely heterozygous. They do not "breed true to type" and, although seed propagation is possible, it is not practical. Date palms produced from true seed normally produce fruit that is of such poor quality that it is fit only for feeding camels. This means that the only practical method of propagation is vegetative but, unfortunately, this too is difficult because basal suckers must be used, and these are produced only by mature palms at a rate of only three or four a year. To produce a large clonal population from one palm thus requires many decades of intensive propagation. Casual propagation requires much more time.

Breeding dates is equally difficult. How, for example, do you select a male parent on the basis of fruit quality, when fruits occur only on female plants? It can be done, but it requires a lot of very patient experimental work. This sort of work is not made any easier by the long generation time in dates, which is 6-7 years from seed to flowering.

Given all these difficulties, we must recognise that the prehistoric farmers who domesticated dates did a fantastic job. Dates are cultivated in the desert belt that stretches from Morocco in the west, to Pakistan and India in the east. Throughout this area, there are hundreds of different clones. Each one of these clones has been selected, and carefully preserved, from among hundreds, probably thousands, if not tens of thousands, of useless palms that were grown from seed. While it is possible that a few clones are relatively modern, the majority are ancient. A few may even have survived from Neolithic times. In spite of the slow rate of vegetative propagation, and the even slower rate of producing new clones, there are about 100,000,000 cultivated date palms in the world.

Dates have excellent levels of horizontal resistance to all their old encounter parasites. However, in Morocco and Algeria, a new encounter disease called Bayoud, caused by the microscopic fungus Fusarium oxysporum f.sp. albedinis, is killing thousands of high quality palms every year, and is spreading inexorably across the Sahara Desert to the East. No one knows where the disease came from, but its relatively recent arrival on the west coast of Morocco suggests a New World origin. Seedling palms are mostly very resistant, but the high quality, cultivated clones are mostly very susceptible. However, a few resistant clones of reasonably good quality are known.

Figs

In Turkey, a clone of the edible fig (Ficus carica) called Sari Lop has been grown for at least two millennia. In his botanical writings, the ancient Roman author Pliny the Elder (23-79 A.D.) mentioned the clone Dottato by name, and this clone is still widely cultivated in Italy. Another clone, Verdone has been grown in the countries of the Adriatic for many centuries.

These ancient clones of figs have many species of parasite but none of them is serious. Furthermore, fig trees are deciduous, and it is thought that gene-for-gene relationships occur with some of their leaf parasites. Nevertheless, these clones have been cultivated for many centuries without any use of crop protection chemicals, and there has never been any suggestion of a resistance failure. They have horizontal resistance that is durable, complete, and comprehensive, and which is in no way compromised by an original, additional protection from vertical resistance.

Garlic

Garlic (Allium sativum) is one of the oldest cultivated plants of all, being widely recorded in ancient Chinese, Indian, Sumerian and Egyptian cultures. Garlic never sets seeds. It can be propagated vegetatively, from individual "cloves", and from inflorescence bulbils, but in no other way. No one knows when garlic lost the ability to form, true seeds, but it was probably thousands rather than hundreds of years ago. Equally, no one has been able to identify the wild progenitor of cultivated garlic with any certainty. This indicates quite clearly that all the existing clones of garlic are very ancient indeed. There are many of these cultivated clones, differing widely in their agronomic and culinary qualities. They are all ancient and, for centuries, they were cultivated without any crop protection chemicals whatever. Every clone has high levels of horizontal resistance to all its parasites, and these resistances have endured for millennia.

Modern garlic farmers often treat their crops with crop protection chemicals in order to obtain improved yields and quality. However, it is likely that many of these clones have suffered an environmental erosion of horizontal resistance (Chapter 13), by being cultivated in an environment that differs considerably from the original. There may also be some new encounter parasites involved.

Ginger

The origin of ginger (Zingiber officinale) is unknown. It has been cultivated in tropical Asia since antiquity, but no wild forms are known. This is the most important spice in Chinese cuisine, and it is a major component of Indian curries. Ginger was known to the ancient Greeks and Romans, and it has long been an important spice in Europe, where it was originally used to disguise the taste of rancid flour, hence the term gingerbread. It is now cultivated throughout the tropics. Its propagation is exclusively vegetative, and only a few clones are known. There are no serious parasites of ginger but, in modern commercial cultivation, crop protection chemicals are sometimes used to control minor pests and diseases.

Grapes

There are more than twenty five million acres of vineyards in the world, producing mostly wine, but also table grapes and raisins. All these grapes (Vitis vinifera), without exception, are clones, and the great majority of them are ancient. It is thought that there may be as many as ten thousand different clones, but a mere dozen clones are responsible for the great wines of the world. And most of them have been cultivated for centuries, if not millennia, without any use of crop protection chemicals. These clones quite obviously had horizontal resistances that were durable, complete, and comprehensive.

Then, in the nineteenth century, a new encounter parasite was introduced to Europe from North America. This was a root-infesting aphid traditionally called Phylloxera vitifoliae, but now the ivory-tower taxonomists have most irritatingly re-named it Daktulosphaira vitifoliae. European grapes were so susceptible to it, that the European wine industry was threatened with ruin. The problem was solved by introducing American species of grape to Europe. Scions of the classic wine grapes were then grafted on to rootstocks of these American grapes, which are highly resistant to the Phylloxera. It should perhaps be added that this resistance is horizontal, and that it has now endured in Europe, without any suggestion of failure, for more than a century.

Soon after the discovery of resistant rootstocks, large quantities of American vines were imported into Europe and, inevitably, other new encounter parasites were imported with them. The worst of these was the downy mildew (Plasmopora viticold) which threatened the wine industry with ruin for a second time. As we have seen (Chapter 18), this problem was solved by Millardet when he discovered Bordeaux mixture.

These grape parasites emphasise the importance of making a clear distinction between old encounter and new encounter parasites. Viticulture has been so plagued by new encounter parasites, for more than a century, that people tend to forget that, for several millennia, it was a parasite-free crop, and a pesticide-free crop as well.

Perhaps more than any other crop, the classic wine grapes indicate that there need be no conflict between high levels of horizontal resistance, and a quality of product which, at its best, is so exquisite that it is impossible to envisage improvement.

Hops

Throughout the Northern Hemisphere, only about eight cultivars of hops (Humulus lupulus) are cultivated. The figure is not clear because some cultivars are mixtures of different, but very similar, clones. Nevertheless, hops are propagated vegetatively, and the clones are mostly ancient. In Britain, the Golding hop has been cultivated for at least 250 years, but the very popular Fuggle hop, which was a new seedling discovered by chance in 1861, now occupies about 80% of the total acreage. In continental Europe, the Hallertauer type dominates southern Germany, and the Saaz type is predominant in the former Czechoslovakia. The traditional beers of these various regions differ accordingly.

The only serious parasites of hops appear to be either new encounter parasites, or to occur on new cultivars that were inadequately tested for resistance during the breeding process. Hop fields, with their expensive systems of supporting wires, are regarded as permanent installations, because the hop plant is a long lasting perennial. Nonetheless, the above ground parts of the plant have discontinuous pathosystems, because they die back to below ground level each Fall. Incredible though it may seem to us, with our modern knowledge, some hop breeding in the past has involved vertical resistance. It is almost beyond belief that anyone should want to breed a long-term perennial crop for temporary resistance. This is yet another indication of how the Mendelian school of genetics has dominated the whole of crop science.

Horseradish

When grated, and mixed with oil, vinegar, and salt, the roots of Armoracia rusticana produce a hot condiment known as horseradish sauce. This crop has to be propagated vegetatively because fertile seeds are very rare. Apparently, horseradish has a hybrid origin, and this may explain why most of its seeds are sterile. Even if a fertile seed does occur, it does not breed true to type, and it will produce a plant of dubious agricultural value. Consequently, most clones of horseradish are many centuries old, and they have been successfully cultivated without crop protection chemicals for the whole of their history.

Olives

An olive tree (Olea europaea) lives for many centuries, and it is thought that a few trees that were planted by the ancient Romans may still be alive. When olives are grown from seed, the variation is so great that it is almost impossible to obtain a tree superior to existing cultivars. This means that olives must always be propagated vegetatively, using suckers that develop at the base of the trunk. However, these suckers occur infrequently, and olive propagation is a slow process. For this reason, many of the older olive orchards contain a mixture of clones. This propagation problem has recently been solved by rooting cuttings in mist propagators.

Most olive clones are very ancient indeed, and the age of some of them should be measured in millennia rather than centuries. They have been successfully cultivated for all of this time without any use of crop protection chemicals. More recently, some growers have started to use crop protection chemicals, and have obtained yield and quality increases that are economic. However, this does not detract from the fact that all olive clones have sufficient horizontal resistance to permit an economic cultivation without pesticides, and that this has been true for the whole of their long history.

Pineapple

Like bananas, pineapples (Ananas comosus) are normally seedless, and they must be propagated vegetatively. The clones are somewhat unstable, and tend to produce mutants with some frequency. As a consequence, many clones are known. However, one clone, consisting of a number of closely similar mutants, dominates pineapple cultivation, worldwide. This is "Cayenne" which was first taken to Europe (for greenhouse cultivation) in 1820, but is believed to have originated in Venezuela many centuries earlier. Modern cultivators complain that this clone is susceptible to several parasites, particularly the mealy bug wilt, which results from the destruction of the roots by the insect Dysmicoccus brevipes. It is now thought that much of this susceptibility may result from an environmental erosion of horizontal resistance, or from a loss of biological controls. "Cayenne" dominates world production because of its excellent yield and quality, and its slight susceptibility to parasites does not prevent this domination. Most other cultivars have considerably higher levels of resistance to the few known parasites of pineapple.

Saffron

Saffron (Crocus sativa) is one of the finest spices of them all, and is the basis of French bouillabaisse, Spanish paella, English saffron buns, Jewish gilderne, Russian challah, Indian zaffrani chawal, and Persian sholezard. Saffron is also the most expensive spice of them all, because it consists of the anthers of a crocus flower, and it is the most labour-intensive of all crops to harvest.

The saffron crocus does not occur in the wild, and this is an indication of its antiquity. Like garlic, the cultivated crocus does not set seed, and it can be propagated only by corms. Multiplication of the crop is a very slow process because only two or three new corms are formed each year at the base of the old corm. It is not known how many clones exist but it is quite clear that all of them are ancient, and that they have been cultivated for millennia without any use of crop protection chemicals.

Sisal

Sisal (Agave sisalana) occurs wild in the semi-arid areas of Mexico, and other parts of Central America, where it has a natural vegetative propagation, and a very limited seed production. This crop was introduced to East Africa in 1893 and, until the world demand for its fibre collapsed with the appearance of nylon, this area was the largest commercial producer. It is thought that the entire crop of East Africa consisted of a single clone or, at most, two or three indistinguishable clones. There are no important parasites of this clone, in spite of the fact that it was grown in a wide range of environments within East Africa, where it could be expected to have suffered an environmental erosion of horizontal resistance. However, the clone, which is still cultivated on a reduced scale, may be vulnerable to reencounter parasites.

Turmeric

This tropical plant (Curcuma longa), which is a botanical relative of ginger, produces yellow underground stems that are used for dyes and spices. The spice is the basis of all curries. Wild turmeric does not occur, and the cultivated clones never form seed. Like garlic (above), the cultivated clones are of very ancient origin, and were derived by vegetative propagation from an unknown wild progenitor. There are not many of these clones, and they are usually named after their place of cultivation in India. Although several parasites of turmeric are known, none of them is serious enough to hinder cultivation, and crop protection chemicals are unnecessary.

Vanilla

This spice (Vanilla planifolia) is the only orchid that is cultivated for purposes other than providing ornamental blooms. It is a native of Mexico, and it was being cultivated by the Aztecs when the Spanish arrived in 1520. The crop is propagated vegetatively and it is thought that only a few, very ancient clones exist. There are no serious parasites of vanilla.

Yams

The botanical family of yams is so old, in evolutionary terms, that it had spread to all the main continents before they were separated by continental drift. Consequently, this is one of the very few crops that was domesticated in the Americas, Africa, and Asia, although different species were domesticated in each continent. Yams are cultivated for their starchy tubers. About twenty million tons of tubers are produced annually, with about two thirds of this coming from West Africa. The present discussion concerns the West African yam (Dioscorea rotundata).

Like the date palm (above), yams are dioecious. However, the male and female plants both produce tubers, and both are cultivated. Some of the clones never form flowers, and none of them form seed under the normal conditions of cultivation. Consequently, new clones of high yield and quality are likely to be discovered and preserved by cultivators only very infrequently. It follows that, like garlic, horseradish, and turmeric, these non-renewable clones are ancient. There are no serious parasites of yams. They have resistance which is durable, complete, and comprehensive, in spite of their tropical rain forest environment that is continuously warm and wet.

PART THREE Solutions

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CHAPTER TWENTY-FOUR
Plant Breeding Clubs

Introduction

Plant breeding for vertical resistance is both complex and difficult. It requires large and expensive research institutes staffed by highly specialised scientists. The biometrical approach to plant breeding, however, which works with horizontal resistances, is much less complex, less specialised, less expensive, and less difficult. While biometrical breeding is usually beyond the capacity of a private individual, it is well within the scope of a group of determined amateurs, who decide to organise themselves into a plant breeding club. These amateur plant breeders might be members of a growers' association, farmers, hobby gardeners, environmentalists, or any group of activists concerned about the world food problem, and the pollution caused by pesticides. Plant breeding clubs would serve several important functions.

First, an abundance of clubs would collectively expose the whole subject of crop science to public scrutiny. Because of its technical nature, and a general lack of public interest, this branch of science has been left to its own devices for far too long. It is a fundamental requirement of science that every experimental result, and every new idea, must be exposed to the widest possible public examination, doubt, criticism, and testing. Had the public at large taken more interest in crop science, it is unlikely that the Mendelian school of genetics could have dominated agricultural education, plant breeding, and the control of crop parasites, so totally, so unnecessarily, and so inappropriately, for some ninety years. Crop scientists have had these ninety years in which to examine the possibilities that are postulated in this book. With a few notable exceptions, they have not done so. It seems indisputable, therefore, that this branch of science needs some stimulation from outside, and the fresh, clean, invigorating wind of competition. Such competition is most likely to come from plant breeding clubs.

Second, plant breeding clubs appear to be the only way of defeating the commercial vested interests that favour the status quo. These vested interests positively require susceptibility to crop pests and diseases. They involve the certified seed industries, and the crop pesticide industries. These vested interests maintain an unnecessarily high cost of food, and the environmental pollution caused by pesticides. They can be vanquished only by some very effective competition. To be effective, this competition must produce a wide range of new, high-yielding, high quality cultivars with parasite resistance that is durable, complete, and comprehensive. In its turn, this requires three things. First is the formation of sturdily independent breeding clubs, made up of concerned, determined, individuals who are free to breed crops in any way they choose. These new plant breeding clubs must also be efficient. And there must be many of them.

Third, as we have seen, pre-harvest crop parasites are destroying an estimated twenty percent of all crop production, particularly food production. This loss of food is enough to feed about one billion people, and it occurs in spite of an extravagant use of crop protection chemicals. While plant breeding clubs cannot be expected to eliminate this loss entirely, they will go a long way towards reducing it. Furthermore, successful plant breeding clubs are likely to increase crop yields above their present levels, quite apart from reducing the losses due to parasites. Breeding clubs could thus be an important factor in alleviating the world food problem.

Fourth, these breeding clubs will have the general function of reducing or, in some crops, even eliminating, the use of crop protection chemicals. Without question, this appears to be the best way of reducing the environmental pollution that results from these chemicals.

Last, clubs can be fun. They can also provide a sense of achievement for activists, a source of new friends with interests similar to one's own, and a new sense of purpose for amateur breeders. These amateurs may range from commercial farmers to gardeners who just love growing plants, but who were previously involved only in their own private gardens. Plant breeding clubs have an added attraction in that they have the potential to earn large sums in plant breeders' royalties. The odds are rather better than most lotteries.

A Typical Plant Breeding Club

A plant breeding club would be formed by dedicated individuals who are concerned about the world food problem, and the environment. Most of the members would be either farmers or amateur gardeners who are prepared to undertake the actual work of breeding plants. However, a few members would be professionals, with expertise in fields such as science, farming, law, and accounting. The club would also have elected officials such as a president, secretary, and treasurer. And there would be a club constitution and club rules. Depending on the country concerned, a club make take one of various forms, such as a private club, a society, a corporation, or a foundation.

Most clubs would specialise in one species of crop which would normally be an important food crop, and one that is commonly cultivated in their locality. The objective would be to produce high yielding, high quality, new cultivars of that food crop, with high levels of durable resistance to all the locally important pests and diseases.

Aims & Objectives

Different clubs may well have different objectives. A club made up of farmers, for example, may want new cultivars simply because the farmers concerned are dissatisfied with the commercially available cultivars. A club made up of environmentalists might be primarily concerned about reducing the pollution caused by pesticides. Another club may be interested in helping the poorer, non-industrial countries. A university club might be concerned chiefly with teaching students by practical example. Some clubs may be interested mainly in gaining plant breeders' royalties. Most clubs would have a combination of these various objectives.

The ultimate aim of a breeding club should be to breed high quality, competitive cultivars that can be cultivated successfully and economically, without any significant losses from parasites, with the absolute minimum use of crop protection chemicals (other than herbicides), and without any need of certified seed. The club would achieve this by accumulating high levels of horizontal resistance to all locally important parasites, while maintaining high yields, high quality of crop product, and high levels of agronomic suitability.

The club would recognise, however, that this ultimate aim may be unattainable, and that it would, in any event, be a long-term objective. However, the components of this ultimate aim are all quantitative variables that differ in degree. In the shorter term, therefore, the club would aim to produce gradual improvements in all of these components. A new cultivar produced by the club would thus be superior to an older cultivar in most of its attributes, and the breeding process would be cumulative and progressive.

LISA

In the recent past, rapid growth in the human population has led to an emphasis on total agricultural production, with little regard to the methods of that production, or to the sustainability of the agricultural system producing it. With improving prospects of stabilising human population growth, there is now a new emphasis on LISA.

LISA is an acronym for "Low-Input Sustainable Agriculture". The low input refers to the costs of production, which should be minimal. These costs refer particularly to energy-extravagant cultivation practices, such as deep ploughing, and the expense of using susceptible cultivars which require both crop protection chemicals and seed certified free from parasites. Another major cost results from the use of artificial fertilisers which are often used to excess. The use of fertilisers is essential if we are to feed the world, but an excessive use, leading to the pollution of ground waters and rivers, is unnecessary.

The sustainable component of LISA refers to all aspects of the environment, which must not be damaged in any way. This means there must be no soil erosion, no undue depletion of soil nutrients, no damage to the soil structure, no build-up of harmful residues in the soil, no depletion of the ground water, no loss of biological controls, no damage to non-target organisms, no build-up of crop parasites, particularly soil-borne parasites, no pollution from crop protection chemicals, herbicides, or fertilisers, no atmospheric pollution from the burning of crop residues, and so on. Sustainability also refers to the cultivars themselves. The failure of vertically resistant cultivars, whose resistances have been matched, does not represent sustainability.

The foundation stone of LISA is obviously resistance to crop parasites that is comprehensive, complete, and permanent. In a word, it is the proper utilisation of horizontal resistance.

Plant Breeders' Rights

The concept behind plant breeders' rights is the concept of a patent, or copyright, otherwise known as intellectual property protection. Most of the industrial countries now have legislation controlling plant breeders' rights. The general purpose of this legislation is to promote innovation, and private plant breeding, by protecting and rewarding private initiative with copyrights and royalties.

There is a widespread fear that modern plant breeding is so complex that it can be undertaken only by large institutes, staffed with many highly qualified scientists, and costing millions each year to run. Furthermore, until plant breeders' rights were established, there was no way in which these expensive institutes could recover the costs of their plant breeding. This has meant that virtually all plant breeding during the present century has been undertaken by governments, or government-funded universities and research institutes. The only possible private plant breeding, therefore, has involved the production of hybrid seed in open-pollinated crop plants, such as maize, in which the hybrid seed can be used only once (Chapter 20). The spectacular progress of private research in producing hybrid maize seed has demonstrated the potential of private plant breeding.

Because of its expense and complexity, it is now feared that non-governmental plant breeding can be undertaken only by very large, and very wealthy, corporations, such as the big pesticide manufacturers. In its turn, this has led to a fear that the new plant breeders' rights will encourage restrictive cartels in crop varieties and farmers' seeds, rather than innovation in plant breeding. Another fear is that the poor, developing countries may be required to pay plant breeders' royalties to the rich industrial countries.

However, these fears result from pedigree breeding methods, which do indeed require large and expensive institutes, and which produce cultivars with a wide climatic adaptation.

But, if we use the biometricians' breeding methods, there is a very different picture. This quantitative plant breeding does not require large and expensive institutes, and it is well within the capability of a group of resolute amateurs who have organised themselves into a plant breeding club. Furthermore, this alternative kind of breeding uses on-site selection, and produces cultivars with local adaptation, and a limited climatic adaptation. There can then be no question of the poor countries having to pay royalties to the rich.

Depending on the country concerned, plant breeders' rights are granted by a government, with respect to a specified cultivar, to the owner of that cultivar, for a period that may vary between 15 and 22 years. These rights reward the private breeder for the initiative, expense and work expended in breeding that cultivar. They do this by prohibiting anyone else from propagating and/or selling that cultivar, unless licensed to do so, and they entitle the owner to a royalty on the sale of all propagating material. The comparison with book copyrights and royalties is a close one. Once the patent or copyright expires, the intellectual property enters the public domain.

Just as private individuals are allowed to make photocopies of copyrighted writing, or tape recordings of copyrighted music, for their own private use, and not for re-sale, so a farmer can use some of his own harvested material of a protected cultivar for propagation purposes on his own farm. But he may not sell any of it, unless licensed to do so. This is the so-called farmer's privilege. Equally, any breeder may use a protected cultivar as parent material in a breeding program. This is the equivalent of new writing being influenced by older, copyrighted writing, or of a scientific paper making reference to earlier papers. Intellectual property always has a parentage and, if it is any good, it produces a progeny as well.

The licensing authority registers all trade in protected cultivars, and controls the collection and distribution of royalties. In order to obtain plant breeders' rights in Europe, an applicant must supply a sample of the new cultivar to the appropriate authorities who will subject it to field and laboratory tests, in order to confirm that it conforms with the necessary requirements. In the U.S.A., however, plant breeders' rights are called patents and, like patents on mechanical inventions, they are based on descriptions rather than actual tests. This system is perhaps less effective than the European.

Allocation of Breeders' Royalties

Each club must reach its own decisions concerning the allocation of any royalties that it may earn. In general, royalties should serve three functions within a breeding club. The first is the financial support of the club itself, including both the existing activities and possible expansion. Second is the incentive to individual club members. The active member who actually discovered the winning cultivar should receive a significant proportion of the royalties as his or her own personal reward. Finally, the remaining monies should be shared equally among all the members.

It must be remembered that an exceptionally successful cultivar can earn royalties amounting to millions each year. A successful cultivar could also earn royalties for many years running. It must also be remembered that, the more breeding clubs there are, the poorer will be the chances of earning royalties. But let us recognise also that money is not the primary motivation in the forming of a breeding club. And if the prospects of earning royalties have been greatly reduced, this can only mean that the whole idea of breeding clubs has been immensely successful.

Basic Organisation

One of the first decisions to be made by a breeding club should be a choice between either a centralised or a decentralised organisation.

The centralised organisation would require a small club farm with enough land to carry a single screening population of about 100,000 plants, depending on the crop species being improved, as well as various field trials, greenhouse work, etc. Such an organisation could be operated by about a dozen active members who would have to devote full-time hard work at peak periods, such as sowing, inoculating, weeding, screening, and crossing. Active members would have to be willing to devote evenings, weekends, and holidays to the club activities. All the active members would be jointly responsible for these club operations, and would share the expenses, work, rewards, and satisfaction equally.

The decentralised organisation would involve perhaps one hundred active members, each with enough farm or private garden space to grow and screen one thousand or more plants in a corresponding number of small screening populations. Depending on the crop being improved, each active member may also require a small greenhouse, and various types of equipment. Each active member would then be independently responsible for the work of growing the screening population, selecting the best plant(s) from within it, and submitting selection(s) to the club jury. Each individual member who produced a winning cultivar would be entitled to a significant proportion of the rewards and satisfaction.

The choice between the two strategies will depend on a number of factors. A centralised organisation will suit a small club with only a few members who enjoy working cooperatively, and who enjoy each other's company. This organisation is also better suited to extensive crops, such as wheat, which require relatively little attention to each plant. A decentralised organisation will be preferred by a large club with members who are individualists. This organisation is better suited to intensive crops, such as potatoes, or apples, in which each plant requires considerable individual attention.

Constitution

Each club should have a constitution comparable to those of other private or professional societies. The constitution would be open to amendment and, with experience, it would gradually be improved. Eventually, it should be possible to publish examples of a model constitution that newly formed clubs can adopt in toto as their own.

Size of Club

The number of active members in a club would vary with the species of crop being improved. Some of the larger clubs, of course, may decide to work on more than one species of crop, or on several categories of cultivar within one species of crop (e.g., white, red, and black haricot beans).

As we have seen, intensive crops would require many active members because all the plants of the screening population must be handled individually. Extensive crops, on the other hand, will need only a few active members, because much of the screening population can be handled in bulk until the final selections are made. Some crops, such as beans, are approximately halfway between these two situations and a choice of organisation, and club size, is then possible.

Categories of Membership

A breeding club will normally have various categories of membership as follows:

Active members: Active members are those who undertake the actual breeding work, at their own expense, and possibly using their own facilities, such as farms, greenhouses, equipment, and gardens. The number of active members in a club will vary widely, depending mainly on the basic organisation (see above). It will also depend very largely on the species of crop, the amount of time that each member is able to devote to club activities, and the labour-saving facilities available to the club. Some clubs might have as few as half a dozen, while others might have 50-100 active members. The club as a whole should be able to screen many thousands of seedlings each summer. However, this figure is likely to vary considerably with different crops.

Club officers: Although elected, some of the club officers should preferably be professionals. Thus, a biologist, a lawyer, and an accountant could assume responsibility for scientific, legal, and financial affairs. However, the president, chairman, and secretary would normally be elected from the ranks of the unspecialised members.

Farmer members: Some clubs that are made up of amateur gardeners may choose to have a few farmer members. These would be one or two farmers able to contribute the use of field space and farm machinery far beyond the capacity of the private gardens of the many active members. Some of the more important functions of farmer members would be to multiply potential new cultivars, and to conduct field trials to make final selections among them.

Passive members: Passive members are members who lack the skill, time, or facilities to undertake actual breeding work, but who nevertheless wish to support the club with membership fees, and to earn a share of any royalties that the club may earn. Passive members would require several years of membership before being allowed to earn royalties. The primary function of passive membership is to provide the club with additional funds during the critical years before breeders' royalties are being earned. Passive members should be regarded as benefactors or, if the club succeeds in earning royalties, as sleeping partners, and providers of venture capital.

Professional members: Professional members would each have an area of special expertise which may be scientific, agricultural, horticultural, legal, financial, mechanical, or administrative. Their primary function is to ensure that the club is well run in all its professional aspects.

Research members: Research members are those who prefer to investigate specific problems rather than undertake the more routine tasks of breeding. A club might have several research members, possibly working competitively, who are given problems of special urgency or acuity to solve. Research members might be scientists themselves or, alternatively, they should have useful scientific contacts. Their investigations might involve field or laboratory experiments, library research, or the locating and consulting of specialists.

Technician members: Technician members would have uncommon technical skills that enable them to undertake various specialised tasks (e.g., laboratory work) that is beyond the normal expertise of active members. A technician member need not necessarily be professionally qualified, and any active member may learn the skills involved, with a view to becoming a technician member. Technician members would normally rate as active members, in terms of membership privileges, but would be excused the routine breeding tasks of the active members.

Qualifications for Membership

The qualifications for membership should be twofold. Members should normally be farmers or keen amateur gardeners who are prepared to contribute time, and a knowledge of plants, to the club activities. They should also be sincerely concerned about the wider implications of private enterprise in plant breeding.

All new members would have to be elected, and would have to pay the prescribed membership fees. However, these fees may well vary for the different categories of membership.

Anyone wishing to become an active member must undertake to work with an agreed number of seedlings each season and, if there is a decentralised organisation, must own adequate facilties, such as a small greenhouse, and enough land, to accommodate such work.

Anyone wishing to become a passive member is required only to be elected, and to pay the required membership fees. Anyone wishing to become a farmer member is required to contribute field space and farm machinery for operations that are beyond the private facilities of the active members. However, volunteer active members would normally assist the farmer in this cooperative work.

Anyone wishing to become a technician member must have the ability to do the relevant field, laboratory, greenhouse, or other technical work. Anyone wishing to become a professional member must be qualified to contribute professional expertise. The fields of expertise are legal, financial, administrative, mechanical (i.e., farm machinery), and scientific.

Any member may undertake research. But anyone wishing to become a research member, excused from the more routine activities of the club, should be able to offer a particular line of investigation, with some indication of expertise in that direction. A research member would naturally be expected to produce solutions to the various problems that the club might encounter.

Obligations of Membership

The obligations of membership should be clearly stated in the club constitution, and the club rules. There should be a constitutional means of expelling members who conspicuously neglect their obligations.

Membership Fees

The club should have both entry fees, and annual membership fees. These fees should be calculated to cover the club costs until such time as breeders' royalties are earned. Membership fees are unlikely to exceed the sum that most people are prepared to spend on a cherished hobby. If the club is successful in producing one or more popular new cultivars, the membership fees can be abolished, and the club can also pay its members their share of royalties.

Passive members should be required to pay membership fees that are considerably higher than those of active members. The difference should correspond roughly to the value of the work that each active member contributes each screening season. Active, farmer, professional, and research members, as well as club officers, would pay lower membership fees, commensurate with their non-financial contributions to the club. However, all members should have equal voting rights, equal ownership rights in club property, and equal rights to the general share of breeders' royalties.

Breeding Strategy

The club should be breeding for horizontal resistance that is both complete and comprehensive. This will require the biometricians' system of breeding, involving a system of mass selection designed to produce changes in the frequency of polygenes controlling continuously variable characters.

If the crop is derived from a discontinuous wild pathosystem (e.g., an annual or biennial species, or a deciduous tree or shrub), gene-for-gene relationships may occur, and the vertical resistances must be genetically eliminated, or epidemiologically inactivated, during the screening process. This will normally be done with the one-pathotype technique (Chapter 25).

If the crop is derived from a continuous wild pathosystem (e.g., an evergreen perennial), gene-for-gene relationships will not occur, and precautions against vertical resistance will not be necessary. (The only apparent exception to this rule is arabica coffee; see Chapter 21).

There will have to be on-site screening which is conducted in the area of future cultivation, during the time of year of future cultivation, and according to the farming methods of future cultivation. These future methods of cultivation may be different from the current methods (e.g., changed fertiliser or irrigation use.).

Depending on the crop, it may be necessary to have a crossing generation and/or a multiplication generation between each screening generation. Single seed descent, family selection, and late selection are recommended for many seed-propagated crops. (See Chapter 25 for more detailed descriptions of these procedures).

Hands-On Experience

It is now a cliché in the computer world that it is impossible to learn about computers from books and manuals. The only way to learn is with "hands-on" experience. The same is true of plant breeding, particularly when breeding for horizontal resistance. Undoubtedly, there will be teething troubles and difficulties, but none of them will be insuperable. Club members who are absolute beginners should charge ahead anyway. They will quickly gain confidence as hands-on experience shows them how easy the biometricians' plant breeding really is. They will also make mistakes but, at worst, these will only waste some time and money. And learning from mistakes is part of the hands-on experience. A perusal of all the techniques listed below will reveal that none of them is difficult. Every one of them can be mastered quite quickly, and with only a little practice, by any determined amateur.

Many people who are computer illiterate find that the prospect of learning to use computers is a daunting task. Those with the courage to tackle this challenge soon find that computers are easy to use, and are great savers of time and labour. Computers are also fun, and they can quickly become fascinating, and totally absorbing. They can also be very rewarding, in every sense of this word. The same is true of plant breeding.

Prepare for Disappointments

Do not expect any new cultivar to be perfect, however promising it may appear at first. Once a new cultivar is being cultivated, various defects are likely to become apparent. There may be an undue susceptibility to a very minor pest, which then becomes a nuisance. Or other characters of yield, quality of crop product, or agronomic suitability may be imperfect. Occasionally, a very promising cultivar will later prove to have a defect that is quite unacceptable commercially. What is important is that the club's new cultivars will need less protection from crop protection chemicals, and possibly no protection at all. The whole point about horizontal resistance breeding is that it is cumulative and progressive. The earliest cultivars will be little more than a step in the right direction.

It will probably prove impossible to produce the perfect cultivar, ideal in every single respect. But the combined efforts of many breeders' clubs will eventually get very close to it.

Club Property

The club might either lease, or purchase, a small farm, using membership fees to pay rent, or to pay off a bank loan. This farm may need a greenhouse large enough to handle the basic club activities, such as cross-pollination, and the maintenance of designated hosts and parasites. The farm should also have enough land for screening activities (if there is a centralised organisation), and for field trials, multiplication plots, and similar activities.

The farm will also require equipment such as farm machinery, and a simple laboratory for culturing parasites will be desirable. The club might also own other property, presumably located on the club farm, such as a meeting room, library, or cafeteria. Other club property would include various kinds of tools, including office, scientific, and farming equipment. Some clubs would require members to contribute a part of their own farms, gardens, and greenhouses to the club activities.

If the club can afford it, there might be a professional farm manager, employed either on a consultancy basis, or full-time. A retired professional may be willing to do this work, possibly in exchange for free occupancy of the farmhouse.

Ownership of Cultivars and Breeders' Rights

Any new cultivars produced by the club should be the sole property of the club. The club should also be the sole owner of the breeders' rights, and to the royalties earned from its cultivars. However, the club would be legally required to share these royalties among its members, according to the club constitution.

Complaints from Neighbours

One of the oldest of agricultural disputes is caused by the farmer who neglects his weeding, and allows weed seeds to blow on to his neighbours' land. Similar disputes can arise from breeders clubs which deliberately encourage pests and diseases which can then spread on to their neighbours' crops. These disputes can become acrimonious and they may even lead to legal battles. However, in principle, most farmer neighbours will be pleased to learn of the club's aims and objectives, and will be willing to cooperate.

The best means of avoiding this kind of dispute is for a club officer to make prior visits to the various nearby farmers and explain exactly what the club is doing, and why. The basic explanations are as follows: (i) Soil-borne parasites will not normally spread to the neighbours' land. (ii) Water-borne parasites may spread in surface drainage water, or in a stream or river that is supplying irrigation water, but this is a relatively rare occurrence, and can usually be controlled or avoided. (iii) Minor wind-borne parasites do not matter. (iv) Major wind-borne parasites are around anyway, regardless of anything the club might do and, if the farmer is using pesticide controls, these should not matter. If necessary, the club could accept responsibility for any extra expense or work required for additional pesticide controls. (v) If the farmer is using a cultivar with a vertical resistance that breaks down during the club activities, it should be explained that the designated pathotypes used by the club are all common races that have been around for some time. The club cannot be blamed for a normal failure of vertical resistance on someone else's land.

It may also be possible for the club to isolate its work to some extent. For example, the screening plots might be located in the middle of a large field or farm growing a different species of crop. In general, however, the requirements of on-site screening restrict the possibilities of isolation in both time and space.

Illegal Parasites

In most countries, working with some parasites is illegal because they are under legislative control. For example, it is illegal to work with potato wart disease (Synchytrium endobioticum) in much of Europe and North America, or with the Colorado potato beetle (Leptinotarsa decemlineata) in Britain, and the golden nematode (Globodera rostochiensis) in most of Canada. Active members should never attempt to break the law in this respect, and they must accept that their new cultivars will be susceptible, and possibly vulnerable, to these alien parasites. Should the foreign parasite ever be accidentally introduced, and become established, the breeders can take this parasite into account, and adequate resistance can probably be accumulated within a few years.

It must be clearly recognised that this limitation can occasionally restrict the geographic range of club cultivars. For example, potatoes that were bred by a club in England would have no resistance to the Colorado beetle, and they might have a reduced value in continental Europe or North America for this reason. Conversely, of course, potatoes that were bred in continental Europe, and were highly resistant to Colorado beetle, could be grown in Britain. Provided that these new cultivars were sufficiently popular to replace all the old cultivars, the crop vulnerability caused by this insect would then disappear, and the need for protective legislation would also disappear.

Newsletters

The club may care to have a newsletter for the dissemination of information among its members. A special club officer could be appointed as editor and production manager. Clubs with similar interests may also care to exchange newsletters. Most clubs are likely to own a computer, and desktop publishing, e-mail, and bulletin boards can be very useful in this respect.

Associations of Clubs

If the idea of breeding clubs becomes popular, it may be possible to form national associations of breeding clubs and, eventually, perhaps, an international association.

Professional Societies

It is often possible for private breeding clubs to obtain membership in professional societies or associations, such as national and international plant breeding, plant pathological, entomological, horticultural, agricultural, and forestry societies. Even without membership, these associations will often sell teaching supplies (e.g., photographic slides and posters of parasite symptoms, microscope slides of parasites, leaflets, books), and some offer services, such as providing lecturers, specimen identification, and specialist advice.

Specialist Advisors

Crop scientists are likely to be asked for help from breeding clubs. In giving advice, they must ensure that they are fully familiar with the new concepts and techniques of horizontal resistance breeding. Equally, club members must not allow themselves to be influenced by the preconceived ideas of a member of the Mendelian school of genetics. Neither of these problems is quite as easy as it sounds.

Possibly the most difficult change for scientists who have been trained in the tradition of the Mendelian school is to get away from the deeply ingrained ideas of vertical resistance breeding. First, forget about needing a source of resistance. Also, genetic transfers of horizontal resistance are impossible, so forget about gene-transfer techniques and back-crossing. And forget about breeding for only one resistance at a time. A cultivar must be resistant to all its locally important parasites. Remember too that horizontal resistance is quantitative, and roughly a dozen breeding cycles will be required for its full development. And, perhaps the most important of all, do not overlook the grossly misleading effects of parasite interference, biological anarchy, and population immunity.

And how can a club member decide whether a specialist is offering sound advice? Cross-examine him on the various points in the previous paragraph. If he seems ill-informed, or ill-inclined to consider these views, find another specialist.

Scientific Publication

Individual club members, or teams of club members, may make a discovery that justifies publication in a scientific journal. The rules following such publication should be the same as in a scientific institute. That is, the paper should be published under its author(s)' name(s) but the name of the club, in which the authors did their research, should be acknowledged. This is because the credit for scientific discoveries goes to the individuals who made them, but credit should also be given to the source of their research funds and facilities.

Financial Audits

The club treasurer would be responsible for keeping the club books, which should be audited by professional auditors at regular intervals.

University Breeding Clubs

Various universities and agricultural colleges may care to form breeding clubs, with the teachers having long-term control, and students being the active members. The club activities would constitute an official university course, from which the professor earned a teaching credit, and the students each earned a course credit. The course would be designed primarily for students who intended to become farmers, and it would teach them how to breed crops for horizontal resistance. The key point of university clubs is that these students would earn life membership in the club and, throughout their lives, would be entitled to receive samples of all the new cultivars produced by that club. Life membership would also ensure high rates of farmer interest, and farmer participation, in the club's activities. After two or three decades, with several thousand active farmer members, and perhaps hundreds of new cultivars, a single university club could have an enormous influence on a farming region. This, indeed, would elegantly match the original intention of the land-grant colleges of the United States.

There should be a separate university club for each species of major crop in the region. Even one successful club could bring great prestige to a university. It might also earn considerable sums in breeders' rights, which would be shared equitably between the university club and its members.

The teaching institutes concerned do not need technical advice from a book such as this, other than a general warning to shake off the way of thinking associated with the Mendelian school of genetics.

Mexico

Mexico does not have plant breeders' legislation but it does have some excellent universities. Universidad Autónoma de Chapingo inaugurated the first plant breeding club in the world, on March 17, 1995. It is a bean (Phaseolus vulgaris) breeding club and it has student members from all over the country. The students at this university are mostly farmer's sons and daughters who plan to return to their family farms when qualified. They find the very concept of a breeding club, with life membership, very appealing. Such a club provides hands-on experience for the students, who can continue to test and cultivate club cultivars, on their own farms, for the rest of their lives. This provides the club with farmer participation of the most effective kind. These clubs also provide professors with an opportunity for long-term plant breeding research, which is impossible with the normal research grants that last for only three to five years, with no guarantee of renewal. Clearly, a successful club could bring great credit to the university itself, and it could be of major benefit to the nation.

Charitable Clubs

Charitable breeding clubs would be organised with a view to helping non-industrial, tropical countries. They would function in much the same way that missionary societies operated during the nineteenth century. That is, they would collect funds in the home country, and send plant breeders to one or more of the poorer, non-industrial countries in order to assist in the production of new subsistence cultivars, and new cultivars of cash crops. This kind of activity could also be a prime activity of breeding clubs operated by universities in non-industrial countries. And it could be a distant aim of ordinary breeding clubs in industrial nations, should they ever win really big money from breeders' royalties.

There are two special reasons for this charitable activity. First, tropical crops are divided into cash crops, such as tea, coffee, cocoa, sugarcane, and rubber, and subsistence food crops, such as maize, rice, sorghum, millets, sweet potatoes, cassava, beans, and yams. Subsistence crops are grown to feed the farmer and his family and, apart from an occasional sale of surpluses, they earn no money. Traditionally, the cash crops have always earned enough money to finance their own research, usually with a small, nationally imposed export tax. But the subsistence crops earn no money at all, and have suffered from a dearth of research. These are the crops that feed the people who may constitute as much as 90% of the population of a non-industrial country. Recently, the International Research Centres have been doing research on these subsistence food crops but they have been plagued by all the problems associated with the Mendelian school and scientific monopolies (Chapter 19). These international research centres also need some competition, because science thrives on competition, and suffocates without it. Furthermore, farmers in non-industrial countries need help. Far more help, indeed, than the International Research Centres, and the farmers' own national governments, can be expected to provide.

If it so desires, a wealthy breeding club, or one that is supported by a wealthy foundation, can be charitable in another way. It can refrain from copyrighting its new cultivars, and they will then be in the public domain, available to everyone, free of royalties.

Tropical Farmer Participation Schemes

Some subsistence crops in non-industrial countries are amenable to farmer participation schemes. These schemes would have to be organised by a central breeding station in the country concerned. The setting up and operation of such a station would normally be undertaken by the government, but it could also be undertaken by a charitable breeding club, working with government permission.

For these reasons, Chapter 27 concerns tropical farmer participation schemes.

CHAPTER TWENTY-FIVE
Techniques

Notes for Readers:

About 130 different techniques are described in this section. The only reasonable way to list these techniques seemed to be in alphabetical order, even though this can be irritating at times. I have tried to avoid the obsessively strict ordering found in military parlance (e.g., "Soap, toilet, officers, commissioned, for the use of") and have felt free to index under adjectives. Equally, I wanted to avoid the more asinine type of cookbook index entry in which beans are listed under "H" (i.e., "How to cook beans"). So it is entirely possible that my listing will please no one. If a half-remembered technique cannot be found, try the main index of this book, where there is extensive cross-referencing.

Technical terms are used in these descriptions, but every one of them is explained and defined, either when used in the text or, more commonly, in the glossary which starts on page 405. Some repetition has proved inevitable and readers are asked to be patient with this.

Bees

When a club is working with a bee-pollinated species of crop, the use of beehives in, or near, the population that is to be randomly cross-pollinated, can be very effective. A club may choose to have an apiarist member, or it may invite a friendly apiarist to devote one or more hives to the club activities.

Bees will visit any flower that provides nectar, and it is the breeder's function to ensure that only desirable plants of the crop being improved are available to the bees. This can be achieved in one of two ways. One method is to have a special crossing generation, grown well away from other crops of the same species, in either time or space, and with its own beehive. However, this method is likely to waste every alternate screening season, and thereby double the duration of the breeding program.

The other method is to use the screening generation as the crossing generation also. In this case, there must be a negative screening to remove all the unselected plants, or their inflorescences, before flowering starts. This means that there may be rather few plants left for the bees to visit. Although this is mainly a problem for the bees, it may encourage them to go elsewhere. The problem can be solved by planting a surround of a different species of plant which the bees like just as well. There will then be enough bees to visit every flower in the screening population also.

With crops that are normally self-pollinated, but which can also be cross-pollinated by bees (e.g., beans), a marker gene will be necessary to identify the seeds or plants that are the result of cross-pollination. (See marker genes, below). This is one of the few instances when single-gene Mendelian characters can be really useful.

Breeding Parents

A breeding parent is a plant that has been selected as one of the best in a breeding cycle, and which is to become one of the parents of the next breeding cycle.

Bulk Breeding

A method of breeding self-pollinated plants, in which there is late rather than early selection (see below). A large sample of a variable population is self-pollinated for several generations, without any selection, to produce a mixed population that is highly representative of the original, but in which every individual has a fairly high degree of homozygosity. The screening is conducted on this relatively homozygous population. For various technical reasons, this late selection is more efficient than the early selection of the more traditional breeding. An alternative, and generally preferable, method of late selection involves single seed descent (see below).

Catalogues

The choosing of equipment can often be difficult, either because there is a plethora of options available in an industrial country, or because there is a dearth of options in a non-industrial country. Breeding clubs should obtain as many manufacturer's catalogues as possible. Catalogues are a rich source of ideas and information about labour-saving devices, and one judicious purchase can often eliminate hundreds of hours of tedious work. Equipment varies considerably in price and quality, and a specialist should be consulted before any expensive purchases are made. Novel uses for equipment, or equipment that proves to be exceptionally useful, should be recommended in the club newsletter. A regular exchange of information between clubs can be very useful in this respect.

Categories of Parasite

Parasites can be classified in a variety of ways, quite apart from their taxonomic classification. They can be grouped according to their method of dispersal. Thus soil-borne, air-borne, water-borne (i.e., with irrigation), seed-borne parasites. They can be classified according to the number of reproduction cycles they go through in each epidemic cycle, each season. Thus there are monocyclic (one cycle), oligocyclic (few-cycle), and polycyclic (many-cycle) parasites. Another classification concerns the type of damage that they cause. Thus diseases can be grouped into wilts, smuts, rusts, blights, rots, and galls, while insect pests can be grouped into stem borers, leaf miners, sucking bugs, root eaters, leaf eaters, and so on. Parasites also differ widely in the frequency of their parasitism, and the injury of their parasitism.

The techniques of culturing and inoculation differ considerably with these various categories of parasite, and the ease of screening also varies. The details are beyond the scope of this book, and specialists should be consulted before the breeding program is launched.

Cereals, Selection Procedures

Beek (see bibliography) tested four different selection procedures for cereals. These were (i) Single Plant Selection (SPS), which is here called early selection; (ii) Bulk Seed Selection (BS), which is called bulk breeding in this book; (iii) Line Selection (LS), which is called family selection here; and (iv) Natural Selection (NS) which leaves all selection to nature. The original publication should be consulted for details which are beyond the scope of this book. It will suffice that all procedures produced results, but the family selection was the most effective. This is a matter in which breeding clubs should consult experts and, possibly, undertake some research.

Clonal Multiplication

In a vegetatively propagated crop, the selected clones of each screening season are multiplied for test purposes. The main tests are designed to reveal a potential new cultivar. They include field trials on the club farm to determine resistance under field conditions, agronomic suitability, and the yield and quality of crop product that the clone produces when it is propagated vegetatively, and without crop protection chemicals. Any clone that survives the various tests is then submitted to the appropriate author