Document(s) 30 of 33

It is often possible, and highly profitable, to exploit an existing plant population that has genetic diversity. Most modern crops are unsuitable for this purpose because they have genetic uniformity, being pure lines, clones, or hybrid varieties. But there are still many plant populations which do exhibit genetic diversity. In commercial agriculture, these are mainly the fodder plants, such as grasses, and various fodder legumes, including clovers, alfalfa, etc. Many subsistence crops in the tropics are landraces, and can also be exploited in this way, while subsistence clonal crops often contain a wide variety of different clones. There are also various tropical tree crops that can be used for both positive and negative screening. Positive screening identifies the best individuals for propagation elsewhere. Negative screening protects an existing population because it identifies the most susceptible individuals with a view to taking them out, ending parasite interference, and allowing population immunity to operate.

There are various precautions to be observed when screening existing populations. The first, and most obvious, is that it is pointless to select individual plants that look good only because they have a functioning vertical resistance. If there are gene-for-gene relationships, it is best to select plants with slight parasitism, rather than plants with no parasitism. This is usually a reliable indication of the horizontal nature of the resistance.

A second point is that parasite interference will be operating in a mixed population. Consequently, any selected individual can be expected to perform rather better when grown as a uniform population well away from susceptible individuals, and free from parasite interference. The real danger of this situation is that the original population may appear to be so parasitised that any thought of selecting within it is dismissed as unrealistic.

A third and obvious precaution is that the existing population must be entirely free of crop protection chemicals.

It should perhaps be mentioned that selection within existing populations has been the standard method of crop improvement since the dawn of agriculture. And this improvement was almost invariably the work of farmers. The following examples consequently represent only the very small tip of a very large iceberg.

Cocoa

Negative screening can be used only occasionally for parasite control but, when it is feasible, it is likely to be very useful indeed. There are two requirements. First, the technique will function best with a tree crop and, second, the crop must be genetically diverse. A cocoa crop that is severely diseased with witch's broom disease (Crinipellis perniciosa) provides an example. This disease produces a proliferation of the shoot growing points, resulting in an unsightly bunch of twigs like the sweeping end of a witch's broom.

This control technique relies on a normal distribution of resistance within the genetically diverse crop. That is, there is a minority of highly resistant trees, and another minority of highly susceptible trees. The majority, or mode, are halfway between these two extremes. The more commonly used screening technique involves a positive selection by the identification and propagation of the most resistant trees. However, this requires the planting of a new crop, and the eventual destruction of the old crop. Negative selection involves the identification and destruction of the most susceptible trees. The control method works because parasite interference is eliminated, and population immunity then operates (Chapter 14).

Think of each tree as a plot in a field trial. A highly susceptible tree is surrounded by more resistant trees. Because of parasite interference, each of those surrounding trees has many times more disease than if there were no interference. The trees beyond them have less disease, but they still have more disease than they would if there were no interference. If the susceptible tree is taken out and burnt, and the witch's brooms on the surrounding trees are pruned out and also burnt, the interference will stop. On average, the surrounding trees will have a medium level of horizontal resistance. This level is probably enough to provide population immunity, and to control the disease, when there is no parasite interference.

By identifying and eradicating a minority (perhaps 1-3%) of the most susceptible trees in the crop as a whole, and pruning out all diseased branches, parasite interference is eliminated, and the disease will be permanently controlled. In practice, an experimental approach will probably be necessary. The first eradication may not achieve a complete control, and a second eradication may be required in order to remove the minority of the next most susceptible trees.

Most tree crops are propagated in genetically uniform populations, either as clones (e.g., stone and pome fruits, olives, figs, dates, grapes) or pure lines (e.g., arabica coffee). Other perennial crops (e.g., currants, hops, banana, sugarcane, pineapples, black pepper, yams) are also cultivated as clones. This is why the technique of negative screening is of limited application. However, it is likely to be useful in open-pollinated, seed-propagated, tropical tree crops, such as cocoa, cashews, mangoes, and tea. The technique may also prove useful in young plantation forests.

Coconut

There is a disease of coconuts (Cocos nucifera) in the Philippines called cadang-cadang, which means "death-death". This disease was first observed in 1926 on San Miguel Island, off the east coast of southern Luzon, near the town of Legaspi. This small island had a single, commercial coconut plantation of 200,000 coconut palms. Within twenty years, all but eighty of them had been killed by cadang-cadang. The total number of palms killed on Luzon is difficult to estimate but, by now, is probably approaching twenty million.

In 1945, A.E. Bigornia, a Philippine scientist, and a little known but very wise plant pathologist, visited San Miguel Island, and decided that the eighty remaining palms must be resistant as, indeed, they undoubtedly are. Because coconuts have a continuous pathosystem, this must be horizontal resistance. Bigornia collected nuts from the best of these palms and planted them on mainland Luzon where they now form a new, resistant landrace.

The Philippines has an excellent hybrid coconut scheme, in which tall palms are crossed with dwarf palms to produce high-yielding hybrids, in a manner similar to hybrid maize. Bigornia's new landrace is an obvious tall parent in this hybrid scheme.

Coffee

The identification of coffee trees resistant to coffee berry disease in the genetically diverse populations of Ethiopia has already been described (Chapter 21), and only a brief reiteration is necessary here. About one tree in a thousand was resistant, and rather more than five hundred resistant trees were identified. In other words, about half a million trees had to be examined in order to find the perfect tree called "741". This may sound like a lot of work but, in fact, it is only a small fraction of the work involved in a formal tree breeding program. Even more important, it produced spectacular results in only a small fraction of the time required for a formal breeding program, in a tree crop with a generation time of three years.

It is also worth reiterating that the work of Doughty (Chapter 21) in re-synthesising Coffea arabica should be repeated. Because new allotetraploids are genetically stable, they can be used immediately as new cultivars, provided that their other attributes are satisfactory. If many new allotetraploids can be produced, this approach is the one most likely to produce new cultivars with comprehensive and complete horizontal resistance in a very short time.

Pasture Species

Many species of pasture grasses and legumes are open-pollinated. In the industrial countries, most of them have already been improved and breeding clubs should do fairly careful investigations before launching an improvement program.

Landraces

Most subsistence agriculture in the tropics involves landraces of the various seed-propagated food crops. Both the yield and the quality of these crops can usually be improved selecting within those landraces. This is often a method of obtaining useful results well in advance of the more fundamental improvements that emerge from a more formal breeding program. It should perhaps be added that most tropical landraces have excellent levels of horizontal resistance to all the locally important parasites. The breeding objective must consequently be to increase the yield and, possibly, the quality and agronomic suitability, without any loss of this resistance. This is the exact converse of breeding modern industrial crops for horizontal resistance, where the objective is to increase the resistance without any loss of yield, quality, or agronomic suitability.

Rice

One of the most spectacular rice cultures in the world belongs to the Igorot people, who live in the northern mountains of Luzon, in the Philippines, where the sides of entire mountains have been terraced to make rice paddies. These stone-faced terraces are ancient, and many generations of farmers have contributed to the enormous task of their construction, during the course of some fifteen centuries. These terraces are justly famous, and most people will have seen pictures of them, soaring up the sides of high mountains, with the highest terraces often lost in the clouds.

This is an area of high rainfall, and the paddies are filled by rain, with any surplus water being allowed to drain from terrace to terrace down the mountain. The people live in villages, and each village has its own temple, and its own priests. One of the duties of the priests is to go into the rice fields each season, just before the rice is harvested. They select the best individual plants, and take them to the temple, where they are carefully preserved, because they are the seed of the next crop.

This process of selecting the best plants for seed has continued for centuries. It is, of course, an excellent method of crop improvement, and it is no less than recurrent mass selection. These rice varieties are landraces, and they are made up of many, closely similar, but nonetheless different genetic lines. Each landrace is perfectly balanced with its own agro-ecosystem. It has adequate resistance to all the locally important pests and diseases, and it has the maximum yield that is possible with this traditional farming method. Each landrace also has exactly the cooking and eating qualities that the people like most.

Now let us suppose that the plant breeders want to make changes. This can be illustrated by a story, no doubt apocryphal, that foreign scientists visited these mountain villages and advised the people that, if they applied nitrogenous fertilizer to their rice crops, they would double their yields. So some of the people broke with their ancient traditions, and used this fertilizer. The rice yields were indeed doubled but, unfortunately, the plants were so luxuriant, and their maturation was so delayed, that they were totally destroyed by a disease called blast, and the doubled yields were reduced to nothing.

Landraces that are in balance with the local agro-ecosystem will lose that balance if the farming system is changed in any important way, such as the addition of nitrogenous fertilizer. If these villagers want to change their farming system to one of high nitrogen applications, they must make the change very slowly. Each season they must apply only a little more nitrogen than in the previous season. And, before each harvest, the priests must select the best plants as seed for the next season. Provided that this process is given enough time, there will be no disruption, and no loss of agro-ecosystem balance. This illustrates how very profoundly recurrent mass selection can change plant populations.

Rice is a self-pollinating cereal and this means that it cross-pollinates only rarely. Recurrent mass selection is consequently slow in a self-pollinating crop. But, if the breeders artificially cross-pollinate the selected plants each generation, the whole process is quick, and it can be completed in a few years.

There is another story, no doubt apocryphal also, that scientists advised these mountain people that, if they grew the new miracle rices of the green revolution, and used nitrogenous fertilizer as well, they would double their yields. A few farmers tried these new varieties and, with fertilizer, the yields were indeed doubled. But, unfortunately, the cooking qualities of the new varieties were so different from the traditional rice, that no one would eat them.

The moral of this story is that subsistence farmers are wise and cautious people, who are usually less likely to make mistakes over their food supply than are visiting scientists, who are often a little brash and, perhaps, a bit too confident. And the local priests who select the next season's seed are also wise and cautious people. They may not know any science, but their ancient traditions are more reliable, and often more appropriate, than the ideas of a foreign scientist, probably trained in the Mendelian school of genetics, and in the spirit of an industrialised, mechanised agriculture.

Rimpau

To the best of our knowledge, a European farmer, called Rimpau, who lived in Schlanstedt, was the first scientifically recorded person to employ recurrent mass selection for the purposes of crop improvement. He worked with rye, which is open-pollinated, and he started in 1866. At each harvest, he would collect the best looking heads and keep them for seed and, after twenty years, his rye was famous as the "Schlanstedt Rye", with long heads and kernels that were nearly double the size of the unimproved, local, rye landraces.

There are several points of interest in Rimpau's work. First, because rye is open-pollinated, it is genetically flexible and genetically diverse. Consequently, it can respond to selection pressures during cultivation. In this regard, it is similar to the subsistence maize crops of tropical Africa (Chapter 20).

Second, Rimpau apparently made no effort to select his male parents. Had he conducted a negative screening, to eliminate all the worst plants that would later produce undesirable pollen, he would have had a more rapid genetic advance. As it was, his screening work took him twenty years.

Third, Rimpau's work was typical of plant breeding before the recognition of Mendel's laws of inheritance in 1900. Had Rimpau selected any single-gene characters, such as vertical resistances, he would not have recognised the fact. His Schlanstedt rye probably possessed a number of different vertical resistances, and it almost certainly had systems of locking against several different parasites. These systems of locking were probably not as effective as those in a well balanced, wild pathosystem, but they were undoubtedly superior to the monolock (Chapter 7) of modern agriculture.

It is also interesting that Rimpau was doing on-site selection. Had the Schlanstedt rye been grown in a markedly different agro-ecosystem, it would have performed less well.

Lastly, Rimpau's work, and his example, were quite definitely on the side of the biometricians. As a result, most modern crop scientists, having been trained in the Mendelian school, do not even know his name.

Rubber

Rubber (Hevea brasiliensis) is a tree that is native to the Amazon basin, in an area where, it is said, they have only two seasons each year. In one season, it rains every day. In the other season, it rains all day. The Amazon, which is reputed to hold one fifth of all the world's fresh water, runs roughly along the line of the equator. Rubber thus grows in an area that is constantly warm and wet.

In spite of this constant, warm, tropical humidity, the rubber tree is deciduous (Chapter 6), and each tree is leafless for about one month each year. The leaf pathosystems of rubber are thus discontinuous, and rubber has vertical resistance to a disease called South American Leaf Blight (SALE) caused by the microscopic fungus Microcyclus ulei. This disease caused one of the very few defeats of Henry Ford, of "Model T" fame.

Ford decided to produce his own rubber, and to manufacture his own tyres, for his motor cars. To this end, the Ford Motor Company established a rubber plantation in Brazil, near Boim, on the Tapajoz River, in 1928, and they called it Fordlandia. But the plantation failed because so many of the trees died of leaf blight. In 1934, Ford established a second plantation at Belterra, also on the Rio Tapajoz, but this too failed. So did smaller plantations in other parts of the Amazon basin.

There is a gene-for-gene relationship, and a system of locking, that obviously functions in the wild pathosystem of SALB. And, because every tree is apparently matched sooner or later in each leaf cycle, each tree obviously has horizontal resistance to SALB. However, both kinds of resistance evolved to function in a dense, tropical, rain forest in which rubber trees occur with a maximum population density of only eight per acre. This means that the SALB spores have great difficulty in finding a host, let alone a matching host.

The effectiveness of both kinds of resistance was lost in the rubber plantations of Fordlandia and Belterra. The trees were so close together, and the spore density was so high, that every biochemical lock was matched early in the season and, and every tree was bombarded with spores. After a few years, the most susceptible trees died from an excessive loss of leaf.

However, many trees survived in both Belterra and Fordlandia, and these now constitute a wonderful screening population for scientists who are looking for both high yields, and high levels of horizontal resistance to SALB, as well as resistance to other pests and diseases.

Tea

The tea crop is a vast hybrid swarm between the two species Thea sinensis and Thea assamensis. The variation in this hybrid swarm is so great that it is believed that no two tea bushes, grown from true seed, are identical. This means that tea crops grown from true seed are very variable, and that about 60% of the yield comes from perhaps 30% of the bushes. Furthermore, the plucked leaf has to be fermented in order to make black tea, and the fermentation times vary considerably between different tea bushes. With variable tea, it is inevitable that some of the leaf is over-fermented, and some is under-fermented. This uneven fermentation reduces the cup quality of the tea.

Tea is consequently a crop that cries out for the vegetative propagation of selected clones. However, vegetative propagation from cuttings became possible only with the relatively recent development of mist propagators (Chapter 25).

Tea clones are produced by selecting promising bushes within a variable tea crop grown from true seed. As with all plant selection work, the easiest tests are conducted first, when there are many plants to test, and the most difficult, laborious, and expensive tests are conducted last, when there are only a few remaining plants to test. It is estimated that about one million seedling tea bushes must be screened in order to obtain one really good clone.

The first test is a simple visual assessment, made by skilled people walking through the crop, and about one bush in a thousand is marked as being promising. More detailed, and more difficult, subsequent tests are for yield, cup quality, rooting ability, and resistance to pests and diseases.

For example, a major disease of tea in S.E. Asia is blister blight, caused by the microscopic fungus Exobasidium vexans. This disease is normally controlled by spraying with a fungicide, and some ingenious disease forecasting schemes have been worked out to let the tea growers know when to spray. It seems that no one has attempted to breed resistant tea, for the simple reason that no genetic source of resistance could be found. However, tea is derived from a continuous wild pathosystem (Chapter 6), and no gene-for-gene relationships occur. But there is wide variation in the susceptibility to blister blight, and the identification of resistant trees for the production of new clones would appear to be a logical development. However, such screening would have to be conducted in unsprayed crops, and the misleading effects of parasite interference (Chapter 14) would have to be taken into account.

After many years of standard breeding work, designed to produce improved seedlings by the crossing of selected clones, the best seedling progenies were found to produce fifty percent more than unimproved seedling tea. But the best clones yield twice as much again, and their cup quality is greatly superior. This point is well illustrated by tea growing in East Africa.

Tea growing was started in Kenya in the 1920s. In those days, it was believed that tea could be grown only on a plantation scale, and the local small farmers were accordingly forbidden by law to grow tea. It was argued that their product would inevitably be inferior, and that this would damage the good name of Kenya tea. The big plantations, of course, were owned by British companies, and the fact that tea grown by native Kenyans would constitute competition was never mentioned. It was also argued that each plantation must have its own tea factory and that, for this reason alone, African small-holders could not grow tea.

In the 1960s, the Tea Research Institute in Kenya produced a remarkable new tea clone called "6/8". Clone 6/8 was not much use to the big companies because all their land was already planted with seedling tea. Once planted, a tea crop is good for a hundred years or more, and replanting is very expensive. But this was the time of Kenya's independence, and the law forbidding native farmers from growing tea was repealed. African small-holders were then positively encouraged to grow tea, and many cooperatively owned tea factories were built to process their crops.

These small-holder tea crops consist mainly of clone 6/8, and today it is the small-holders who are producing the best yields of the best quality tea. Tea produced from clone 6/8 regularly wins top prices in the London market, and it is in great demand for blending with inferior teas. Today, it is the big commercial plantations in Kenya, with their seedling tea, that are producing the inferior product, which is damaging the good name of Kenya tea. And it is the small-holders, the "peasants" who are producing some of the best tea in the world. This must surely rate as poetic justice of a rare and transparent quality.

Document(s) 30 of 33