Document(s) 25 of 33

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In spite of the fact that coffee is an old world crop, there are no early historical references to it.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 It seems that the first historical reference to coffee is an Arabian one, dating from the ninth century A.D.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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