Some Important Diseases of Coffee

Frederick L. Wellman.

It has been commonly said that coffee (Coffea arabica) is a tree practically free from disease. Actually, the coffee plant is subject to more than 40 diseases ailments due to lack of minor elements, virus troubles, mild bacterial infections of roots and fruits, and attacks by fungi and parasitic flowering plants. A century of effort has been expended on agronomic and horticultural problems in coffee, but only in the past 50 years has intensive work been done on its diseases.

In 1952 the Office of Foreign Agricultural Relations (now the Foreign Agricultural Service) of the Department of Agriculture sent a mission to study coffee diseases in all parts of the world. The mission was sponsored by the Point IV program and financed by the Institute of Inter-American Affairs. Information gathered on the trip has been incorporated into this chapter, which is based primarily on study and experience in Latin America.

COFFEE RUST, also called the oriental leaf disease, is by all odds the most serious disease of coffee. It does not occur in the Western Hemisphere, maybe just by pure luck. There are two species of rust: The classic Hemileia vastatrix, which is so destructive and is found in most of the coffee regions of Africa, the Near East, India, Asia, and the Pacific Islands, and H. coffeicola, an equally dangerous rust but still confined to the Cameroons of West Africa and the nearby island of Sao Tome. This discussion deals with the first species.

Ceylon, once one of the world's greatest coffee-producing countries, has had to become an exporter of tea when its coffee crops failed because of rust. The disease was first discovered in Ceylon in 1867. Between 1879 and 1893, after it had become well established, exports of coffee dropped to less than 7 percent of former shipments. In the Philippines in 1881 rust cut the harvest 35 percent. In Java, too, rust practically wiped out the coffee plantations, and the planters turned to hevea rubber as a substitute crop.

The first symptoms are small, yellowish, translucent, oily spots on the leaves. They expand into rather large, round spots and early show a powdery coating of spores on the under surface. As the spores mature, the spots gradually turn bright orange to red. With age the lesions become brown and surrounded with a yellow rusted band. Defoliation occurs to such an extent that many trees retain only two or three pairs of leaves on their branches where they might ordinarily have 15 or 20. Such affected trees are stunted, cannot produce, and usually die after a few years.

The disease was not present in 1953 in the Western Hemisphere. It was brought to Puerto Rico in 1903 but prompt and drastic action by O. W. Barrett, the horticulturist there who also dealt with pathology, destroyed all diseased material. He saved untold wealth for the coffee industry in the American Tropics and for centers of world trade.

In some eastern countries, notably the French Cameroon, Kenya, and Tanganyika in Africa, and in the states of Mysore and Coorg and in the Nilgiri Hills of southern India, growers control the rust by two or three annual spray applications of bordeaux mixture. The sprays are put on just before the heavy monsoon rains begin and again in the short dry spell that intervenes before the light monsoon rains fall. Sometimes a third spray is used after the light rains end. In countries that have extremely dry seasons, few rust spores are produced in these periods. Likewise little new leaf surface is developed by the coffee trees. When rains begin, both the parasite and the coffee take on new life, but by well-timed sprays the planters plan to keep about 70 percent of their coffee foliage free from rust infections.

Where the seasons are not so well defined, such as in Ceylon, Java, Malaya, and the Philippines, weekly or monthly sprayings are needed to control rust. They increase tremendously the cost of growing coffee. Some plantations in some of those countries have been moved to high altitudes, where cool temperatures reduce the inherent producing capacity of the coffee tree but do permit it to grow with less trouble from the rust.

Spores of the rust are long-lived, withstand drying and other vicissitudes, and may be easily transported on live plants or as invisible dust from one country to another. Quarantines have been instituted, and many research workers in the western Tropics have repeated warnings of the danger. All varieties of coffee grown commercially in the Americas are highly susceptible to Hemileia. Increasing transportation between East and West by air as well as by sea multiply the hazards of reintroducing the disease and establishing it this time on western shores.

For more than a half century it has been known that highly tolerant and rust-resistant types of coffee exist. Those better strains, one after another, have been brought to afflicted areas in the Orient. There, at first, they were grown with success, only to succumb later to the rust. Beginning some 30 years ago intensive study was devoted to the phenomenon in India. It was learned that the rust, like others, had biologic races, which attacked new coffee varieties that had been selected for their resistance to the old populations of rust. Moreover, the races were probably the result of mutations, as no one has ever been able to discover an alternate host relationship, which ordinarily is fruitful in producing new races of rust.

Coffee breeders have been able to secure trees with enough resistance in them to be in time the basis for developing improved and acceptable coffee types that will grow well in the presence of rust. Through the work of the mission that studied the rust in the Eastern Hemisphere, seeds from all the rust-resistant coffees, and many more, have been obtained for growing in the Americas. After careful disinfection and other prophylactic measures to assure freedom from all diseases and insects, these new coffees are now growing in the Western Hemisphere. They are insurance against the time that the rust comes to this part of the world if it does.

THE AMERICAN LEAF SPOT is recognized as the most serious disease of coffee in the Occident. It was discovered and studied by N. Saenz in Colombia in 1876. He sent herbarium species from Colombia and Costa Rica to Europe for identification. The causal fungus, Mycena citricolor (Omphalia flavida), is an inhabitant of wet mountains and woodlands and has a phenomenally wide range of wild hosts, from which it has spread to coffee. Its attack has been particularly severe because coffee culture has been concentrated in cool, moist mountain regions. It may cause losses of 75 percent or more of the crop in some districts. In Costa Rica it takes an annual toll of about 20 percent of the crop: It occurs in all leading coffee countries of the Americas.

The first symptom is a small, dark area on a leaf. In the center of the spot is a yellow infection body. The tiny lesion grows into a round, grayish spot. On the spot are produced a number of fine, yellowish, hairlike stalks, which at one time are tipped with a large, pear-shaped fruiting body. The infection body is not always present in field material but is large enough to be seen with the unaided eye. It is readily detached by water and carried by splashing droplets of rain practically the only means of distribution. The disease progresses slowly in coffee plantations. Wide roadways, rows of closely set banana plants, thick hedges of old shade trees, and narrow fields of annual crops are barriers against its movement. Rarely in nature does the fungus produce the characteristic brilliant-yellow, miniature mushrooms. They bear few spores. Some bear none at, all.

The large infection bodies carry the disease from plant to plant. They cause excessive defoliation and fruit drop. They also attack flowers and green stems and produce lesions on fruits. The trees finally die if serious infection continues. Because the fungus has a comparatively narrow temperature range, the disease is more severe in cool highland areas and does not occur in warm places at low altitudes. It grows fairly well under conditions as cool as 54 F.; well at 61 to 75 , reaching an optimum at 75 ; and poorly at 83 . At 86 to 90 it stops growing.

The disease thus far is confined to the Americas. With care it can probably be kept from spreading to the oriental Tropics. The disease can be restrained somewhat in countries of its origin if the coffee is grown at warm, low altitudes, and if the dense protecting shade trees are thinned by severe pruning.

Spraying with bordeaux mixture has been recommended. Tests of other fungicides have been started. It is ordinarily considered that costs of materials, employment of untrained labor, and difficulties of terrain make adequate and regular spray applications practically impossible.

Several years of research under Point IV in cooperative stations by the Office of Foreign Agricultural Relations have resulted in developing a control that does not employ spray methods but is accomplished by removing leaves. During the first 2 weeks of the rainy season, diseased trees are stripped of all leaves, flowers, and fruits, which fall to the ground, carrying the inoculum with them. There the fungus is destroyed by natural means by insects, slugs, and bacterial action before the 6 weeks elapse that are required for new leaves to appear on the denuded trees. The current crop is sacrificed, of course, but the larger harvests in the following years compensate for the destruction of the one poor crop. If careful watch is maintained and reinfected trees are defoliated as soon as they are found in the old treated area, the disease apparently may be kept in check indefinitely.

DIEBACK, or anthracnose, caused by the fungus Colletotrichum coffeanum, is common in all countries where coffee is grown. It apparently causes injuries at irregular times to seedlings in the seedbed and nursery, to old bearing trees, and to new supplies transplanted in the field. In the places where it has been studied, the fungus has been found to be present on all trees. Losses are hard to measure, because the disease is still not well understood, but the trouble doubtless cuts substantially into profits year after year.

First symptoms of anthracnose are the dark lesions, usually large, which appear on seedling leaves. On weakened or older plants leaf lesions of anthracnose often spread into the stem tissues to which the leaves are attached. Those parts are killed, and the blackened dieback spreads down into branches and stems. When conditions of moisture and temperature favor the disease, it produces profuse masses of sticky spores. The spores are spread by rains, insects, and other means. A variable interval elapses between the period of inoculation with fungus spores and the visible appearance of disease symptoms. Latent infections, which involve the presence of the organism in apparently healthy plants, may therefore occur. Intensive dieback of stems often is found that is unrelated to Colletotrichum infection under conditions adverse to tree growth. In Africa and India dieback is often due to plant exhaustion, following heavy crops. That is true also to some extent in tropical America. Instances are encountered in which the condition known as dieback results as a complex of both widespread infection by the causal fungus and from unfavorable growth conditions or physiological disturbances of infected plants.