Coffee is a crop of the tropics surpassed only by oil in its value as a world commodity. For centuries, it has been a significant import crop in Europe and economically important to the European countries that ruled tropical colonies. It remains an important crop to the independent nations created from those colonies.
Coffee became a popular drink in Europe in the 1600s when contaminated drinking water limited people to fermented beverages or those made with boiled water, such as tea or coffee. Coffee houses were major social centers in England in the 1650s. The Dutch were the first major European coffee importers, transporting coffee from their colonial plantations in Ceylon (now Sri Lanka), Java, and Sumatra.
Coffee with the berries that contain the seeds or "beans" that are harvested and roasted.
The fungal parasite probably arose in southern Ethiopia, the origin of the coffee plant itself. It is a Basidiomycete, a fungal group containing many important plant parasites, and belongs to a subgroup known as the rusts. The rusts are such important plant pathogens that they are discussed in detail in Chapter 10, and the complete biology of the coffee rust fungus is explained there. It is sufficient at this point to consider the problem of a rapidly reproducing fungus capable of infecting the foliage of the coffee tree, a nondeciduous, perennial plant that grows in a frost-free climate.
Symptoms of coffee rust infection (Hemileia vastatrix).
A single tiny rust pustule on a coffee tree leaf can produce 150,000 spores, and a single leaf can contain hundreds of pustules. When the coffee rust fungus, Hemileia vastatrix, reached Ceylon in 1875, nearly 400,000 acres (160,000 hectares) were covered with coffee trees. No effective chemical fungicides were available to protect the foliage, so the fungus was able to colonize the leaves until nearly all the trees were defoliated. The spores produced on the leaves are quite resistant to desiccation, unlike the sporangia of P. infestans, and are capable of long-distance movement in a viable state. They easily moved through the acres of coffee trees, feasting on the banquet prepared by unsuspecting plantation owners. In 1870, Ceylon was exporting 100 million pounds (45 million kilograms) of coffee a year. By 1889, production was down to 5 million pounds (2.3 million kilograms). In less than 20 years, many coffee plantations were destroyed, and production had essentially ceased.
World distribution of the coffee rust fungus, with the dates it was first discovered.
Billboard near the airport of Bogata, Columbia, waning about the danger of importing the coffee rust fungus, before its introduction to South America.
What will be the consequences of the importation of such a dangerous pathogen? Frequent fungicide applications will be necessary to protect the highly susceptible cultivars of C. arabica, which produce the best quality of coffee. Chemical inputs are particularly demanding for small growers, who now must purchase fungicides and spraying equipment. Trees must be grown at lower densities to allow complete fungicide coverage of the susceptible foliage. Wider spacing of trees also increases air movement between the trees. When the foliage dries more quickly, infections are reduced since, like almost all fungal spores, rust spores require water for germination. Chemical inputs and changes in planting practices increase the costs of production and hence the price to consumers.
Rust-resistant cultivars of C. arabica and other species such as C. canephora exist, but the crop is of poorer quality. Plant breeders must often struggle with the problem of combining desirable genetic traits for crop quality with genes for resistance in the same plant. Rust fungi are capable of producing many genetically different races, and 32 races of H. vastatrix have been detected. It is always particularly difficult to find durable resistance to a pathogen when the crop is a perennial growing in a frost-free environment. The pathogen population is not reduced by winter stresses, and replanting with new cultivars is expensive and infrequent. Resistance that is effective against all races of the parasite remains the long-term goal. In the meantime, fungicide applications are becoming part of the routine production practices on coffee plantations in the Western Hemisphere.RETURN TO "What is Plant Pathology and Why is it Important"RETURN TO APSnet FEATURE STORY