coffee rust

coffee rust, devastating foliar disease of coffee plants caused by the fungus Hemileia vastatrix. Coffee rust has long been known in coffee-growing areas of Africa, the Near East, India, Asia, and Australasia, and it destroyed the once-flourishing coffee plantations of Ceylon (now Sri Lanka) and Java in the late 1800s. In 1970 the disease was discovered to be widespread in Brazil, the first known infected area in the Western Hemisphere, and it has continued to spread throughout the Americas. An epidemic that began in Central America and the Caribbean in 2011 ultimately damaged crops on about 70 percent of farms in Latin America and caused more than $3 billion in damage by 2021. Coffee rust was detected in Hawaii for the first time in late 2020 and threatens the coffee industry on the islands. See also list of plant diseases.

The symptoms of coffee rust include small, yellowish, oily spots on the upper leaf surface that expand into larger round spots that turn bright orange to red and finally brown with a yellow border. The rust pustules are powdery and orange-yellow on the underleaf surface. Later the pustules turn black. Rusted leaves drop so that affected trees are virtually denuded; such trees have significantly lower coffee yields and usually die within a few years.

Coffee rust can be partially controlled by the timely application of fungicide sprays during wet seasons. Plantations in some areas have been moved to higher and cooler altitudes, 1,800 to 2,100 meters (6,000 to 7,000 feet), at which the rust fungus has difficulty reproducing, though global warming is expected to further the spread of the disease into these areas. There is evidence that shade-grown coffees, which are not grown as monocultures, are somewhat less susceptible, as the agroforestry practice of mixing tree crops greatly slows the spread of the disease. Additionally, resistant varieties of Robusta coffee (Coffea canephora) have been developed, but the beans are generally considered to be of lower quality than those of the vulnerable Arabica plants (C. arabica). One resistant variety, Lempira, was widely planted in Honduras but lost its resistance to the disease in 2017, resulting in crop losses in that country. No resistant varieties have shown to be impervious to all races of the fungus.