M. Grabowski, University of Minnesota Extension
Coffee rust is caused by the fungus Hemileia vastatrix which causes powdery yellow to orange spots on leaves. These leaf spots eventually turn brown and infected leaves drop from the coffee plant prematurely. Severe defoliation by coffee rust weakens the coffee plant, reduces yield and can eventually kill the coffee plant.
The powdery yellow spores that form on infected leaves spread easily on wind and splashing water. It is estimated that a single rust pustule on a coffee leaf contains 150,000 spores and a single leaf contains hundreds of pustules. Spores need moisture on the leaf surface to start a new infection and in warm rainy weather disease can spread at an incredible rate.
In 1875, the coffee rust fungus was found in Ceylon (now Sri Lanka) which contained nearly 400,000 acres planted in coffee. At the time little was known about coffee rust and the disease essentially wiped out coffee production in Ceylon, Indonesia and the Philippines. Coffee production was moved to the Americas and quarantines successfully kept the pathogen at bay for 100 years. In 1970 however the coffee rust fungus was found in Brazil and was identified in 1975 in Nicaragua. Since then La Roya (coffee rust) has spread throughout Central and South America.
Coffee rust was not as severe in the Americas as many had first predicted. In part coffee was grown at cooler higher altitudes that did not favor growth of the rust fungus. In addition American coffee growers had many more management options available to them than the plantation owners of Ceylon 100 years earlier.
Unfortunately, there are many different races of the coffee rust fungus. Races are strains of the pathogen that are unaffected by the resistance genes within a specific cultivar, so a cultivar resistant to one race of the rust fungus may not be resistant to another race. Breeders struggle to combine genes for disease resistance in a plant that produces high quality coffee in a way that new races of the rust fungus will not be able to overcome the plants resistance.
Fungicides are also used to manage coffee rust on coffee farms. Fungicides protect leaves from infection. Unfortunately fungicides can be quite expensive and need to be repeatedly applied when weather conditions are favorable for disease. Cultural control practices like pruning and spacing plants in wide rows can help leaves dry quickly after rain, there by reducing disease. Unfortunately these management strategies have little affect in a very rainy year.
This years epidemic of coffee rust in Central and South America has made headlines. The coffee crop from Colombia through Mexico is severely affected by the disease and coffee rust is being reported at higher altitudes than normal. In February Guatemala declared a state of emergency, reporting over 70% of the coffee crop infected with the disease. The coffee harvest is expected to be reduced both this year and the next.