What is coffee leaf rust?

Coffee leaf rust - caused by the fungus Hemileia vastatrix - is the single most destructive coffee disease in history. It attacks the leaves of coffee plants, producing orange, powdery spore deposits on leaf undersides and causing defoliation that reduces photosynthesis and fruit production. In severe cases it kills the tree.

CLR is present in every major Arabica-growing country. It spreads through wind and rain, thrives in warm humid conditions, and can move through a region with devastating speed. The Central American outbreak of 2012–2014 destroyed an estimated 40% of the crop in some countries. Sri Lanka's entire Arabica industry was effectively eliminated by it in the 1870s - the event that converted the island from coffee to tea.

Every disease-resistant cultivar in this glossary - Catimor, Castillo, Ruiru 11, F1 hybrids - was developed, at least in part, in response to CLR. It's the single biggest driver of coffee breeding research worldwide, and with climate change expanding the conditions it thrives in, that research is more urgent than ever.