Which fungus causes coffee leaf rust, a major disease of coffee plants?

The fungus that causes coffee leaf rust, a major disease of coffee plants, is Hemileia vastatrix.

Hemileia vastatrix is an obligate parasitic rust fungus in the order Pucciniales. It infects coffee plants, especially Coffea species, and produces powdery yellow-orange urediniospores on the undersides of leaves. Infected leaves lose photosynthetic capacity and may drop prematurely, reducing flowering, berry production, and sometimes plant survival.

The disease was first reported in major coffee-growing regions of Sri Lanka, then called Ceylon, in 1867. English mycologists Berkeley and Broome formally described and named the fungus from material collected there. A severe epidemic helped undermine Ceylon’s coffee industry, encouraging a shift toward tea cultivation.

Puccinia graminis is also a rust fungus, but it attacks cereal crops such as wheat and barley. Cryphonectria parasitica causes chestnut blight, while Botrytis cinerea causes grey mould. These are plant pathogens, but none is the causal agent of coffee leaf rust.