Until the late 1800s Britain ran a profitable coffee industry in Ceylon, now Sri Lanka, and a fungus wiped it out, pushing the colony and the country toward tea. That fungus, Hemileia vastatrix, is coffee leaf rust, first described by Berkeley and Broome in 1869 from Ceylon samples and nicknamed Devastating Emily by planters. It is an obligate parasite that enters leaf pores within 24 to 48 hours, feeds through haustoria, and releases 300,000 to 400,000 spores from a single lesion. The tree dies of starvation after dropping its leaves, and the disease still costs an estimated 15% of world coffee yields every year.

Scientists long assumed the rust simply cloned itself, since no alternate host was ever found, yet it kept producing new races. The answer turned out to be cryptosexuality: two nuclei inside a single spore fuse and recombine. The rust spread from East Africa to India in 1870, Java in 1878, Brazil in 1970, and Maui in late 2020. The 2012 to 2014 Latin American epidemic caused about $1 billion in damage, and in Honduras 10,000 family farms lost their entire production. Farmers now combine pruning and spacing, copper sprays, and resistant F1 hybrids such as Starmaya, bred by CIRAD and grown from seed.

• Dry spores resist UV light and can ride high altitude winds for up to six weeks.
• The fungus needs free water on the leaf and prefers 16 to 28 degrees Celsius, which is why high farms were safer.
• Before the 2012 outbreak, 82% of farms in the affected region were certified organic and planted with susceptible Caturra, Bourbon, and Typica.
• Fungicides can reach half of a farmer’s production costs.
• Resistant plants kill their own cells around an infection, starving the rust through hypersensitive cell death.