In Spanish they call it la roya, in English it's coffee rust, and an outbreak of the fungus (Hemileia vastatrix) has hit Central American and Mexican coffee farmers hard over the past few years.
The fungus attacks the leaves of the coffee tree, restricting photosynthesis and resulting in fewer cherries of poorer quality. The effect on coffee yield is significant – Central American output fell 20 per cent in 2012-2013, prompting the governments of Honduras, Costa Rica and Guatemala to declare states of emergency.
Mexican farmers had one of their worst seasons ever in 2013-2014, with the harvest falling by 18 per cent. A similar fall is forecast for the 2014-2015 season by AMECAFE, Mexico's national coffee association.
Andy Gelman from Omar and the Marvellous Coffee Bird has just returned from Central America, where he saw the effect of la roya first-hand.
"We skipped into El Salvador, to Procafe, the government coffee research institute. The trees there had no leaves, no cherries – it was like a ghost town." El Salvador has been hit particularly hard, he says.
As Gelman points out, "The disease eats away at the leaf and reduces the ability of the plant to ripen cherries. They go from green to yellow to over-ripe without hitting the red sweet spot."
Arabica – grown for specialty coffee throughout the region – is particularly susceptible.
Climate change also seems to be playing a role. Previously rust occurred at altitudes below 800 metres, but with warming it has spread to plantations above 1000 metres, according to Juana Barrera, an agricultural engineer at Colegio de la Frontera Sur in Mexico.
In Guatemala, says Gelman, many farmers have lost their organic certification because they've had to spray fungicides to battle rust.
Others have planted rust-resistant varieties, such as catimor (an arabica-robusta hybrid) – but these are more susceptible to another fungus, ojo de gallo, or rooster's eye, he says. Ojo de gallo attacks the bean inside the cherry as well as the leaves, making them soft. When farmers process the cherries, they fall apart.
Even without these problems, Gelman thinks prices for specialty coffee are only going one way – up: "More and more roasters are fighting over a dwindling supply of specialty coffee," he says. "I'm glad I work with a broker (MTC) who can buy large quantities, which is more attractive to farmers."