Coffee rust has ravaged Latin American plantations for several years, leading to reductions in annual coffee production of up to 30 percent in some countries and threatening the livelihoods of hundreds of thousands of small-scale farmers in the region.

A new study by University of Michigan researchers suggests that the coffee plants themselves may hold biological weapons that could someday be harnessed in the fight against the coffee rust fungal pathogen.

Those potential weapons are themselves fungi, a surprisingly diverse community of more than 300 species of them – including 15 likely fungal parasites – living on coffee leaves, within or alongside the yellow blotches that mark coffee rust lesions.

Using an old-fashioned handheld paper punch, U-M researchers collected leaf samples from both infected and uninfected coffee leaves at coffee farms in Chiapas, Mexico, and in Puerto Rico.

They found up to 69 fungal species living on a single quarter-inch-diameter leaf disc from uninfected leaves and up to 62 species on rust-infected leaf discs, according to Timothy James, a U-M mycologist and lead author of a paper published online Nov. 13, 2015 in Applied and Environmental Microbiology, a journal of the American Society for Microbiology.

"Latin America is experiencing unprecedented epidemics of coffee rust, so identification of its natural enemies could aid in developing management strategies or in pinpointing species that could be used for biocontrol," said James, an associate professor in the Department of Ecology and Evolutionary Biology.

"We recovered a surprisingly high fungal diversity in extraordinarily small samples of coffee leaf material – more than 300 species in a sampling area that, in total, was much smaller than the average size of a single coffee leaf. These hyper-diverse communities highlight the complexity of fungal diversity of unknown ecological function within these leaves."

James' co-authors on the paper include U-M faculty members Ivette Perfecto of the School of Natural Resources and Environment and John Vandermeer of EEB, who have operated research plots at an organic coffee farm in southern Chiapas, Mexico, for nearly 20 years. The fourth author is U-M research fellow John Marino, an EEB alumnus.

Michigan News press release