Effect of shade trees on microclimate conditions and coffee leaf rust

Abstract

Coffee leaf rust is a common agricultural disease in the tropics, caused by the fungal pathogen Hemileia vastatrix. Recent epidemics throughout Latin America has caused devastating economic loss and re-invigorated research into new disease management. Because H. vastatrix uses wind for transmission and humidity for germination, I propose that coffee leaf rust disease dynamics can be influenced by shade trees that modify the abiotic environment. Specifically, I hypothesize that tree stands will disrupt wind transmission of fungal spores but that increasing canopy cover will reduce evaporation, increase local humidity, and increase rust germination. I explored the effect of trees on rust density on a highland coffee farm in Chiapas, Mexico where I measured and modeled the influence of tree density, canopy cover, evaporation rate, and coffee plant density on disease incidence and severity.

Coffee plants were significantly less likely to become infected at higher tree and coffee plant densities, but canopy cover increased the likelihood of infection. The proportion of leaves infected was influenced only by higher coffee densities, and evaporation rates had no correlation with infection or other structural variables. These results suggest that vegetation structures – including both trees and coffee plants themselves – reduce the probability of plants contracting the coffee leaf rust, potentially by blocking spore dispersal. However, once infected, the disease severity is not influenced by humidity, as previously proposed. I suspect that areas with higher coffee densities are more likely to contain different varieties, some of which are more resistant than others, so that disease severity is influenced by the dilution effect. These results suggest that tree stands have complex, multidimensional effects on the coffee leaf rust, and that their use in disease management may not be straightforward.