COS 142-1 - Effect of shade trees on American leaf spot disease incidence and intensity in a coffee agroforestry system

Background/Question/Methods

American leaf spot disease caused by Mycena citricolor is a coffee fungal disease in Latin America that can lead to yield losses. Native to the area, American leaf spot disease can infect not only coffee, but also weeds and shade trees, making it hard to control. In this study, we investigate the potential effects of shade trees on American leaf spot disease in an organic coffee agroforestry system in southern Mexico based on seven months of sampling for disease incidence and intensity and data on shade tree distribution.

Results/Conclusions

Although low variation in disease intensities is explained by shade tree diversity, general linear model shows significantly negative relationship of diversity with the disease and no effect of tree density. We then compared disease incidence and intensity on coffee bushes under Inga spp., nitrogen fixing trees commonly found in coffee farms, Alchornea latifolia, a second common shade tree in the study site, and other shade trees including fruit trees. Chi-squared test and Kruskal-Wallis rank sum test show no significant difference in disease incidence for different tree types. T-test by tree types shows significantly higher disease intensities under Inga spp. than other shade trees, possibly due to the favorable microclimate conditions under Inga trees in the farm. The results suggest that higher shade tree density does not necessarily promote fungal diseases, while diversity could play a positive role on pathogen control in coffee agroforestry system. A possible trade-off between nutrient addition by Inga spp. and shade levels for nitrogen fixing trees may be present, and further studies examining independent factors associated with ecosystem functions of shade trees can inform management in coffee farms for fungal disease control.