Factors Impacting American Leaf Spot Disease Incidence and Intensity in a Shaded Coffee Farm in Chiapas, Mexico
Creators: name: Su, Chenyang
Publisher: name: My University
Year: 2021
Subjects: coffee
agroforestry
fungal diseases
american leaf spot

description: American leaf spot disease, known in Latin America as ojo de gallo, is a coffee fungal disease caused by the fungal agent Mycena citricolor. Recent increase in the disease and its ability to cause severe yield losses in coffee raised questions about our understanding of the disease dynamics in coffee farms, and our ability to predict and prepare for possible outbreaks and avoid the economic losses similar to those caused by recent outbreaks of the coffee leaf rust across Latin America. In this thesis, I investigate some of the factors that might impact patterns of American leaf spot disease in an organic shaded coffee farm in Chiapas, Mexico. In chapter 1, I assess the co-occurrence pattern of American leaf spot (ALS) disease with the coffee leaf rust, another fungal disease caused by Hemileia vastatrix, which caused severe outbreaks in Latin America since 2012/2013. Based on seven months of sampling of ALS and coffee leaf rust incidence and intensity in an organic coffee farm in Chiapas, Mexico, results from chi-squared analyses of contingency tables of disease presence suggest that the incidences of the two diseases are not independent. Significantly fewer coffee plants are infected with both diseases than expected at random, while significantly higher number of coffee plants are infected with only one disease than expected at random. The patterns show evidence that there may be trade-offs between the control of the two pathogens. Specifically, the planting of coffee leaf rust resistant varieties may be more susceptible to American leaf spot disease and may lead to an outbreak of this fungal disease. In chapter 2, I explore the possible effects of shade trees and a dominant ant on ALS. As nitrogen fixing trees can usually add nutrients to the soil and possibly improve plant health, I proposed that coffee plants under nitrogen fixing shade trees will have a lower incidence and intensity of ALS. In addition, due to the complex ecological interactions that the dominant ant, Azteca sericeasur, can have with many organisms in the system and maintaining stability through biocontrol, I hypothesized that trees with A. sericeasur nests will have a negative impact on ALS disease. Using data of American leaf spot incidence and intensity collected in Finca Irlanda, Chiapas, Mexico, along with shade tree types and Azteca ant nest presence, I ran chi-squared analyses, Kruskal-Wallis rank sum test, and t-tests to understand the disease patterns with respect to three shade tree types (Inga spp [a n-fixing genus in the family Fabacea], Alchornea latifolia [the second most common species after Inga], and all other trees). I found that shade tree types do not significantly impact American leaf spot incidence and intensity in the same way. Coffee plants around Inga spp., the nitrogen fixing trees in the coffee farm, have significantly higher disease intensities than other shade trees but are not different from Alchornea. The presence of Azteca nests in shade trees nearby coffee bushes did not have a significant effect on the incidence of ALS. The results suggest that shade trees have complex impact on American leaf spot disease, and multiple factors, including nutrients, shade level and microclimate, and possible ecological interactions can have a combined effect. Future research focusing on independent factors associated to shade trees can further improve our understanding of American leaf spot disease patterns.