Plant leaves are complex and dynamic ecosystems inhabited by a vast array of pathogenic and non-pathogenic microorganisms. While pathogens are clearly the causal agents of disease, biotic interactions involving non-pathogenic microorganisms - endophytes - are increasingly recognized as influencing plant disease expression. A better understanding of the factors structuring microbial communities in the leaves of wild plants will inform testable hypotheses regarding the role of endophytes in plant disease. Here we analyze fungal ITS1 amplicons from leaves of Populus trichocarpa(black cottonwood) collected across its native range in the Pacific Northwest to elucidate how host identity, geography and climate affect fungal community structure. In particular, we examine the interplay between pathogenic and nonpathogenic fungi and explore their relationship with the environment and disease outcomes.
We found that Populus trichocarpa hosts a multitude of foliar fungi (1400 OTUs), yet its leaves are highly uneven and dominated by few species (~15 OTUs). Individual host tree, geography, and precipitation significantly influenced the structure of fungal communities (PERMANOVA). In particular, communities varied most strongly east versus west of the Cascade Mountains, and fungal OTU richness was positively correlated with long-term regional precipitation (i.e. wetter areas host more OTUs than more arid sites). We also found that leaves with more similar endophyte communities hosted more similar pathogen communities (Mantel test), and that the relative abundances of several endophytes were either positively or negatively correlated with the severity of disease caused by several different fungal pathogens (e.g., Melampsora leaf rust, Sphaerulina leaf spot). Our results suggest that endophytes are acting as pathogen facilitators or pathogen antagonists within leaves, and reinforce the hypothesis that endophytic communities (not just pathogens) are important for modulating disease severity. Our ongoing research seeks to validate putative disease modifying endophytes using controlled inoculation experiments.