Foliar fungi – pathogens, endophytes, epiphytes – form taxonomically diverse communities that affect plant health and productivity. The composition of foliar fungal communities is variable at spatial scales both small (e.g., individual plants) and large (e.g., continents). However, few studies have focused on how environmental factors and host plant traits influence the composition and temporal variability of these communities. Moreover, predicting how nonpathogenic members of these communities affect the plant host remains a challenge.
In Chapter two we used ITS metabarcoding to characterize foliar fungal communities of Populus trichocarpa in two consecutive years at the same sites located across its native range in the Pacific Northwest of North America. We used multivariate analyses to test for and differentiate spatial and environmental factors affecting community composition, and tested whether the magnitude of year-to-year variation in community composition varied among environments. We found that climate explained more variation in community composition than geographic distance, although the majority of variation was shared, and that the year-to-year variability of communities depended on the environmental context, with greater variability in the drier sites located east of the Cascade Range.
In Chapter three we used ITS metabarcoding and multivariate analyses to test whether the influence of intraspecific host genetic variation on the foliar fungal community diminished over the course of one growing season. We utilized 12 P. trichocarpa genotypes that vary in two functional traits: phenology of bud-burst (early vs. late) and Sphaerulina leaf spot resistance (resistant vs. susceptible). We found that both of these traits drove differences in community composition among trees, but that the strength of the effect diminished through time as the communities converged. Our results suggest that for the leaves of deciduous plants, intraspecific host genetic variability may have its strongest impact on microbial community composition early in assembly.
In Chapter four we tested whether Cladosporium endophyte phylogeny can be used to predict endophyte effects on P. trichocarpa leaf rust disease severity caused by Melampsora × columbiana. We used multilocus sequence typing to infer phylogenetic relationships among 96 Cladosporium endophyte isolates collected from wild P. trichocarpa trees throughout its native range. We then conducted a double-inoculation leaf-disk assay (endophyte inoculated first, then rust pathogen) for a subset of 50 Cladosporium isolates to characterize disease modification for the endophyte isolates; data on endophytes parasitizing rust was collected simultaneously for each isolate. We found that Cladosporium phylogeny was a significant predictor of rust disease severity and was also correlated with rust mycoparasitism, demonstrating that fungal endophyte phylogenetic relatedness can help predict differences in endophyte function.