Genetic basis of pathogen community structure for foundation tree species in a common garden and in the wild

1. Genetic variation within and among foundation plant species is known to affect arthropod, plant and soil microbial communities. We hypothesized that the same would be expected for pathogen communities, which have typically been studied only as individual pathogen species. 2. In a common garden in Utah, USA, we first tested how genetic differences within and among Populus angustifolia, P. fremontii and their interspecific hybrid P. 9 hinckleyana affect a fungal leaf pathogen community. Next, we tested how Populus genetic differences at the level of species and hybrids affect fungal leaf pathogen communities in the wild, specifically in a natural Populus hybridization zone (13 river km) and throughout the larger Weber River watershed (150 river km). 3. In the common garden, genetic variation both within and among Populus species and hybrids significantly affected the structure (i.e. species abundances and composition) of pathogen communities. In the wild, genetic variation among Populus species and hybrids also significantly affected pathogen communities, though not as strongly as was found in the common garden environment. Stand-level density of the susceptible Populus species most strongly affected the structure of pathogen communities in the hybrid zone. 4. Synthesis. Plant species and genotypic variation can affect the local and geographic distribution of pathogen communities in a similar fashion as other diverse organisms (e.g. arthropods, plants, soil microbes), both within a relatively controlled common garden environment and in the wild.