Species interactions are often recognized as key drivers of biological invasions (i.e., the enemy-release hypothesis). However, little consideration has been given to how these interactions vary geographically, despite expectations that the strength of species interactions involving native species should evolve to exhibit a latitudinal gradient. In contrast, invasive species may not have had time to evolve a parallel gradient in interaction strength, which could result in large-scale heterogeneity in community resistance/susceptibility to invasive species. To address this hypothesis, I focused on the unique study system of foliar fungal endophytes and pathogens of Phragmites australis. P. australis is a macrophytic grass with conspecific native and exotic genotypes widely distributed throughout North America. Moreover, despite their ability to exert both positive (e.g., endophytes) and negative (e.g., pathogens) impacts, fungi have received little attention from invasion biologists. We surveyed 35 sites from South Florida to Maine to examine variation in community structure and impact of foliar fungi based on plant genotype and site latitude. At each site we measured a range of site descriptors and plant traits, quantified pathogen damage to leaf tissue, and collected 20 healthy and diseased leaf tissue samples for isolation and identification of emergent fungi on potato dextrose agar.
Pathogen damage to leaf tissue at the site level was low (average of 0.4 - 11.2 % of leaf area damaged per site) and did not vary among P. australis genotypes, contrary to predictions of the enemy-release hypothesis. However, pathogen damage to leaves increased with site latitude for invasive genotypes, but showed no relationship with latitude for native genotypes. The presence of these non-parallel latitudinal gradients suggests that there is heterogeneity in community resistance/susceptibility to the invasive species at a geographic scale (i.e., exotic genotypes have lower fungal pathogen damage than native genotypes at latitudes less than 40°, whereas native genotypes are at an advantage at higher latitudes). A total of 168 morphospecies were identified from 1,062 fungal isolates obtained from leaf tissue samples. The foliar fungi community differed strongly among P. australis genotypes, between healthy and diseased leaves, and based upon geographic location. Candidate species for investigation as potential genotype-specific fungal pathogen biological control agents were identified from highly damaged leaves, and potentially beneficial endophytes were identified from sites with vigorous P. australis growth and low pathogen damage.