Grassland ecosystems are host to a complex food web of host-pathogen interactions mediated by abiotic and biotic factors. Abiotic factors such as nutrient availability can influence pathogen populations, whereas biotic factors like host plant abundance can influence disease spread. Physiological characteristics of the host can also alter the host’s likelihood of infection. Specifically, stomata act as an important entry point for certain plant pathogens because they provide a natural entry point. Here, we tested whether host abundance or stomatal density affects disease prevalence across a long-term gradient of nitrogen addition at Cedar Creek Ecosystem Science Reserve (East Bethel, MN). In this experiment, N was added at rates similar to ambient deposition (1g, 5g, and 10g m-2 year-1; ambient deposition ~1g m-2 year-1) since 2007. We focused on two dominant grasses, an introduced, cold-season C3 perennial (Agropyron (Elymus) repens) and a native, warm-season C4 perennial (Andropogon gerardii). We measured percent cover of all species biweekly throughout the growing season. For each species, we scored 30 plants for disease and prepared stomatal peels from five individuals in each treatment at three timepoints (June, July, August) during the growing season.
Results/Conclusions
Within each host species, nitrogen availability did not correlate with disease prevalence except in the lowest nitrogen treatment. In this treatment, A. gerardii infection prevalence was significantly greater than in the control treatment (p = 0.034). A. repens infection prevalence was not significantly different in any treatment. Across all treatments, we found greater percent cover of A. repens early in the growing season, but greater percent cover of A. gerardii later in the growing season. Despite these trends, disease prevalence in both species increased across the growing season, suggesting a link between host abundance and infection in A. gerardii but not A. repens.
Stomatal density was generally negatively correlated with infection; however, we found positive correlations between stomatal density and infection at the high nitrogen addition rate for A. repens and the intermediate nitrogen addition rate for A. gerardii. These results suggest that increased stomatal density does not act as a driving factor of infection in either species. Taken together, these results suggest a complex relationship between disease prevalence and abiotic and biotic factors in this grassland ecosystem.