LB 4-33 - Dilution of Foliar Pathogens in Response to Species Richness and Phylogenetic Dispersion Gradients

There is evidence demonstrating that dilution of foliar pathogens occurs with increasing diversity. However, the relative role of plant community composition via species richness and phylogenetic relatedness is less understood. Closely related plant species are more likely to share specialist pathogens, thus driving the dilution effect. We collected disease incidence data from a biodiversity experiment to measure foliar pathogen dilution across plant species richness (plots with 1, 2, 3, 5, and 6 species) and phylogenetic dispersion (under and over-dispersed) treatments across 240 plots. The experiment used 17 plant species, 6 Asteraceae, 6 Fabaceae, and 5 Poaceae, with every possible treatment combination. To collect disease incidence data, we randomly selected 4 individuals per species in each plot, then recorded disease using a scale 1-10; this was done in July of both 2021 and 2022. We used mixed linear models to measure response of foliar disease to the planting design. In 2022 we cultured foliar fungi from leaves of 3 species with highest disease incidence. In spring 2023, we performed Koch’s postulates greenhouse test of the cultures to confirm their pathogenicity. We inoculated fungal broth media onto the original host plant species (all Fabaceae), as well as a grass and a composite, to test whether the putative pathogens have a phylogenetic preference. Analyses were conducted using mixed linear models, with plant biomass as the response. Our full mixed linear model shows significant differences between years (p < 0.001), plant species (p < 0.001), as well as the interaction of plant species with plant species richness (p < 0.001), phylogenetic dispersion (p < 0.001), and precipitation (p < 0.001). Due to the strong species effects, we ran post-hoc linear models on the subsetted data for each species. We found significant decreasing relationships between disease incidence and plant species richness for 6 of the plant species (p < 0.05). In addition, 4 species had significantly lower disease incidence in the over-dispersed treatment compared to under-dispersed (p < 0.05). Greenhouse experiment results to follow, est. July 2023. Our findings provide evidence to support the dilution effect, as well as potential pathogen accumulation in low species richness, e.g. monoculture. This evidence supports the idea that host availability as well as pathogen specialization, influences the amount of disease found in plants. This provides insight into the role of resident microbial pathogens in native plant community diversity maintenance, which is accumulating consideration in novel restoration and conservation efforts, making it important information for All Ecologists!