Plant community ecologists have long been interested in whether seedling pathogens are a mechanism for maintaining forest diversity. This mechanism rests on the assumption that pathogens are host-specific, killing certain tree species and not others. However, studies identifying potential seedling pathogens have often found many fungi that can infect multiple hosts. Not only are most of these isolated fungi generalists, but seedlings are often simultaneously infected by multiple fungi. Could these generalist fungi still have differential impacts on seedling survival, or could combinations of fungi have non-additive impacts? We employ a hierarchical Bayesian model of fungal impacts on seedling survival, involving a model selection framework (reversible jump Markov Chain Monte Carlo) that allows us to evaluate how combinations of fungal symbionts affect survival without assuming that these effects are additive. Using a dataset of fungal symbionts of six temperate tree species, we look for cryptic specialization within fungal species.  

Results/Conclusions
We assess the impacts of different fungi individually and in combination on survival probabilities of seedlings given infection, and find some evidence of specialization. Two tree species (Liquidambar styraciflua and Nyssa sylvatica) are negatively impacted by the fungi tested only when unique combinations of fungi are present. One species (Acer barbatum) is not negatively impacted by any of the fungi included in this model. Some symbiotic fungi, though isolated from multiple hosts, do not have equivalent impacts on the survival of different hosts. Others (for example, Pilidiella sp. A) have minimal impacts on all six hosts and are likely not acting as pathogens. We discuss the Janzen-Connell hypothesis in the context of these results, but also consider ways in which generalist fungi may impact plant diversity. We also discuss how modeling in this more epidemiological framework can help to understand the functions of symbiotic fungi capable of multiple lifestyles.