Fungal and oomycete plant pathogens are believed to be an important mechanism maintaining tree species diversity via control of seedling growth and survival. In this study, we characterized the fungi and oomycetes found in dead or dying seedlings of thirteen southeastern U.S. forest tree species in a North Carolina mixed hardwood forest. Seedlings planted in the forest were surveyed weekly for mortality over two growing seasons, and dead or dying seedlings were collected and assayed for pathogens. We used culture-based methods to isolate fungi and oomycetes, and DNA sequencing to identify them to the genus or species level.  

Results/Conclusions
The most common pathogen identified was Colletotrichum acutatum, a known generalist pathogen of many commercially-grown tree species. C. acutatum was isolated from over 30% of all seedlings sampled, and found on 12 of the 13 tree species planted.  To better understand what factors influence infection with C. acutatum, we developed a hierarchical Bayesian model of fungal infection transmission using temperature, soil moisture, and distance from conspecific and heterospecific adult trees as explanatory covariates. Although soil moisture may play an important role in infection probability, we do not find support for the distance from conspecifics hypothesis (Janzen-Connell processes). Like C. acutatum, the majority of the fungi and oomycetes isolated in this study are shared by multiple hosts. These generalist pathogens may still have a sizeable impact on plant community structure through differential effects on growth and mortality.