Spatial variation in soil-borne disease dynamics of a temperate tree, Prunus serotina.

Soil-borne pathogens are posited to maintain forest diversity. However, their in situ impact and spatial variation are largely unknown. We examined spatial patterns of pathogenic activity in a deciduous forest using a common garden experiment and also in a natural experiment around replicated trees, and we quantified Pythium (a soil-borne pathogen) density around individual Prunus serotina trees. In both experiments, P. serotina seedling survival was 52-57% greater in plots treated with a metalaxyl-based fungicide specific to oomycetes (i.e., Pythium) than in untreated plots. Disease dynamics were not density dependent, but pathogenic activity and Pythium density were spatially variable. In the common garden and natural experiments, pathogenic activity of soil inoculum varied among trees, while in the natural experiment disease dynamics were also distance dependent and pathogenic activity decreased away from P. serotina trees. Disease and Pythium density were not always related but displayed considerable spatial variation. We found that Pythium density did not vary with distance away from P. serotina trees but did vary among trees. Understanding the spatial complexity of soil-borne pathogens is critical to accurately characterizing their effects on populations and ultimately on forest diversity.