Soil feedback and pathogen activity in Prunus serotina throughout its native range

1 Oomycete soil pathogens are known to have a negative effect on Prunus serotina seedling establishment and to promote tree diversity in a deciduous forest in Indiana, USA. Here, we investigate whether negative feedbacks operate widely in its native range in eastern USA. 2 In laboratory experiments, soil sterilization was used to test the net effect of soil biota (pathogens and mutualists) and fungicide treatments to test the effects of soil pathogens (oomycetes) on survival of P. serotina seedlings in soils from 22 P. serotina populations throughout the eastern USA. 3 In soil associated with P. serotina trees, there was a significant positive effect of both soil sterilization and fungicide on seedling survival. The two effects were positively correlated, suggesting that oomycetes were responsible for the observed mortality of seedlings in untreated soils relative to sterilized soils. 4 We determined the host-specificity of these interactions by comparing the effects of the soil biota associated with conspecific and heterospecific trees. There was no interaction between the effects of soil origin and soil sterilization, or of soil origin and fungicide, on seedling survival, although an effect of soil origin on the relative oomycete effect suggested that soil pathogens associated with conspecifics had a more negative influence than those from heterospecifics. 5 Fungicide treatment decreased pre-emergence mortality of P. serotina seedlings at two of three field sites in the northern USA. 6 The overall consistency between the laboratory experiments and the field experiment strongly suggests that oomycete soil pathogens have a negative effect on the survival of P. serotina seedlings throughout its native range in the eastern USA. 7 Soil-borne pathogens therefore appear to regulate the densities of a common tree species (P. serotina) at larger geographical scales than previously described, providing additional evidence of the important role that soil biota play in regulating plant populations and structuring plant communities.