Interspecific interactions between the Sudden Oak Death pathogen Phytophthora ramorum and two sympatric Phytophthora species in varying ecological conditions

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Even when introduced invasive pathogens lack their natural predators or competitors, they must still interact with other organisms in their introduced range. Sudden Oak Death (SOD), caused by Phytophthora ramorum (Oomycota), is an introduced disease causing large-scale tree mortality. Two additional Phytophthora species, Phytophthora nemorosa and Phytophthora pseudosyringae, cause significantly lower oak mortality, yet they also commonly colonize leaves of Umbellularia californica, the major transmissive host of SOD in California. We combined field surveys and inoculation experiments to understand disease prevalence dynamics and competitive interactions among these pathogen species. Despite the broader geographic distribution of P. nemorosa with respect to that of P. ramorum, our results suggest that P. nemorosa exhibits a narrower ecological amplitude and, in any given region, occupies fewer sites than P. ramorum. Our results additionally suggest that, perhaps due to priority effects, P. nemorosa can persist at levels comparable to those of P. ramorum in ecologically suitable plots when climate favors P. ramorum dormancy. However, P. ramorum prevalence increases to levels higher than those of the competing species when abundant rainfall triggers its sporulation. Understanding the determinants and outcomes of competition between these species has important implications for understanding the epidemiology and possible control strategies for Sudden Oak Death.

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