Impacts of Novel Soil-borne Phytophthora Pathogens in Oak Riparian Restoration Sites

David Mitchell, Dept. of Plant Sciences, University of California, Davis

Valerie Eviner, Plant Sciences Department, University of California, Davis

Sebastian Fajardo, Plant Pathology Department, University of California, Davis

David Rizzo, Plant Pathology Department, University of California, Davis

Novel Phytophthora plant pathogens have been introduced into restoration sites in the San Francisco Bay Area. These soil- and water-borne pathogens pose a threat to restoration plantings and can disperse to adjacent ecosystems. Several of the Phytophthora taxa in these sites can be pathogens of oaks, among other native plants. We investigated relationships between Quercus agrifolia and Quercus lobata seedling health, soil properties, and soil Phytophthora detection in riparian restoration sites in Alameda and San Mateo Counties. We hypothesized that Phytophthora detection would be associated with soils prone to waterlogging, lower soil organic matter, and with seedlings with disease symptoms or that had died. Soils around seedling root zones were sampled and baited for Phytophthora in spring 2019. We did not find Phytophthora presence to relate to seedling health or survival; few relationships with soil factors could be demonstrated due to low detection rate. Oak seedling health and survival were better explained by soil variables related to water availability and limited root zone. This work was the first stage in a larger study to assess Phytophthora impacts and management in these riparian sites. Subsequent work will increase sampling to better quantify Phytophthora presence and focus on interactions between disease and water-related stresses in these sites.