UC Davis

Invasive Phytophthora species can potentially be destabilizing to whole ecosystems with detrimental effects on biodiversity and on ecosystem services. Studies have shown that Phytophthora species may be introduced into natural areas through outplanting of infested native plant nursery stock. The Angeles National Forest (ANF), located in Southern California, utilizes thousands of nursery-grown native plants for landscape restoration of heavily disturbed areas. Previous pathogen testing performed on ANF restoration sites detected several Phytophthora species. Little is known about the ecology and biology of Phytophthora species in drier regions of the world, thus a baseline of Phytophthora distribution and diversity is needed in ANF lands. Between 2018-2021 forty sites were selected, and soil samples were taken from plant rhizospheres, riverbeds and off-road vehicle tracks in chaparral and oak woodland areas. From these surveys, fourteen species of Phytophthora were detected, including three undescribed species and one hybrid species. Phytophthora species were found in both chaparral and oak woodland areas with a higher frequency in riparian areas. Selfing species (homothallic), capable of readily producing oospores, were more abundant in drier chaparral areas. Phytophthora species were also detected in off-road tracks, dirt trails, and riverbeds, indicating potential natural and anthropogenic-associated routes of dispersal. Pathogenicity tests were conducted to test theaggressiveness of detected Phytophthora species towards common chaparral plant species. Phytophthora cactorum, P. multivora, P. crassamura and P. chlamydospora, were all capable of causing disease on Adenostoma fasciculatum, Eriogonum fasciculatum, Salvia mellifera and Eriodictyon crassifolium. A. fasciculatum was determined to be the most susceptible plant species, especially to P. multivora and P. cactorum. Although the Angeles National Forest is among the driest and most fire prone areas in the United States, these Mediterranean areas harbor a large diversity of Phytophthora species indicating a potential risk for the native and endemic Californian chaparral vegetation. The long-term consequences of the presence of Phytophthora species in these locations still needs to be understood.