Phytophthora Now Found in Drier Ecosystems of Marin County, CA: Is Chaparral Phytophthora’s Next Target?

Sudden Oak Death is a devastating plant disease in California that has killed many native trees. Phytophthora ramorum is the causal agent of SOD, belongs to the oomycetes (or ‘water molds’) and thrives in moist environments. Symptoms similar to those caused by P. ramorum were recently detected on manzanitas (genus Arctostaphylos) in chaparral communities in drier and sunnier areas in Marin County, CA. To better understand the pathogen’s spreading ability, it is important to determine if these similar symptoms seen in chaparral plants are indeed P. ramorum. Is this pathogen shifting toward these drier environments and if so, what are the potential consequences? During an internship from summer through fall of 2019 for the National Ornamentals Research Site at Dominican University of California (NORS-DUC), I monitored plant disease symptoms of chaparral and native plants of Marin County, CA, with a focus on manzanitas. Monitoring included data collection, photo-documentation, and leaf sample collection in natural environments throughout the county. Samples were then brought to NORS-DUC for an Immunostrip (Agdia) test that detects Phytophthora presence in the leaves. Surprisingly, 7 out of 48 samples collected were infected (15%). For further characterization, symptomatic leaf tissue was plated on selective media to isolate the plant pathogens. Although results are still preliminary, I was able to provide some evidence that Phytophthora sp. might spread to new environments and threaten the native plant community