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R-21-63 Meeting 21-14 May 12, 2021 AGENDA ITEM 8 AGENDA ITEM

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R-21-63 Meeting 21-14 May 12, 2021 AGENDA ITEM 8 AGENDA ITEM R-21-63 Meeting 21-14 May 12, 2021 AGENDA ITEM 8 AGENDA ITEM Oregon State University Phytophthora Research: Assessment of Phytophthora Soilborne Pathogens at Midpeninsula Regional Open Space District Restoration Sites GENERAL MANAGER’S RECOMMENDATIONS Receive Oregon State University’s Phytophthora Research Presentation on the Assessment of Phytophthora Soilborne Pathogens at Restoration Sites. Provide feedback on recommendations and next steps. No formal Board action necessary. SUMMARY Invasive plant pathogens in the genus Phytophthora cause significant economic and ecological damage to horticultural and agricultural industries, and native wildlands. Phytophthora species can cause negative impacts across many different plant families in a variety of native habitats when introduced into the wildlands. These species have been introduced into California wildlands via infected native plant nursery stock and through other disturbances such as soil importation. Oregon State University’s (OSU) Dr. Jennifer Parke and Dr. Ebba Peterson, have completed their research on Phytophthora in Midpeninsula Regional Open Space District (District) lands and will provide key findings and recommendations for future management actions to minimize the risk of impacts to the natural environment caused by Phytophthora. Background Soilborne Phytophthoras are a group of water molds that infect plants. There are over 150 described Phytophthora species, including the sudden oak death pathogen (Phytophthora ramorum; SOD). Although not known for certain, most experts believe that some types of Phytophthoras are native to California. They spread via spores in water, soil, or plant debris; some species are airborne. In 2004, the District began to address SOD through staff trainings, use of Best Management Practices (BMPs), updating contract language, SOD Blitzes (coordinated with the University of California, Berkeley), and by supporting research on District lands and other locations. Since 2012, numerous species of soilborne Phytophthoras have been identified in native plant nurseries and revegetation areas of California. Some were inadvertently introduced into District preserves through use of infected nursery stock and other disturbances. Starting in 2014, District staff ceased all native plant nursery stock installation in District Preserves for several years after learning about these plant pathogens and the destruction they are capable of causing at restoration sites and wildlands. District Natural Resources (NR) staff have Rev. 3/15/21 R-21-63 Page 2 worked closely with native plant suppliers to ensure plants received for revegetation projects are grown utilizing phytosanitary BMPs and are tested for Phytophthora prior to being planted in District Preserves. Beginning in 2017, nursery plants grown with these BMPs that test negative have been installed at revegetation sites following the District’s “Guidelines for Minimizing Phytophthora Contamination” BMPs (Attachment 2). These BMPs were compiled by NR staff specifically for planting projects to protect sensitive sites and prevent movement of Phytophthoras from known contaminated sites. In 2016, spot sampling and testing completed by Phytosphere Research (a District Consultant) identified several soilborne Phytophthora species in several District Preserves, including the Skyline Ridge Tree Farm in Skyline Ridge Open Space Preserve (OSP), and the Mount Umunhum Summit and Bald Mountain parking lot in Sierra Azul OSP. To determine the presence and distribution of soilborne Phytophthora species at all District revegetation sites previously planted with nursery stock, NR staff released a Request for Proposals to twelve researchers and consultants to sample and test for Phytophthora species and determine what features may influence establishment and spread of these pathogens. On June 28, 2017, the District’s Board of Directors (Board) authorized an agreement with OSU to test District revegetation sites for soil diseases (R-17-85) using two methods: soil baiting1 to isolate Phytophthora into culture, and the detection of Phytophthora DNA directly from the soil. DISCUSSION In December 2017 and 2018, OSU sampled and tested sites in 10 preserves. Sites were classified into one of four categories: 1) revegetation sites previously planted with nursery stock between 1993-2014, 2) recent revegetation sites previously planted with nursery stock between 2017-2018, 3) sites where future revegetation projects are planned, and 4) disturbed but non-remediated sites where no revegetation projects are planned. The objectives of the OSU sampling, testing, analysis, and research were to: 1. Determine the presence and distribution of soilborne Phytophthora pathogens in representative revegetation sites on District preserves. 2. Identify conditions allowing for the designation of existing and future planting sites as either a high or low risk of Phytophthora introduction and establishment. 3. Provide recommendations for management of sites with Phytophthora contamination, and protective actions for uncontaminated sites. Presence and Distribution of Soilborne Phytophthora Pathogens Twenty (20) species of Phytophthora were cultured from District samples, including one provisionally new species (P. aff. ilicis2), one species recently discovered in California but not known to spread from restoration plantings (P. taxon asparagi), and one species that had not been detected in North America previously (P. boehmeriae). Some sites had a large number of taxa that were detected with DNA sequencing, despite having a low number of species detected with baiting. Importantly, this method found DNA of some pathogenic species of concern, which were not detected by baiting. Such DNA-only detections indicate either that the 1 Baiting is a method which uses a growing medium, typically a pear, that Phytophthora present in the soil infect and grow on. 2 Aff. (Latin: affinis) literally means “related to” and is used in scientific name for species that have not officially described in the scientific literature. In this case, the new species is closely related to P. ilicis. R-21-63 Page 3 Phytophthora sp. is present but could not be baited from these sites, or it may be remnant DNA from introductions of the pathogen that did not persist. Risk of Phytophthora Introduction and Establishment Several Phytophthora species were detected on District Preserves that are known to cause severe disease elsewhere and are considered high risk to native habitats. For many of the other Phytophthora species found, the amount of damage to native habitats is fairly unknown. Unlike SOD (a species that spreads via air, i.e., ‘airborne’), the majority of soilborne Phytophthoras are not known to cause widespread plant death. However, many of these species, including many detected in OSU's sampling and analysis, are likely contributing to plant health decline associated with other environmental stressors. Some are likely native species thought to cause minor or sporadic disease, and other species can cause severe symptoms in nursery settings but are less likely to persist in native habitats. Given the different risks posed by each Phytophthora, OSU ranked each project area taking into account not only its total diversity, but also the potential for each detected species to cause harm. The risk ratings of each Phytophthora (low to very high risk of causing disease on District lands) were based on published literature and best professional judgement. These rankings can be used to create management recommendations for preventing the introduction of Phytophthora into District Preserves, containing Phytophthora species in heavily infested sites, and protecting less infected sites with a lower diversity of Phytophthora species. Each Phytophthora was categorized into four risk classes (very high, high, moderate, or low) based on its ability to cause disease. Project areas with the highest infestation scores were at Pulgas Ridge OSP at the Blue Oak parking lot and forest site, the Mindego Gateway parking lot in Russian Ridge OSP, Skyline Ridge Tree Farm in Skyline Ridge OSP, and the La Honda Creek OSP site near the Event Center. Project areas with the lowest risk infestation scores were in El Corte de Madera OSP, Sierra Azul OSP, and Bear Creek Redwoods OSP. There are Phytophthora species present at the lower risk sites, however these may be less pathogenic and less diverse. In general, OSU found that sites with hardwood, chaparral, and shrub vegetation had higher infestation scores, as did riparian areas. These sites tended to have a greater proportion of samples in which Phytophthora was detected, and greater diversity of high to very high-risk species. Rocky sites and those with limited forb cover had the lowest infestation scores, as did Douglas-fir or redwood dominant habitats when not adjacent to streams. These areas had lower species diversity; when higher-risk species were detected they were often limited to prior revegetation project sites. This could indicate that these sites have plant genera or other site factors that limit Phytophthora establishment. Recommendations for Management OSU developed three main methods (outlined below) to prevent further spread of Phytophthoras and reduce their impacts in District preserves and wildlands: 1. Exclusion and Prevention Prevention of introducing Phytophthora species into District Preserves should be a top priority. Risk for introduction can be greatly reduced by: • minimizing importation or movement of soils; • using direct
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