Safety, Sanitation, Pathogens, and Pests

Friday, October 21 at 3:00-4:40 pm, Cedar Room

Session Description: The susceptibility and resistance of native species will be evaluated through a combination of ecological techniques, population genetics, genomics, and management. This session will focus on pathogens and pests in horticulture and those currently threatening native California flora.

Session Chairs: Diana Benner (Phytophthora Working Group/The Watershed Nursery, Richmond, Ca, USA) and Susan Frankel (USFS, CA, USA)

30.1    Saving an Endangered Species from Extinction due to Phytophthora

Betty Young (CNPS Milo Baker Chapter, CNPS Healthy Nurseries and Habitats Committee, Sonoma, CA, USA)

Phytophthora was verified in late 2019 at the CNPS-owned Vine Hill preserve containing the last remaining wild population of Arctostaphylos densiflora, a CNPS and State listed 1B Federal C2 species. In early 2020 plant pathologists verified that the whole 1.6 acre site was infected. What are we doing to save this species? What does this work and genetic testing of each manzanita at the site tell us about how Phytophthora spread through the site? 

Our ongoing work has started to grapple with these dilemmas. Since the initial determination of Phytophthora and obtaining CDFWS Collecting permit, we have propagated clones and treated the site to slow the spread of Phytophthora. Genetic testing disclosed that there are 19 genetic lines of A. densiflora. Original propagation captured 3 lines; additional propagation has been undertaken to capture the other 16 genetic lines. We are completing agreements to plant clones at two other Sonoma County preserves with similar soil type. CNPS has just hired a botanist to write a Recovery Plan.The ongoing question, is this another site where infected nursery plants were planted or was the site infected when CNPS bought the property or have the many botanists that have visited the site brought the disease with them?

30.2    Growing the Extra Mile: The SFPUC Sunol Native Nursery

Mia Ingolia (San Francisco Public Utilities Commission, San Francisco, CA, USA)

After the San Francisco Public Utilities Commission unintentionally introduced Phytophthora spp. into their habitat restoration sites in 2014, they discontinued the use of container plants at all sites across their 60,000+ acres of watershed lands. Recognizing the difficulty associated with direct seeding and an on-going need for plants in restoration, conservation, and landscaping projects, the SFPUC designed and constructed a state-of-the-art nursery facility using the best management practices established by the Phytophthoras in Native Habitats Work Group. This talk will outline how the best management practices were incorporated into the construction and operations of the facility and talk about the challenges and successes at the nursery during its four years of operations.

30.3    Ongoing Phytophthora Risk Management in the Golden Gate Parks Nurseries: New Considerations and Strategic Resilience

Martine Glaros (Golden Gate National Parks Conservancy, San Francisco, CA, USA)

Since 2015, the Golden Gate National Parks Nurseries have undergone a deep transformation to prevent the spread of Phytophthora via nursery container stock. The journey from near ignorance about Phytophthora pathogens, to implementation of extensive phytosanitary protocols was a tremendous, resource-intensive effort that resulted in measurable success. A key barometer for this success is data from in-house testing, in which the nurseries, over the last five years, have had no Phytophthora detections in our container plants, with the one exception of a positive Phytophthora detection in 2020 on Fragaria vesca (wild strawberry) cuttings originating from a contaminated field source.

Into the present day, the Park Nurseries’ relationship with Phytophthora prevention is ever evolving. Here we address the questions, challenges, and decision-making we continue to face in the realm of phytosanitation, especially in an environment of shifting resources. Downstream effects from the Covid-19 pandemic presented us with both difficulties and opportunities to consider our methodologies. How have we continued to assess the effectiveness of our best management practices in alignment with science? Having created thorough and resource-intensive practices, how have we adapted our protocols to limited resources? This latter question has become especially relevant to Phytophthora testing which is both a critical indicator that we rely on to ensure the effectiveness of our BMPs, and is also an extremely labor-intensive process. We will share our story as an ongoing case study, and as a call to continued conversations about the deeper questions confronting nurseries pursuing Phytophthora risk-management.

30.4    Phytophthora species surveys of burnt areas of Angeles national forest reveal species-specific patterns of distribution and variability of pathogenicity towards common native chaparral

Sebastian Fajardo (UC Davis, US Forest Service, National Fish and Wildlife Foundation, Becas Chile (Chilean scholarship), Davis, CA, USA)

The oomycete genus Phytophthora, belonging to the family Peronosporaceae and kingdom Straminipila are amongst the most highly damaging invasive plant pathogens in the world. In the attempt to renew and restore degraded habitats through the practice of restoration, Phytophthora species could potentially be introduced into natural ecosystems by unintentionally outplanting infested plant stock. Between 2018-2021 Phytophthora surveys were done in areas of the Angeles National Forest, Southern California, to assess Phytophthora distribution in areas adjacent to restored areas from which Phytophthora spp. were previously detected. From these surveys 15 species of Phytophthora were associated with chaparral-grassland and oak woodland areas, many areas with a historical fire past. Common associated host in these areas were Adenostoma fasciculatum, Quercus agrifolia and Salix spp. Detected Phytophthora spp. were found in both upland and riverbed areas, and strongly associated with riparian areas of both chaparral and oak woodland areas. Phytophthora spp. were also encountered in water ways and off-road tracks and trails, indicating a potential natural and anthropogenic-associated routes of dispersal. Pathogenicity tests were performed using the encountered Phytophthora species on common native chaparral plants, which identified A. fasciculatum as a highly susceptible host. These tests have also indicated that P. crassamura, P. cactorum and P. multivora are all capable of infecting Eriodictyon crassifolium, A. fasciculatum, S. mellifera and Eriogonum fasciculatum and Q. agrifolia. Thus, a high risk for disease exists for the plant hosts associated with the presence of Phytophthora species. Phytophthora species can infect common Southern California chaparral plants and potentially disperse to adjacent areas through water ways and soil movement. Phytophthora spp. also have resistant structures (oogonia), that allow survival under dry and arid conditions.

30.5    Phytophthora damage on California native plants: Identification, monitoring and management

David Rizzo (Department of Plant Pathology, University of California-Davis, Davis, CA, USA) 

Over the past decade, California native plant nursery stock has become recognized as commonly harboring Phytophthora pathogens. When outplanted, infected nursery plants may inadvertently lead to new infestations in restoration areas or landscapes, causing detrimental changes to many plant communities, including those with rare plants. Symptoms and damage from Phytophthora species can be difficult to distinguish from other disease and pest injury, from changes in plant appearance that occur during seasonal drought. Dieback and mortality on trees and shrubs will be discussed with an emphasis on patterns and conditions most common for Phytophthora development. Sampling techniques required for laboratory identification will be explained. Approaches for monitoring disease spread in restoration areas will be discussed using case studies from southern and northern California. The potential for pathogen spread on wild-collected seed will also be discussed. Other management approaches, including the shortcomings of chemical treatments, will be presented.