California’s 2017 citizen scientist-based sudden oak death surveys (SOD Blitzes) revealed that urban areas and popular tourist destinations are experiencing sharp increases in the presence of Phytophthora ramorum (the pathogen known to cause SOD) as a result of the previous two wet winters. Blitz efforts this year documented a three-fold increase in overall infection rates in those areas sampled since the drought ended in 2015, with 13% of samples found positive (the highest to date since the blitzes began in 2008).
Urban areas of the San Francisco Peninsula (including the western part of Redwood City) and East Bay have had significant increases in pathogen levels from previous years, putting oaks at high risk for infection for the first time. The pathogen is also now established in the Carmel Valley, with multiple confirmations in valley floor urban areas and sporadic locations on the drier northern side of the valley. Sonoma County has had an increase in urban and rural outbreaks, with the disease reemerging near Cloverdale and found to be at epidemic levels east of Healdsburg, near Santa Rosa and Glen Ellen. In southern Sonoma County, the pathogen has become established for the first time in the more rural areas west and east of Petaluma. Western San Mateo County also has increased pathogen levels.
Several popular public destinations have been found to have substantial infestations, including the Point Reyes National Park Visitor Center near Point Reyes Station, the San Francisco Presidio, the UC Berkeley Campus and Botanical Gardens, and the UC Santa Cruz (UCSC) Arboretum. In the Presidio, 10 positive sites were found in 2 distinct areas of the park – the southeastern corner and the northern boundary. At the UCSC Arboretum, four manzanita (Arctostaphylos) species, including two rare species, were severely affected by SOD and had extensive dieback.
“2017 Blitz results remind us that climate change and the actions of people play an important role in pathogen spread and establishment. Pathogen colonization in rural locations, highly visited places, and urban areas mean that sites previously thought to be peripheral to SOD will likely have to face disease impacts and management decisions. Once regarded as a marginal host, extensive manzanita dieback or mortality suggests that climate and changing conditions may result in evolving host impacts. Implementing preventative strategies in high-risk areas as well as policies at popular tourist sites that minimize the risk of spreading SOD long distances are essential steps in helping to safeguard uninfested areas. Novel strategies and a strong educational effort need to be deployed if we are to successfully address the ever-increasing intertwining of people and this pathogen,” said Matteo Garbelotto, UC Berkeley Forest Pathology and Mycology Cooperative Extension Specialist and Adjunct Professor and SOD Blitz founder.
Blitz findings also determined San Luis Obispo County is still uninfested, confirming that that the 2016 positive detections in the county were false positives. The determination was made after laboratory analysis of 2017 survey samples revealed no infection after using two different DNA assay tests, DNA sequencing, and culturing for the pathogen.
“The value of SOD Blitzes is the fact that surveys are repeated year after year. Repeated sampling facilitates use of the data for modeling disease spread as well as to confirm or debunk first reports, as has occurred with San Luis Obispo County data,” continued Garbelotto.
Celebrating its 10th anniversary, the 2017 SOD Blitz was the largest to date in terms of area covered and was the first to include three tribal land surveys. An estimated 300 volunteers surveyed nearly 15,000 trees and submitted leaf samples from approximately 2,000 symptomatic trees to the Garbelotto lab for pathogen testing. Counties surveyed included Siskiyou, Trinity, Humboldt, Mendocino, Lake, Sonoma, Napa, Solano, Marin, San Francisco, Alameda, Contra Costa, San Mateo, Santa Clara, Santa Cruz, Monterey, and San Luis Obispo.
“Blitz results this year are likely to be amplified in 2018, when we expect to find increased infection levels resulting from ‘young’ infections in 2017 that will have matured enough to be recovered in 2018. Anticipating what that could mean for next year, we hope to motivate more volunteers than ever to participate in the 2018 Blitzes!” concluded Garbelotto.
SOD Blitz Workshops are being offered this fall from 10/25 to 11/17 to discuss Blitz results as well as new recommendations for protecting oaks. Workshops are intended for the general public, tree care professionals, and land managers (see www.sodblitz.org for details). Data collected from the Blitzes (both positive and negative samples) have also be uploaded to the SOD Blitz map (www.sodblitz.org ) as well as to SODmap (www.SODmap.org) and the free SODmap mobile app, which can serve as an informative management tool for those in impacted communities
The SOD Blitz surveys were made possible thanks to funding from the US Forest Service State and Private Forestry, the National Science Foundation, the Gordon and Betty Moore Foundation, and the PG&E Foundation. The Blitzes are organized by the UC Berkeley Garbelotto lab in collaboration with government and non-government organizations, including the National Park Service, Presidio Trust, San Francisco Public Utility Commission, Midpeninsula Regional Open Space District, Save Mount Diablo, Land Trust of Santa Cruz County, East Bay Regional Parks, Santa Lucia Conservancy, Sonoma State University, UC Santa Cruz Arboretum, Los Padres National Forest, City and County of San Francisco Department of Recreation and Parks, UC Berkeley Botanical Garden, and California Native Plant Society.