Sudden Oak Death (SOD), a serious exotic tree disease, threatens the survival of tanoak and several oak species in California. Currently, SOD infects trees in 14 coastal California counties, from Monterey to Humboldt. The disease, which was estimated to have killed over 50 million oaks and tanoaks over two decades, has changed the coastal forests composition in Northern California and Southern Oregon.
Though SOD occurs in patches, the overall infection area continues to grow with each passing year. Researchers had previously discovered that Phytophthora ramorum, the pathogen that causes SOD, most often spreads on the leaves of infected California bay laurel and tanoak. Forest managers can use proactive methods for controlling the disease—including sanitation, chemical treatments, and the targeted removal of bay trees—but such tools are preemptive in nature, only useful before oaks and tanoaks are infected. Timely detection of the disease in these species is therefore critical to slowing the epidemic.
With this in mind, UC Berkeley joined with over 30 local organizers in 2019 to assemble 25 SOD Blitzes staffed by trained volunteers. More than 400 volunteers were taught to identify SOD symptoms and to carefully collect symptomatic leaves from California bay laurels and tanoaks. Armed with this training, the teams surveyed 16,227 trees across 16 California counties, collecting approximately 9,000 leaves from 1,732 symptomatic trees. Samples were sent to the UC Berkeley Forest Pathology and Mycology Laboratory for processing and analysis.