April 15, 2017
Two years ago, my husband and I volunteered to help conduct a sudden oak death (SOD) blitz in Marin County. After training, we surveyed bay laurel trees on Belvedere Island as well as the Tiburon peninsula.
SOD is a plant disease introduced to our California landscapes in the 1980s on imported, infected ornamental plants, most likely rhododendrons and camellias. The scientific name is Phytophthora ramorum; this pathogen is related to Phytophthora infestans that caused the deadly Irish potato famine. SOD has killed more than 1 million California oak trees and infected more than 100 species of plants. The USDA list of SOD host plants and a symptom gallery can be viewed at suddenoakdeath.org. Scientists have discovered SOD is most often transmitted to oaks from nearby infected California bay laurels.
Every spring since 2008, trained volunteers conduct SOD blitzes tagging suspect bay laurel trees, recording GPS location and bagging five leaf samples to send the UC Berkeley Forest Pathology Lab. Transmission of the SOD pathogen occurs without wound to leaf or trunk when spores are dispersed aerially by wind or rain for short distances. Because early detection is pivotal to saving oaks in our landscape, the work of citizen scientists is important. Go to sodblitz.org to look at data and maps showing the spread of SOD and for 2017 calendar information.
In years with lots of rainfall and warmer temperatures, SOD infection rates go up. In 2016, more than 14,000 trees were surveyed and in Marin, the infection rate rose to 10.2 percent from 7.9 percent in 2015. Infected oaks bleed red sap from the trunk as SOD girdles the tree, severing it from nutrients and water and the oak suddenly dies. There is no cure for an infected oak.
The most commonly infected oak species include coast live oak, California black oak, Shreve oak, and canyon live oak. Tanoaks, not technically oaks because their flower differs and pollination is by insect not wind like oaks, are highly susceptible to lethal SOD infection. Scientists hope that by studying traits and genetic blueprints of species of white oaks that show signs of being less susceptible to SOD, they may find ways to help forests help themselves by making things more difficult for SOD. In forests, scientists are stream baiting to monitor the spread of SOD. Uninfected rhododendron leaves are bagged and placed in the stream for about two weeks The leaves are then lab tested and, if found infected with SOD, foresters remove tan oaks, bay laurels and other foliar hosts near the streams to see if it affects the spread of SOD.
If you’re a Marin resident concerned about SOD, the California Oak Mortality Task Force advises looking if bay laurel trees or tan oaks are present in your neighborhood near susceptible oaks. Preventative measures include doing yard work after the rainy season in late summer or fall, including pruning, grading, cutting and spraying a phosphonate fungicide annually to the oak trunk from the ground to 10 feet up in late fall. To help keep the oak healthy, amend the soil about 3 feet out from the trunk every two years with 3 to 5 pounds of granular gypsum (calcium).
Get involved as a citizen scientist. Funded by the US Forest Service and the UC Berkeley Pathology Lab, blitzes depend on citizen scientists to gather data. Oak trees are a keystone tree species in California, important because they provide habitat, shelter and food supply. While lots of research is still needed to determine the ecological impact of SOD, any massive die off of oaks would radically change our forest ecology while increasing the risk of wildfire. You can make a difference saving California oaks for their natural beauty, longevity and importance to a wide variety of environments including mountains, valleys, foothills, canyons and coastal islands.