Sudden oak death is a disease of oak trees caused by an invasive plant pathogen, Phytophthora ramorum. It currently occurs in California counties from Monterey to Humboldt and in a small portion of southwest Oregon. It is estimated to have killed more than 1 million oak and tanoak trees during the last decade. In addition, more than 100 other plant species are susceptible to the pathogen, but most suffer only minor damage limited to leaf spots or twig dieback.
Sudden oak death (SOD) kills California oaks. There is no known cure. But SOD prevention is easy and effective to keep your oaks safe. In oak trees, sudden oak death causes bleeding cankers that girdle trunks, inviting secondary pests, such as Hypoxolon canker and wood boring beetle infestations.
Although Sudden oak death is a forest disease, it is common in urban-wildland interface areas -- places where development meets or intermingles with undeveloped wildland -- and can present many challenges for residential landscapes. Management options include treatment with phosphonate compounds and selective plant removal.
Once Sudden oak death infects oak trees, there is no known way to cure them. Therefore, most of the management practices discussed below are directed at preventing the spread of the disease to new plants or areas and protecting susceptible trees before they are infected.
One phosphonate fungicide, Agri-Fos, is registered as a preventative treatment for P. ramorum for use on individual, high-value tanoak and oak trees. Treatment is not recommended in areas where infested plants are not already present. This treatment is not a cure, but it can help protect trees from infection and suppress disease progression in very early infections. The phosphonate compound can be injected or mixed with a surfactant and sprayed on the trunk for absorption through the bark. Booster treatments need to be made every 1 to 2 years.
Since the treatment must be made to healthy trees and the pathogen's distribution and activity is patchy and somewhat unpredictable, it is difficult to determine which trees need treatment. Generally, you should consider treating healthy, high-value oak or tanoak trees within 150 feet of other infested plants. You also might want to treat healthy, high-value oaks or tanoaks if they are surrounded by healthy California bay laurel.
Best Indoor/Outdoor Pest Solutions president, Randy Williams, has been trained and certified as a professional applicator by the Sudden Oak Death Mortality Task Force on October 31, 2012, and is listed by COMTF as a trained provider.
Preceding information taken from the California Oak Mortality Task Force.