The disease is believed to be caused by a new species of the fungus Phytophthora, which invades susceptible trees through the bark, killing portions of the tree and weakening it so that insects and other fungi can invade. If you visit diseased areas in California, wash mud or soil from your vehicle and shoes before traveling to disease-free areas. This includes mountain bikes ridden in areas with the disease. If you see an oak or tanoak tree with symptoms of SOD, report it to the Oregon Department of Agriculture or Oregon Department of Forestry immediately. In young tanoaks, the first symptom may be a wilting and drooping of all new growth.
Surveys are underway in Southern Oregon to confirm that the state remains free of sudden oak death.