California forestry officials began to notice a significant decline in the Tanoak population in 1995. In 2000 scientists isolated and identified the cause of the decline to be a new disease in California called Sudden Oak Death. The disease was quickly confirmed in eight counties by 2001 and has now been confirmed to be present in southwest Oregon.
Sudden Oak Death is a canker disease that has been found to infect and kill several species of oak. The most susceptible oak species is the Tanoak. The disease has also been found to infect over 100 other plant species, but often with less significant consequences. Sudden Oak Death is an aggressive disease that will infect and kill healthy oak trees.
Sudden Oak Death is most active, and most likely to infect new trees, during periods of cool, moist weather. Spores of the disease may be windblown to healthy trees or may be present in the soil. Spore build-up may occur in an area where plants other than oak trees are infected with the disease. Whenever possible work on oak trees should be performed from June to October when the weather is generally drier and warmer.
A quarantine zone has been established due to the aggressive nature of this disease and the potential for damage to other areas of the country. Movement of regulated materials is allowed within the quarantined area, but movement outside of the quarantine zone is by permit only. The California counties quarantined due to Sudden Oak Death include Alameda, Contra Costa, Humboldt, Lake, Marin, Mendocino, Monterey, Napa, San Francisco, San Mateo, Santa Clara, Santa Cruz, Solano, and Sonoma counties.