Home » Pest Alerts » Diseases
Pest Alerts
Sudden Oak Death
First identified in 1995 in Mill Valley, California, sudden oak death is becoming a major problem in western U.S. forests. The disease, which has so far killed tens of thousands of oaks and tanoaks in California and Oregon, is caused by a funguslike pathogen, Phytophthora ramorum. Scientists have yet to determine the pathogen's place of origin.
Since 1995, sudden oak death has been found in at least 12 counties in the coastal region of California and in one county in southwestern Oregon. It has also been identified on nursery stock in Washington State and numerous European countries. Because the disease also infects some commercial ornamental shrubs, such as rhododendrons and camellias, experts fear that the disease could spread through the nursery trade and threaten deciduous forest species in the East. (In 2002, the USDA set up a quarantine zone in California to regulate the interstate movement of known host species. See the USDA site for more details.)
Tanoak die-off caused by Phytophthora ramorum at Kent Lake, California. Photo courtesy Oregon Department of Agriculture.
At present, 38 woody plant species are known to be susceptible to Phytophthora ramorum. These include such natives as huckleberry (Vaccinium ovatum), bigleaf maple (Acer macrophyllum), manzanita (Arctostaphylos manzanita), California buckeye (Aesculus californica), and coast redwood (Sequoia sempervirens). Symptoms vary from plant to plant and may include leaf spots, needle and tip blight, shoot-tip dieback, and stem cankers.
Oaks and tanoaks are being hit the hardest. Infections in coast live oak (Quercus agrifolia), black oak (Q. kelloggii), Shreve oak (Q. parvula var. shrevei), and tanoak (Lithocarpus densiflorus) are lethal. Bleeding cankers form on trunks and stems, and girdling eventually takes place. Cankered trees can live up to three years, but once crown dieback begins, leaves change color and drop in a matter of weeks (hence the "sudden" in the common name).
Infection of most other susceptible species is less serious and mainly consists of nonlethal foliar and twig damage. However, these species do play an important ecological role—they facilitate the spread of the disease to the more vulnerable plants.
Since the symptoms of sudden oak death are nonspecific, they can be confused with those of other oak disorders such as oak wilt and oak decline. (See photos of some of these disorders on the USDA Forest Service web site.) Only a lab test can confirm the presence of the Phytophthora ramorum pathogen.
No biological or chemical controls have yet been developed for sudden oak death. So far the only way to stop the disease is to cut and burn infected plants.
Related Links
For more information about sudden oak death, visit the following web sites: