Sudden oak death, caused by the pathogen Phytophthora ramorum, is a potentially devastating disease. P. ramorum has been found in nurseries in California, Oregon, Washington, and British Columbia, and more recently in the eastern United States. Sudden oak death causes leaf and shoot blights in most hosts, and is known as Ramorum shoot and leaf blight in the nursery industry. In the forest the fungus can cause bole cankers and tree death. Sudden oak death has not been found in forests of the eastern United States nor has the pathogen, P. ramorum escaped in the eastern United States. The impact this disease could have on the southern Appalachians is unknown but it has the potential to severely alter the biodiversity and decimate the populations of oaks, one of the most abundant groups of trees in the eastern United States. The disease, first found in coastal California in 1995, was thought to be restricted to California but it was discovered in Curry County, Oregon in 2001, and in April 2004, P. ramorum was detected in 14 nurseries in Georgia. In the eastern United States, it has also been detected in Florida, and has recently been found in nurseries in over 20 states. Whether the disease has spread beyond the nurseries is as of yet undetermined.
Figure 1. Under the bark of infected trees, necrotic tissue surrounded by black zone lines is characteristic of sudden oak death. Photo by: Joseph O'Brien, courtesy of forestryimages.org
Sudden oak death causes cankers on the stems of oaks (Fig. 1). Trees often bleed reddish or black ooze from the cankers (Fig. 2). Under the bark of infected trees, necrotic tissue surrounded by black zone lines often can be found (USDA - Forest Service 2002). Cankered trees may survive for one to several seasons. Once crown dieback begins, leaves turn brown within a few weeks. The disease usually does not spread from oak to oak since it rarely produces spores from infected bole cankers. If, however, the host is also a shoot and leaf host, spores will be produced on these infections. However, the pathogen infects many species of shrubs. Two common genera for host plants are Rhododendron spp. (common in the Appalachians) and Camellia spp. (USDA - APHIS 2004). These shrubs are not killed, but spores are readily produced on infected leaves (Fig 3), which display spotting and blighting symptoms. Another common symptom is twig dieback.
Adapted for eXtension.com by Thomas DeGomez, University of Arizona
USDA - APHIS. 2004. Phytophthora ramorum (Pr) also known as Sudden Oak Death, ramorum leaf blight and ramorum dieback. http://www.aphis.usda.gov/ppq/ispm/sod/. [Date accessed: November 10, 2010]
USDA - Forest Service. 2002. Sudden Oak Death, Oak Mortality is caused by a new pathogen, Phytophthora ramorum. USDA - Forest Service, State and Private Forestry Northeastern Area. NA-PR-02-02. 2 p.
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