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Invasive Species Spotlight: Sudden Oak Death (Phytophthora ramorum)

June 19, 2008

Sudden oak death (SOD), or ramorum blight, was first identified in the United States in 1995 in coastal forests in northern California. Since then it has spread to another relatively isolated area in southwest Oregon. It has also been identified in some areas of Europe; scientists are unclear as to the origins of this disease, which has been responsible for the death of thousands of western oak trees.

Forest with oak trees dying of sudden oak death. Photo courtesy of David Rizzo.

This disease is caused by Phytophthora ramorum, a “funguslike” organism that spreads by producing spores transported locally by wind and water or regionally through insect infestations and the sale and acquisition of ornamental trees and shrubs.

Some of the first symptoms of the presence of SOD (http://www.ppdl.purdue.edu/PPDL/pubs/SOD_pictorial_guide.pdf) include a dark red to black tarlike sap oozing from the tree’s bark, and wilting and dieback of foliage and stems. Often, new branches re-sprout on the dying tree in large numbers but eventually wilt and die, too. Once the tree is in this state of decline, it is further susceptible to other insect and environmental problems like bark beetles and drought. An infected tree can die in as little as 1 year from these compounded problems. Although SOD has been found on other plants (http://www.aphis.usda.gov/plant_health/plant_pest_info/pram/downloads/pdf_files/usdaprlist.pdf) like rhododendrons and viburnums, it has not shown to be as certainly fatal to them. Serving as hosts for the pathogen, these plants may suffer leaf spot and twig blight.

Two common oak trees in Illinois, northern red oak (Quercus rubra) and northern pin oak (Quercus paulustris), are known to be extremely susceptible to this disease, which could lead to severe problems for the state. Invasive fungal diseases can be very dangerous. Although this disease has not yet been found in Illinois, it is of great concern for two main reasons. It has the potential to eliminate hosts–much of eastern North America is covered with deciduous forests that include oaks and shrub species that serve as disease reservoirs. Secondly, this pathogen infects several shrub species that grow in the wild and in domestic settings. Ornamental shrubs grown horticulturally could carry the disease to uninfected areas.

Several other pests and diseases can cause the decline of oak trees (http://www.na.fs.fed.us/spfo/pubs/pest_al/sodeast/sodeast.htm), but not many of them that are potentially as troubling as sudden oak death. All suspect trees should be properly diagnosed by pest-management specialists.

For more information, stop by the Illinois CAPS blog (http://www.illinoiscapsprogram.blogspot.com) for all the latest news on invasive pests in Illinois.

(Kelly Estes and Mike Garrett)
Author: Nancy Pataky


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