By Sharon Omahen
University of Georgia
Oak trees in California appear to be suddenly dying, but the disease that's striking them down isn't actually fast acting. University of Georgia researchers are working with other scientists in the Southeast to track the disease and make sure it doesn't head this way.
"Sudden oak death is actually a misnomer," said Jean Williams-Woodward, an extension plant pathologist with the UGA College of Agricultural and Environmental Sciences. "It isn't sudden and it doesn't just affect oak trees."
Actually takes three or more years
Sudden oak death was coined because a number of oak trees appeared to be suddenly dying in California, she said. The disease, caused by the fungus Phytophthora ramorum, actually takes three or more years to cause the death of a tree.
Tanoaks and other true oak species first began showing signs of the disease in 1995 in Marin County, California.
"It appeared as if one year the trees were green and the next they were brown," said Williams-Woodward. "It first struck tanoaks, which many people consider to be a weedy tree, so not many people were concerned."
Concern grew when the disease began to affect live oaks. The disease actually affects diverse plant species other than oaks including, but not limited to, Douglas fir, California bay laurel and camellia.
The disease causes dark, rust-colored cankers that seep at the tree's base. Sudden oak death has also been seen on understory plants such as rhododendron, causing leaf spots or scorch-like symptoms.
"It spreads to other plants when the spores are splashed by water from rain," said Williams-Woodward.
Quarantine in California
To help control the spread of the disease, 12 counties in California and one county in Oregon have been quarantined. Growers there cannot ship plants out unless they are declared disease-free.
"Despite this quarantine, there have been several outbreaks in Oregon, Washington and British Columbia," she said. "These outbreaks are believed to have originated from ornamental plants brought into the U.S. from Europe."
In an effort to keep the disease from entering the Southeastern states, Williams-Woodward and her counterparts in Tennessee, Virginia, North Carolina and South Carolina are surveying nurseries in their states. The survey is being conducted in cooperation with the USDA and the U.S. Forest Service.
"We are sampling high risk plants in high risk areas of each state," she said. "In Georgia, that's the northern part of the state above the fall line. This is the area that has the most oak and rhododendron forests."
The researchers are collecting samples of leaf spot and die back and checking the samples for the disease.
"So far, we have collected 500 samples from six nurseries in Georgia and we haven't detected the pathogen," she said. "Overall, across the five states, we have collected about 2,000 samples and haven't found the disease."
Can it survive in southern climates?
Williams-Woodward believes it's just a matter of time until sudden oak death arrives in the South. How it will fare is unknown.
"We don't know if it would even survive in Georgia," she said. "It prefers cooler, wetter climates so it may not be able to survive the hot, humid weather here."
Research on chemical control is being conducted by researchers in California. However, Williams-Woodward says fungicides are unlikely to truly control Sudden Oak Death.
"Limiting the spread of infected plants and soil from areas known to have the disease is still the best approach to controlling the disease," she said. "Quarantining infected nurseries is another control method."
For now, Williams-Woodward sees some potentially positive outcomes for Georgia growers.
"The California quarantine has opened up a market that could be filled by Georgia growers," she said. "Korea is refusing to accept ornamentals from California. Other countries may impose a similar quarantine. Georgia growers could be filling this need."
(Sharon Omahen is a news editor with the University of Georgia College of Agricultural and Environmental Sciences.)