If there were a color-coded threat-level indicator for plant diseases like the one developed by the Department of Homeland Security for the likelihood of terrorist attacks, Sudden Oak Death Syndrome (SOD) would rate at least orange, for "high risk." SOD, caused by the Phytophthora ramorum fungus, was first identified in California in 2000 in oak trees that had mysteriously died.
Since then, the disease has spread rapidly in coastal forests in California and to a few sites in southwestern Oregon, infecting not only oaks but also other hardwoods, shrubs, and conifers such as coast redwood and Douglas-fir.
What's worse, infected rhododendron, camelia, and other ornamental plants, shipped by West Coast nurseries have spread SOD to 173 sites in 22 states; it also has been found in British Columbia nurseries. This summer, a red oak tree on Long Island in New York was found to have SOD. Shipments of rhododendron plants from Oregon nurseries spread the disease to Connecticut earlier this year. By late October, most of the several thousand infected plants were sold before nursery owners could quarantine them and are untraceable. Connecticut forestry officials fear the disease could spread to the state's forests, where 20 percent of the trees are oaks.
So far, SOD has been found in natural forests in only California and Oregon.
"In California, it's had a big impact," said Katie Palmieri, public information officer for the California Oak Mortality Task Force at the University of California, Berkeley. "We've found it in 14 counties now-we added two counties just in this past year-and hundreds of thousands of trees have been infected."
In the Los Padres National Forest, in the Coast Range in central California, recent aerial surveys found about 8,000 acres where the disease had taken hold in an estimated 119,000 tanoak trees (Lithocarpus densiflorus). Aerial surveys can identify only trees with advanced infections, because SOD symptoms often are not readily apparent in newly infected trees.
The symptoms of the disease vary depending on the host species and may include bleeding from the main steam or trunk, wilted shoots, or a rapid change in foliage color from green to brown.
To date, 64 species and more than 40 genera are known to host SOD, according to Palmieri.
Eastern Forests at Risk
Pat Shea, SOD research coordinator for the USDA Forest Service's Pacific Southwest Research Station in Davis, California, said forests in the East and Southeast United States are at risk of SOD infection.
"We've tested red oak and pin oak, which are the two major oak species in the Southeast, and they are highly susceptible," said Shea. "Now that the disease has made its way to the East in the nursery systems, it is everyone's fear that it will spread into wildland areas."
In Europe, a form of SOD has been found on ornamental plants at more than 350 nurseries in England, The Netherlands, Germany, and eight other countries and has spread to beech, Holm oak, and horse chestnut trees.
Scientists at the Virginia Bioinformatics Institute and the Joint Genome Institute are comparing the DNA of P. ramorum with that of another Phytophthora that causes a disease in soybean plants to further understand SOD and explore potential routes to treatments.
SOD thrives in moist, cool environments. Infections are limited by warm, dry weather and may be limited by cold winter temperatures. It is spread via infected plants or plant tissue (including firewood and pruning debris), rainwater, and soil. Host species such as bay laurel transmit the fungus to other species. P. ramorum spores also can be transported by animals, humans, automobiles, chain saws, and other carriers.
Detecting the presence of the fungus in trees and shrubs is time-consuming: polymerase chain reaction or culture tests take up to 2 weeks. Shea says plant pathologists and other scientists are working to develop a simpler, equally accurate field test.
No treatment has been found for SOD in forest ecosystems. A fungicide, Agri-Fos, is effective in preventing healthy trees from becoming infected and slowing the advance of the disease it its early stages. When applied by a combination of spray and injection, said Shea, Agri-Fos is an effective treatment for individual, high-value trees but is not practical in forest stands.
Shea said Dave Rizzo, an associate professor in the Department of Plant Pathology at the University of California-Davis, offers sobering thoughts about the future of SOD.
"Dave Rizzo, one of our leading researchers, reminds us that it has been only 4 years since SOD was discovered," said Shea. "But you've got to remember that it was 50 years after chestnut blight was introduced [into the US] that it really took off."
The Oak Mortality Task Force's web site, www.suddenoakdeath.org, has a wealth of information about SOD, including best management practices for working in areas where SOD is present and decontamination techniques for preventing the spread of the disease. The Sudden Oak Death Science Symposium II will be held in Monterey, California, January 18-21, 2005. For information, visit http://nature.berkeley.edu/forestry/ sodsymposium.