PORTLAND, OREGON--Humans may be important vectors for sudden oak death, according to new research, raising concerns that the disease ravaging California's oak woodlands may be difficult to control. But a study by another research group has yielded some potentially good news: Fire may limit the spread of the pathogen. Both findings were presented here on 4 August at the annual meeting of the Ecological Society of America.
Since 1994, thousands of oak trees have succumbed to sudden oak death in 14 counties in California and Oregon. The pathogen, the water mold Phytophthora ramorum, has infected more than 25 plant species in the region. Researchers have been alarmed at the prospect of losing California's millions of oaks and of the disease spreading to the vast oak forests of eastern North America and elsewhere. Some had floated the idea that fire might stop the pathogen.
Max Moritz of the University of California, Berkeley, and Dennis Odion of the University of California, Santa Barbara, mapped all available data on the pathogen's presence at different sites in California, as well as historical data on forest fires. When they overlaid the maps, they discovered that the disease was much less prevalent in areas that had burned since 1950. "You almost never see infections in areas that have previously burned," says Moritz. "It's exceedingly rare, which is remarkable." The reason could be that plant defenses against pathogens gradually weaken after a fire, Moritz and Odion speculate--for instance, from stiffer competition among plants as they grow, decreased chemical defenses as they age, or depletion of soil nutrients.The findings suggest that California's fight against forest fires over many decades may have precipitated or accelerated the outbreak, and that perhaps controlled burning could be used to halt it, the authors say. They suggest conducting experiments to test that hypothesis.Meanwhile, John Hall Cushman of Sonoma State University in Rohnert Park, California, showed that soil samples taken along hiking trails have much higher rates of P. ramorum spores than those taken away from the trails, and that plants in areas with high visitation rates, such as public parks, had greater infection rates than those on sites where access is restricted. That suggests humans may spread the disease, says Cushman, who calls the results "exciting and discouraging" at the same time. In theory, the disease could be halted by limiting people's access to unaffected areas, but that would be a very unpopular policy and perhaps impossible to implement, he says.The rapid spread of sudden oak death is "such a dynamic system that a lot of our tools in ecology for understanding and predicting patterns are inadequate," says Rick Ostfeld of the Institute of Ecosystem Studies in Millbrook, New York. That's why the findings on fire and on humans as vectors, he adds, are "both interesting and important" for stopping the disease.