From the Marin County Journal, By Richard Halstead
Scientists studying sudden oak death are watching the late spring rains with trepidation - since new research indicates that production of the spores that cause the virulent plant disease spike during warm, wet weather.
"That's why we're a little leery about what's going to happen this year," said David Rizzo, an associate professor of plant pathology at the University of California at Davis. Rizzo helped identify the pathogen that causes sudden oak death.
The results of sudden oak death research that Rizzo assisted with were published this month in the scientific journal Phytopatholgy. Rizzo and a team headed by U.S. Forest Service plant pathologist Jennifer Davidson measured production of the disease's spores at the Fairfield Osborn Preserve in Sonoma County during 2001, 2002 and 2003.
They found that concentrations of the spores in rainwater increased significantly toward the end of the rainy season in the spring of each of the two complete years that were monitored.
Spore production following late April rains in 2003 was 100 times greater than during the previous two years, the researchers reported. The larger amount of spores corresponded with more than a 10 percent increase in infection among coast live oak at the study site.
This pattern coincides with previous diebacks, Rizzo said. Sudden oak death leaped to the public consciousness by killing thousands of oak and tanoaks in 2000. The state had been deluged by El Nio rains two years before. That is about how long it takes for the disease to kill trees - "even though it is called sudden oak death," Rizzo said.
Due to the lag time, the effect of this spring's rains won't be seen for a couple of years. But the timing is right for increased mortality this summer due to the 2003 rains, Rizzo said.
During recent years, the disease seems to have been in remission in Marin, said Kent Julin, a forester with the Marin County Fire Department.
"We're not experiencing the same level of oak mortality that we had three or four years ago," Julin said.
But Julin said California bay trees in the county are continuing to die, and sudden oak disease has killed a large number of trees, mostly tanoaks, on Bolinas Ridge.
Rizzo said his research indicates that spore levels remain high in tanoak and redwood forests, which tend to be wetter than coast live oak forests, where spore production has fallen.
"Last year, it basically stopped raining in February," Rizzo said. "It got really hot and seemed to knock down levels of the pathogen quite a bit in the coast live oak forests. They haven't really recovered a whole heck of a lot."
The question now is how all this warm, spring rain is affecting spore production, Rizzo said. He and his research team have been gathering samples and should know within days if levels are rising.
"It's certainly a concern," Rizzo said.
