Dozens of volunteers are expected to comb Sonoma County woodlands in two weeks looking for the telltale signs of a tree killer.
They will be hunting for discolored leaves on bay laurels, evidence that those trees harbor the sudden oak death pathogen, which has infected more than 105,000 acres in the county.
Sudden oak death, discovered in Marin County in 1995, has killed more than 3 million tanoak and oak trees in 15 counties from from Monterey to Humboldt, according to UC Berkeley's Forest Pathology Laboratory.
Taking advantage of what is expected to be another dry spring, the lab is focusing its seventh annual Sudden Oak Death Blitz on pinpointing the bay trees that are “reservoirs” of the pathogen, a fungus-like microbe called Phytophthora ramorum.
“We can only determine if there are reservoir trees over a few dry years,” said Lisa Bell, Sonoma County's sudden oak death program coordinator.
Removing those trees could curb the spread of the pathogen, which has infected only 12 percent of the susceptible habitat, said Matteo Garbelotto, head of the Berkeley lab.
The SOD Blitz in Sonoma County will be conducted April 19 and 20 and results will be posted in the fall on the SODMAP, a Google Earth map with overlays that currently shows the results of blitzes each year from 2008 to 2013.
Volunteers in last year's blitz tagged about 400 bay trees in 16 Bay Area communities, including Sonoma County, with the intention of checking them again this year, Bell said.
Last year, with just 6.71 inches of rain from January to June in Santa Rosa, the blitz found an 11.8 percent drop in Sonoma County's rate of sudden oak death infection, compared with 2012.
Santa Rosa has received about double that amount since Jan. 1 this year, but more than 9 inches fell in February, while just 2.38 inches fell in March.
Experts believe the pathogen rides wet spring winds from unharmed host bay trees to four types of oak trees — including coast live oak and black oak common to Sonoma County — as well as tanoaks, which are not a true oak.