Prolonged drought curbed the spread of sudden oak death in parts of Sonoma County and 12 other counties near the coast, but some locales — including west Sonoma County — remained hot spots for the disease that has ravaged the region’s woodlands since 1995.
A relatively arid spring deprived the sudden oak death pathogen of the wet winds it typically rides from host bay laurel trees to vulnerable species of oaks and tanoaks, enabling landowners to pinpoint and possibly eliminate the most persistent sources of the infection.
“If we had one more year of drought it would really be ideal,” said Matteo Garbelotto of the UC Berkeley Forest Pathology and Mycology Lab.
Garbelotto is not hoping for more drought, but he said the evidence is clear that sudden oak death infection rates dropped during both of the two multi-year drought cycles between 2008 and 2014.
In some cases, infected bay trees returned to health “because the pathogen dies,” he said.
Results of the seventh annual Sudden Oak Death Blitz organized by the UC Berkeley lab found both extremes in Sonoma County, where the disease has infested more than 105,000 acres.
In the Cloverdale area, where a new outbreak was confirmed last year, none of the 13 bay trees sampled had the disease this year, dropping from an 11.5 percent infection rate. “I would say it is there but we just didn’t find it,” Garbelotto said, noting the small number of samples.
East of Highway 101, the rate dropped slightly, from 26 percent last year to 23.3 percent of the 305 trees sampled in the blitz, conducted April 19-20 in Sonoma County.
West of the freeway, primarily in the hills from Sebastopol to the coast, the infection rate hit 33.1 percent this year, up from 18.3 percent in 2013.
More than 500 volunteers participated in the blitz, collecting leaves from 2,125 trees in 13 counties, with an overall infection rate of 15.2 percent, 3 percent lower than last year.
Discovered in Marin County in 1995, sudden oak death has killed more than 3 million tanoak and oak trees throughout 15 coastal counties from Monterey to Humboldt. Tan-oak is not a true oak, but is a member of the same botanical family, and could be wiped out in Sonoma County if the disease continues unabated, experts say.
Once infected, about 90 percent of oaks perish, Garbelotto said.
The shrinking rate of infection affords landowners an opportunity to employ one of the few defenses against oak loss: removal of bay trees that harbor but are not harmed by the infection.
Garbelotto singled out one such opportunity at the end of Bittner Lane west of Occidental, where a lone infected bay tree was found with no others nearby. “Now is a good time to think about getting rid of trees that test positive (for the pathogen),” he said.
That’s what nearly 600 homeowners in the Fountaingrove II subdivision are contemplating, while they continue a costly program of spraying hundreds of oaks, said Dennis Searles, president of the subdivision’s open space maintenance association.
Searles and two other residents collected samples from about 20 bay trees during the blitz this year, and four trees east of Crown Hill Drive tested positive. Residents have found up to 15 suspect bay trees, as well as black ooze — the telltale sign of infection — from the bark of some oaks.