Bruce McConnell plucked a leaf from a scruffy little bay tree on a steep, rocky Fountaingrove hillside overlooking the Santa Rosa Plain.
"Here's your clue," he said, pointing to a brownish stain at the edge of the leaf, a telltale sign of the as yet unstoppable disease that is ravaging Sonoma County's woodlands.
Sudden oak death, discovered in Marin County in 1995, has now killed about 257,000 tanoak and oak trees in 14 counties from Monterey to Humboldt, leaving property owners grasping for ways to at least slow the pathogen's onslaught.
"These trees need our help," said McConnell, who can see the bay tree from his home on Shillingford Drive in the Fountaingrove II subdivision.
Nearly 600 homeowners in the upscale development are spending $8,000 a year to spray the trunks of 400 large oaks surrounding their homes with a chemical considered the best defense, but by no means a cure, for the disease.
At least five oaks in Fountaingrove's 200 acres of open space are infected by the pathogen that 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.
"Once your trees become infected, there is no hope — and the danger of the trees or their limbs suddenly crashing onto your home or loved ones becomes a frightening reality," warns the Fountaingrove II Open Space Maintenance Association's website.
A dry spring curbed the rate of infection documented by the sixth annual Sudden Oak Death Blitz organized by UC Berkeley's Forest Pathology Laboratory.
More than 400 volunteers in 16 greater Bay Area communities collected leaves from 2,020 trees, enabling Matteo Garbelotto's lab to calculate an overall 7.38 percent rate of infection.
In Sonoma County, McConnell was one of 46 volunteers who sampled 193 trees, which were determined to have a 5.57 percent infection rate, eighth-lowest among the communities ranging from Mendocino County to San Francisco and the East Bay, and from the Peninsula down to San Luis Obispo.
Rates in other areas ranged from 53.4 percent in the South Skyline area west of Saratoga to zero percent in Atherton, the South East Bay-Sunol area and San Luis Obispo.
In nine areas, the reported rate of infection was lower this year than in 2012, including an 11.8 percent drop in Sonoma County. Santa Rosa posted its second-driest January to June period since 1932, with just 6.71 inches of rain, about one-third of the average for those six months.
Sudden oak death outbreaks "follow the weather patterns," Garbelotto said, noting that infection rates also dropped during two dry years prior to 2011.
A dry spring like 2013 makes a particular impact, he said, since the sudden oak death microbe, Phytophthora ramorum, is most active from March to June.
As soon as timely wet weather returns, the infection will "pick up rapidly," Garbelotto said.
"Sudden oak death is here to stay," he said, adding that it has infected only 12 percent of the susceptible habitat.
More than 105,000 acres in Sonoma County are infected, according to aerial surveys by the U.S. Forest Service.
The blitz, conducted in the county in June, confirmed a sudden oak death outbreak near Cloverdale, a first report of the disease in the Carneros area, and continued large outbreaks in the hills around Santa Rosa, the slopes of the Valley of the Moon and the hills west of Sebastopol.