More than one-third of the nearly 376,000 trees killed by sudden oak death disease along the North Coast are in Sonoma County, with a concentration around Guerneville and Jenner, according to a U.S. Forest Service report.
The aerial survey, conducted by Forest Service officials flying about 1,000 feet above the ground, mapped 375,700 newly dead trees in a coastal swath from southern Humboldt County to Sonoma County and stretching inland over parts of Lake, Napa, Yolo and Solano counties.
An estimated 136,918 of the dead trees, predominantly tanoaks, were in Sonoma County, said Zachary Health, one of the surveyors.
The areas around Guerneville and Jenner "had some of the highest levels of mortality," the report said.
Tanoak is one of three species common to Sonoma County, along with coast live oak and black oak, susceptible to sudden oak death.
The aerial survey, conducted on June 22, July 2 and July 5, reflected a stunning increase in sudden oak death's impact on the North Coast.
More than 315,000 dead tanoak were mapped over 45,000 acres within the survey area this year, compared with just 24,000 trees on 5,300 acres in the same area in the 2011 survey.
Overall, the survey covered 2.8 million acres in portions of the seven counties.
Sudden oak death, discovered in Marin County in 1995, infects 14 counties, from Humboldt to Monterey along the coast, as well as Lake, Napa, Solano, Contra Costa, Alameda and Santa Clara counties.
In Sonoma County, an estimated 105,000 acres are infected with Phytophthora ramorum, the pathogen that causes the disease.
Leaves of bay laurel trees host the pathogen but are not harmed by it, and experts believe that it spreads to vulnerable oaks and tanoaks during wet, windy spring weather.
Once infected, a tree may die quickly or within a year or two, and there is no cure.
The Forest Service conducts aerial surveys every year over California forests to document damage from diseases, pests and other hazards.
The North Coast survey noted extensive bear damage to redwoods in an area north of MacKerricher State Park on the Mendocino County coast.
(You can reach Staff Writer Guy Kovner at 521-5457 or guy.kovner@pressdemocrat.com.)
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