A recently completed survey shows 376,000 dead oak trees across the coastal regions affected by Sudden Oak Death, a pathogen that develops on host plants ranging from the bay laurel to ornamental rhododendrons.

Last year’s survey, like this one aided by volunteers led by scientists at UC Berkeley and the California Oak Mortality Task Force, found 38,000 dead trees across a much smaller area.

“It’s huge. It’s really huge, but it’s not the largest die-off we’ve had,” said Katie Palmieri of the California Oak Mortality Task Force.

Solano County is one of 14 counties known to have Sudden Oak Death. To date, most known cases are along the western border with Napa County, such as in the hills above Green Valley. Coastal clouds often hang long over these Green Valley hills on a summer’s morning. That usually doesn’t happen in other parts of the county.

“Most of our county is really hot,” Assistant Solano County Agricultural Commissioner Simone Hardy said Wednesday. “The organism prefers a cool, damp area.”

Because of budgetary constraints, the county hasn’t done a recent survey for Sudden Oak Death, she said. The county has focused on doing inspections at nurseries to make sure Sudden Oak Death isn’t spread, she said.

The largest die-off came in 2007 after extremely wet springs the previous two years left 830,000 trees dead. The fewest number of dead trees were counted in 2010, when 2,700 trees died after a dry spring in 2009.

The disease that quickly kills trees that can take hundreds of years to grow is present in 14 coastal California counties from Monterey to Humboldt, and just across the border in coastal Oregon.

Oak trees in the Sierra foothills have been spared, and scientists believe drier weather and hotter temperatures create inhospitable conditions for the pathogen.

“Certainly the hosts are present there, but the assumption at this point is that the weather patterns don’t support it,” Palmieri said.

Scientists say that with each passing year the infection sites that once were sporadically spread across a large geographic region have become more contiguous. They fear it eventually could wipe out huge swaths of the gnarly trees that are the defining flora of the state’s golden, rolling hills and whose acorns are a staple for wild deer.

In the current survey the highest rates of new infection in oaks and the beech-family member tan oak were discovered in the Carmel Valley south of Monterey and in the areas around Saratoga west of San Jose. But infected trees also were found inside San Francisco’s urban Golden Gate Park, and in Santa Cruz and the East Bay.

“When individual trees die it’s sad, but when groups of trees die it becomes a huge concern for fire hazard, particularly in the Bay Area, where it’s so developed and we have a wildland-urban interface,” said Tom Smith, a forest pathologist with the California Department of Forestry and Fire Prevention.

Trees are most vulnerable in the spring, and pathogen is spread by wind and in water, which means wet springs are particularly worrisome.

“That’s a huge concern,” Smith said. “The past couple of years we’ve had late spring and early summer rains and that’s ideal for even more deaths in the future. What’s going to happen from this year’s rains?”

Some varieties of oaks are more susceptible than others, and even within those species some are genetically predisposed to resisting infection.

“But with each passing wave of infection we end up with more diseased and dying trees,” Palmieri said.

Chemical preventative treatments must be applied before the oaks are infected, which is why volunteers canvass infected areas every year to look for symptomatic bay laurels and other host plants.

Sudden Oak Death is blamed on a microscopic pathogen introduced into California about 25 years ago, probably on ornamental rhododendrons and camellias. It causes leaf spots in host plants and sometimes entire branches die back. Scientists are still unsure of its origin.

The pathogen gets under the oak’s bark and destroys the cambium – the live outer layer of the tree that transports food. Infected trees cannot be cured die after their food stores are depleted, usually within one to five years.

They can be protected by injecting phosphites, a salt of phosphorus acid.

Barry Eberling contributed to this report. Reach him at 427-6929 or beberling@dailyrepublic.net. Follow him on Twitter at www.twitter.com/beberlingdr.