The deadly pathogen known as sudden oak death is spreading throughout the Bay Area, infecting more trees in more places than have ever been seen before, according to scientists tracking the disease.

The Forest Pathology and Mycology Laboratory at UC Berkeley used 10,000 tree and plant samples collected by 500 citizens between April and June this year to document a dramatic increase in the infection rate from Napa to the Carmel Valley and virtually everywhere in between.

"We found that the number of positives were double and in some cases triple what they were last year," said Matteo Garbelotto, the UC Berkeley forest pathologist who organizes the annual surveys. "We were surprised. That was a big jump."

The findings are part of a major effort over the past four years to involve citizens in the battle against the mysterious pathogen, which has killed hundreds of thousands of oak trees from Big Sur to southern Oregon.

Arborists and ecologists are afraid that sudden oak death could eventually denude California's golden hills of its signature tree. As it is, experts predict as many as 90 percent of California's live oaks and black oaks could die from the disease within 25 years.

The citizen scientists, who were trained to detect disease symptoms, surveyed about 50,000 acres and collected samples from 2,000 oak and bay trees during a series of so-called SOD blitzes.

The information was added to a Bay Area-wide map plotting the tree-strangling microbe's path of destruction. New infestations were found this year in and around neighborhoods throughout the region, including urban and suburban areas of Napa, Berkeley and other locations in southern Alameda County.

The largest infestation was along South Skyline Boulevard, west of Saratoga and Los Gatos, where 97 percent of the specimens that were collected tested positive for the pathogen.

Worse than expected

Garbelotto, who is one of the nation's foremost experts on sudden oak death, said the positive tests in this woodsy region were not a surprise. The area is a known hot spot for the disease. This was, nevertheless, the first comprehensive survey done in the area, he said, and it came out worse than expected.

"We confirmed that it's at epidemic levels," he said, "but we didn't expect to find 97 percent."

The infection rate was also disturbingly high around Woodside, in the Portola Valley, with 70 percent of the samples testing positive. A year ago, 24 percent of the samples tested positive in that area.

Marin County has also been ravaged by the disease, with 53 percent of the samples there testing positive compared with 35 percent a year ago. Menlo Park, Atherton, the Los Altos Hills, the Carmel Valley, Sonoma and Napa counties also had numerous positive tests.

The most surprising result, Garbelotto said, was the fact that the disease had moved into residential areas, including North Berkeley, the Claremont district in Berkeley and the Montclair area of Oakland. Positive tests were recorded in East Richmond, San Leandro and at the Oakland Zoo.

Infected trees were even found in the Sunol area of eastern Alameda County, which was thought to be too dry for the disease.

Disease moves west

"I didn't expect it to be that widespread," Garbelotto said. "It looks like it's coming down the slopes toward the bay. Pretty much from San Leandro to Richmond, we are seeing the disease move westward."

Sudden oak death, first discovered in Mill Valley in 1995, exists in forests and wildlands in 14 California counties and in Curry County, Ore. It kills the big oak trees and the smaller understory tan oaks, which have nearly been wiped out in portions of Big Sur, Jack London State Park, China Camp State Park and Marin Municipal Water District watershed lands near Mount Tamalpais.

The disease, known scientifically as Phytophthora ramorum, has 107 susceptible host plants. Infected California bay laurels are the most effective spreaders of the deadly microbe, but such common garden ornamentals as camellias and rhododendrons can also spread the disease to oaks.

Scientists believe it was an ornamental plant that infected an oak tree in San Francisco's Presidio, prompting the National Park Service to sample virtually every host plant, including ornamentals at several nearby homes. Not a single new infection was found.

"It's the only good news we got this year," Garbelotto said. "At least the Presidio is doing well."

Sudden oak death is known to spread in water and has been detected in numerous waterways, including the Crystal Springs Reservoir. Garbelotto believes heavy rains over the past two years caused the sudden jump in the infection rate reflected in the study this year.

Rains help spread it

"What we are seeing is the cumulative effect of the rain last year and the beginning of this year," he said. "My prediction is that next year will be even worse because then we will have the cumulative effect of a rainy 2010 and all of 2011, which was very wet."

More neighborhood and community group volunteers than ever participated this year in the blitzes and mapping program, which were paid for by the U.S. Forest Service, state and private forestry groups, the National Science Foundation and the Gordon and Betty Moore Foundation. Agdia Inc., donated the diagnostic kits.

"This is the biggest citizen scientist program in the country," Garbelotto said. "It shows what you can accomplish when a lot of people buy in. It is a really good way to tackle an issue that the government can't tackle itself, and it's a great way to educate the public and get them involved."