Extent of sudden oak death spread in California unexpected in dry year

Sudden oak death made unexpected gains this year in California, including in northern Sonoma County, surprising experts who expected the state’s dry weather would have curbed the infection that has killed more than 50 million trees over two decades.

A massive survey by volunteers covered 171,000 acres and collected leaves from more than 2,000 trees in Central and Northern California, revealing a regionwide estimated infection rate of 7.4%, up from 6.1% last year.

An outbreak from Windsor to Cloverdale pushed the rate to 5%, up from 1% last year, while the other two segments of Sonoma County had significantly lower rates.

“The numbers are surprising,” said Matteo Garbelotto, director of the UC Berkeley laboratory that has organized the survey, known as the SOD Blitz, since 2008. “We expected to see a slowing down of the disease. That’s not what happened.”

“It’s the first time we’ve seen an increase in a dry year,” he said in an online webinar.

Oak death infection rates usually match rainfall because the pathogen that causes it — a fungus-like water mold — rides on droplets blown by warm spring rain from the leaves of bay laurel trees, a host species, to susceptible oak and tanoak trees.

Significantly higher infection rates were documented Marin and Mendocino counties, as well as Carmel, the San Francisco Peninsula, the East Bay and Santa Cruz County, where the infection rate jumped to 14.2%, up from 0.4% last year.

Garbelotto said the regionwide rate was most telling because the all-volunteer survey does not cover the same areas ever year.

The uptick in a dry year “is unusual and further proves the disease has truly become more endemic in areas of California that have been infested for 10 or more years,” a SOD Blitz news release said.

There were some anomalies in the observations: Western Sonoma County, a heavily infected area, recorded a decline to 5.1%, down from 9% last year. Eastern Sonoma County’s infection rate was 8.3%, a big drop from 20.4% last year.

Discovered in Marin County in 1995, sudden oak death has killed some 50 million trees from southwest Oregon to Big Sur and infected about 200 million trees in California.

The disease was first detected in Sonoma County at Fairfield Osborn Preserve on Sonoma Mountain in the early 2000s.

Anderson Valley and Route 253, halfway between Boonville and Ukiah, were cited as locations of significant outbreaks, along with Calistoga and the Napa River Trail.

Asked if there was a connection between sudden oak death and wildfires, Garbelotto said infected trees will “burn very hot” even if the leaves are still green because the disease prevents trees from absorbing moisture.

Garbelotto is director of UC Berkeley’s forest pathology and mycology lab, which tests all the leaves collected by the survey volunteers.

There is no cure for sudden oak death, but there is a chemical that can be applied as a preventive treatment, while cutting down bay trees close to prized oaks is a recommended control strategy.

You can reach Staff Writer Guy Kovner at 707-521-5457 or guy.kovner@pressdemocrat.com. On Twitter @guykovner.