Sudden oak death disease found in SLO County for the first time

A California bay laurel at Cal Poly’s Leaning Pine Arboretum.
A California bay laurel at Cal Poly’s Leaning Pine Arboretum. jjohnston@thetribunenews.com

The pathogen that causes sudden oak death — a disease that’s killed millions of trees along the Northern California coast — appears to have arrived in San Luis Obispo County, according to a UC Berkeley study released Sunday.

Phytophthora ramorum, a pathogen known to cause sudden oak death, was found to have infected California bay laurels south of Monterey County for the first time, according to a sudden oak death survey conducted in the area.

UC Berkeley researchers will be working with the California Department of Food and Agriculture and the USDA Animal and Health Inspection Service to “validate the data for regulatory use,” according to a California Forest Pest Council news release.

The spread of sudden oak death (SOD) and the pathogen that causes it is associated with the return to normal or above-normal rainfall along the Central and Northern California coast last winter, according to the study, called the SOD Blitz.

In San Luis Obispo County, survey maps show many of the infected bay laurels were found along Stenner Creek and in Prefumo Canyon in San Luis Obispo and Santa Rosa Creek near Cambria. A few also were found west of Atascadero near Highway 41.


The pathogen also was found for the first time on Mount Diablo in Contra Costa County. In Monterey County, pathogen rates jumped by 27 percent along the Big Sur coastline. New outbreaks also were found in areas of Mendocino, Alameda and San Francisco counties.

“This is the most significant increase in sudden oak death in California since the Blitz program began in 2006,” said Matteo Garbelotto, principal investigator at the UC Berkeley Forest Pathology and Mycology Lab and founder of SOD Blitz. “Whether or not this surge of new infection continues will depend on rainfall levels this coming winter and spring. Significant rain could mean a lot of new infection, whereas a dry year could slow disease spread substantially.”

Katie Harrell, communications director for the California Forest Pest Council, said phytophthora ramorum is normally found first on bay laurel trees before it spreads to oak trees.

The disease is spread through spores, is water-dependent and requires mild conditions to exist, she said. Trees in damp coastal areas are more at risk of contracting the disease than those in drier inland regions, Harrell said.

“Because they got some rain last year, we’re seeing some infection as a result of that,” she said.

Sudden oak death, the disease caused by phytophthora ramorum, has killed about 3 million trees since it was discovered in California in the mid- to late-1990s, Harrell said. The disease “girdles” an oak tree, or cuts off the flow of water and nutrients from its trunk to the rest of the plant, eventually killing it, according to the UC Berkeley Forest Pathology and Mycology Lab.

At this point, it’s tough to tell whether the disease will continue spreading or start killing oak trees in San Luis Obispo County, she said.

“It’s just this kind of wait and see game,” Harrell said.

Lindsey Holden: 805-781-7939, @lindseyholden27