Wet winter in Sonoma County may have helped spread virulent oak disease

Now that the North Coast is finally drying out from an unusually wet winter, concern is growing over the potential rapid spread of sudden oak disease, renewing calls for the public’s help tracking the deadly forest pathogen.

“Now is when we might expect the pathogen to take off a bit,” said Kerry Wininger, a UC Cooperative Extension staffer in Santa Rosa.

Wininger is a local organizer of annual sudden oak death surveys known as the SOD Blitz. This year’s survey occurs from April 25 to 28 across Sonoma and Mendocino counties. Organizers are hoping for a good turnout of volunteers, who will become educated spotters and collectors to help scientists slow the disease’s spread.

A relatively dry winter in 2017-18, coupled with the attention paid to devastating wildfires, appear to have dampened public concern over sudden oak death. But experts say conditions are ripe now for a resurgence of the disease. In addition, there’s heightened worry about a new, more virulent strain of the pathogen gaining a hold on the North Coast and causing more devastation.

Wininger said one of the highlights of this year’s surveys is the unveiling of a new test for the European strain in time to possibly thwart its spread. The new strain has been detected in Oregon.

“We want to nip it in the bud, if it’s here,” Wininger said.

Phytophtora ramorum, the pathogen that causes SOD, most often is spread by water droplets blowing from the leaves of infected bay laurel trees. There is no cure, only preventative measures or destroying oak and tanoak trees that succumb to the pathogen.

Coast live oak, California black oak, Shreve’s oak and canyon live oak also are vulnerable.

Sudden oak death has killed up to 50 million trees from Big Sur to southwest Oregon since its discovery in Marin County in the mid-1990s. The disease typically spreads rapidly after wet winters, such as the one we just experienced on the North Coast.

Sudden oak death was first detected in Sonoma County in the early 2000s at Fairfield Osborn Preserve on Sonoma Mountain. The preserve is among the locations where volunteers will fan out later this month conducting surveys.

According to researchers, the pathogen most often spreads on infected California bay laurel leaves.

By noting their observations of the leaves and sending in samples for testing, volunteers survey takers assist researchers and property owners in taking proactive measures to curtail the pathogen’s spread.

These measures include removing infected bays, chemical treatments and sanitation.

The 2018 survey showed a sharp decline in the rate of infection from the previous year, both in Sonoma County and other counties were tracking efforts are concentrated.

The 2017 data reflected the highest infection rate ever recorded - following two straight wet winters.

Similar conditions exist now.

“I’ve been saying this to people for years - it could get worse,” Wininger said.

In a region still reeling from deadly wildfires and where planning for the next disaster occupies much of the planning and political bandwidth, sudden oak death might strike some as less of a concern. Wininger said she understands the sentiment.

But she also argued that oaks, as a keystone species, are crucial to the overall health of an ecosystem.

“It’s like an arch,” she said. “If you lose a stone in the arch, everything else in the ecosystem is affected.”

Researchers are introducing a new program in which arborists, tree care specialists, preserve managers and government workers can become certified to test for the sudden oak death pathogen. Training for volunteers also will be conducted in Spanish for the first time this year.

Volunteers must attend informational sessions to go over detection methods and areas for collection. Sessions are being held this year in Petaluma, Santa Rosa, Sebastopol, Yorkville (Mendocino County), Penngrove and Sonoma Valley (see box).

Weekend sessions on April 27 and 28 feature guided outings at Galbreath Preserve in Mendocino County and at Fairfield Osborn Preserve. To register, visit ucanr.edu/sodblitz2019.