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Sudden oak death rebounds in Sonoma County, spreads in California

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Sudden oak death reasserted its grip on Sonoma County this year, doubling its presence in the wake of a wet winter, while the pathogen that has sickened and killed millions of trees cropped up alarmingly 300 miles away near the Oregon border.

The identification of two infected tanoak trees in Jedediah Smith Redwoods State Park in Del Norte County hardly constitutes an epidemic, but worries scientists because it could mark the beginning of a bridge that brings to California a European strain of the disease that attacks conifers, possibly including Douglas fir, a mainstay of the lumber industry in both states.

“It’s cause for serious concern,” said Richard Cobb, an assistant professor of forest health at Cal Poly San Luis Obispo. “It points the sudden oak death story in a new direction.”

The European strain, identified as EU1, “kills or damages several conifer tree species and appears more aggressive than the North American lineage,” Oregon’s Department of Forestry said in a letter last year requesting $1million to continue eradicating the European strain found there by cutting and burning infected trees and nearby vegetation.

A state forestry official called the eradication effort a success, noting the infestation had spread up to three times slower than in California. In Oregon, the disease has primarily killed tanoaks, a noncommercial tree species.

California should consider following Oregon’s example to protect the state’s recreational and commercial forest resources from the onslaught of an incurable killer, Cobb said.

The North American strain found in California, dubbed NA1, does no significant damage to Douglas fir trees, he said. In California, the sudden oak death pathogen resides on bay laurel and tanoak trees, signaling its presence with brown and yellow discoloration of the leaves.

Matteo Garbelotto, director of the forest pathology and mycology laboratory at UC Berkeley, said discovery of the two infected tanoaks in Del Norte County was a signature finding of this year’s sudden oak death survey, known as the SOD Blitz, organized by his lab since 2008.

“It’s a good thing that we detected it because the sooner we know, the more options available to minimize the impact of the disease,” he said.

Since the mid-1990s, sudden oak death has killed about 50 million trees from southwest Oregon to Big Sur and infected about 200 million trees in California, Cobb said. About a billion trees are at risk in Oregon and California, concentrated in Del Norte, Humboldt and Mendocino counties.

Discovered in Marin County in 1995, the disease is caused by a pathogen — a fungus-like water mold — spread primarily on droplets blown by warm spring rain from the leaves of bay laurel trees, a host species that abounds among the susceptible oak and tanoak trees.

Sudden oak death was first detected in Sonoma County at Fairfield Osborn Preserve on Sonoma Mountain in the early 2000s. It was discovered in Oregon in 2001 and the European strain appeared there in 2015.

The name is a misnomer as the disease slowly cuts off a tree’s circulatory and growth systems, taking up to four years to kill an oak. There is no cure, but a pesticide called Agri-Fos has been approved as a preventive treatment, while cutting down bay trees close to prized oaks is the main control strategy recommended to landowners.

The scientific jury is out on whether the European strain, found largely in the United Kingdom, poses a threat to Douglas fir, one of the world’s most valuable timber trees. It was first identified in a federal forest four years ago in Curry County, Oregon, which holds the state’s entire sudden oak death infestation, the target of a $1.5 million-a-year disease control program.

Jared Leboldus, an Oregon State University forest pathologist, said the European strain is a potential threat to Douglas fir but no canker, a telltale sign of sudden oak death, has been found on one yet. Still, it can’t be dismissed because of the documented impact on European conifers and the strain’s ability to produce far more spores that make it “more aggressive” than the domestic strain, he said.

Oregon is continuing to wage war on sudden oak death, with a report to the Department of Forestry in February asserting that $30 million worth of forest treatments over the next 20 years could offset the loss of 1,200 jobs by 2028 and $580 million in wages from 2020 to 2038.

There has so far been no impact on Oregon’s annual timber harvest, export or log prices, the consultant’s report said.

Garbelotto said four years is insufficient time to assess the European strain’s impact and remains concerned that Del Norte County may provide a connection between the Oregon and California infestations, with the latter now spanning 16 coastal counties from the Oregon border south to Monterey County.

If either strain in Oregon combines with California’s pathogen, “together they could do more damage than they do individually,” he said.

California can’t mimic Oregon’s sudden oak death control program because of the scope of the Golden State’s infestation, but it might be worth taking steps to confront the disease in Del Norte County, he said.

“I don’t want to paint gloom and doom. The big concern is we don’t know what will happen,” Garbelotto said.

Meanwhile, the SOD Blitz conducted last spring found the rate of sudden oak death infection doubled in Sonoma County, quadrupled in Napa County and rose tenfold in Big Sur and part of the East Bay.

In Sonoma County south of Santa Rosa, the infection rate hit 20.4%, up from 9.7% last year. Throughout the 16-county survey area, the infection rate was nearly 6%, up from 3.5% in 2018.

The Russian River area continued to be an oak death hot spot, while infestations were found in Petaluma, Bennett Valley, Santa Rosa and east of Rohnert Park.

All four trees tested this year near Petaluma’s Helen Putnam Regional Park were infected, while only one of four were last year, suggesting a higher infection rate, said Kerry Wininger, the SOD program coordinator for the UC Cooperative Extension in Sonoma County.

Garbelotto attributed the increased infection rates — based on laboratory analysis of leaves collected from bay laurels and tanoaks — to a wet winter and continued rebound from California’s five-year drought.

“The pathogen has established itself in Sonoma County,” he said. “It will express itself rather easily when we have favorable conditions.”

Santa Rosa got more than 48 inches of rain in the season that ended Sept. 30, about twice as much as the previous season. Most other Sonoma County communities registered a similar increase.

Around the Bay Area, the pathogen, called Phytophthora ramorum, is “becoming naturalized,” Garbelotto said. Dry periods do not thwart the infestation, “it just comes back and continues its job. Even if we don’t see it, it still persists.”

The survey involved 422  c itizen volunteers who observed 16,227 trees in 16 counties, collecting about  9,000 leaves from 1,732 symptomatic trees.

A map showing the survey results is available at sodblitz.org and on the sodmap mobile app. With one of the apps, a user can stand next to a given tree and tap the “RISK” button to determine whether the tree is at risk of contracting sudden oak death.

Officials will discuss the results of the 2019 SOD Blitz, along with treatment and new diagnostic options, at a public meeting at 6 p.m. Tuesday at the UC Cooperative Extension, 133 Aviation Blvd., No. 109 in Santa Rosa.

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

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