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Spring rains brought relief to a drought-weary region, filling North Coast reservoirs and farm ponds and turning grassy hills a glorious emerald green.

But the wet, sometimes windy weather was also ideal for Phytophthora ramorum, the insidious pathogen that causes sudden oak death, a disease that has killed more than 3 million trees in coastal forests from Monterey to Humboldt counties since 1995, when it was discovered in Marin County.

The pathogen can be spread by human footprints and nursery plants, but in nature it rides in water droplets blown from the leaves of bay laurel trees, a host species that abounds in close proximity to the oak and tanoak trees that sudden oak death kills.

California’s four-year drought slowed the disease’s spread considerably, but officials are wary of a widespread rebound, including Sonoma County, owing to a comparatively soggy spring. Santa Rosa has recorded 12.5 inches of rain since March 1, compared with 1.5 inches in the same period last year.

“We think we’ll see a bump this year,” said Lisa Bell, the county’s sudden oak death program coordinator. “We want to see what the pathogen does coming out of a drought.”

The means for that assessment is the SOD Blitz, an annual volunteer effort organized by the UC Berkeley Forest Pathology and Mycology Lab. More than 40 people have signed up for the Sonoma County blitz on Saturday and Sunday, and Bell said she’s hoping for many more volunteers who will be trained to collect samples — primarily bay leaves — that will be analyzed at the Berkeley lab.

Last year more than 500 volunteers took leaves from 2,168 trees in the 15-county region, including about 35 Sonoma County participants who took samples from 65 trees.

The sudden oak death stakes ramped up with the publication of a study last week that said sudden oak death is out of control in California, and curtailing its spread “has been impossible for a number of years.”

The study by scientists from UC Davis, Cambridge University in England and North Carolina State University predicted the spread of sudden oak death will accelerate after 2020, when it reaches the vast forests of the northwest coast. By 2030, the disease could infect 5,400 square miles, a nearly tenfold increase from the estimated 600 square miles infected in 2014, the study said.

Another study published in 2011 by some of the same scientists estimated that 4.4 billion oak and tanoak trees growing on 53 million acres of California and Oregon forests are at risk of infection.

“We can’t get every spore and kill it,” Bell said, agreeing with the finding the sudden oak death is beyond control. But, she said, there is a battle to be waged up north in attempting to prevent the disease from crossing the Humboldt-Del Norte county line.

Sonoma County landowners, Bell said, can attempt to protect valued oaks on their property by the only method cited as effective in last week’s study: the removal of sudden oak death’s host species, primarily bay trees.

At Pepperwood Preserve, a 3,200-acre property in the Mayacmas Mountains northeast of Santa Rosa, a massive coast live oak tree, dead for three years, stands amid a grove of spindly bay trees that almost certainly infected it, preserve manager Michael Gillogly said.

Sudden oak death began claiming tanoaks in the preserve about 15 years ago, and has since killed about 20 coast live oaks and hundreds of tanoaks, a close relative of oak trees. An uncounted number of oaks are showing cankers in the bark close to the ground, a likely symptom of the disease.

“I think it is widespread (in the preserve),” Gillogly said. “It’s here; it’s something we have to live with.”

The preserve has removed bay trees that were close to oaks around the Dwight Center for Conservation Science, a combination visitor center and preserve office with a stunning view of green hills and valleys.

Gillogly said he is also selectively thinning the forest, with chainsaw crews and possibly controlled burns, to eliminate understory vegetation and bolster the health of mature trees.

Sudden oak death is a misnomer, he said, noting the disease typically infects an oak for years, slowly cutting off its circulatory system, but when it dies the leaves can turn brown within a matter of weeks.

There is no cure for sudden oak death, but a pesticide called Agri-Fos has been approved as a preventive treatment.

Landowners who participate in the blitz can sample bay trees on their property to find out if they harbor the pathogen, Bell said. A map showing the infected trees identified in previous blitzes is available online at www.sodmap.org.

The disease didn’t spread much in Sonoma County during the drought, Bell said, but remained entrenched in West County, especially around Occidental, and scattered through hilly, wooded areas east of Highway 101 from Geyserville down to Sonoma.

People who want to participate in this weekend’s local blitz are asked to sign up online at ucanr.edu/blitz2016 and to start by attending an hour-long training session Saturday morning at four locations in Santa Rosa, Graton, Cloverdale and Sonoma (see the website for details).

Leaf samples will be collected Saturday and Sunday and turned in at the training sites by 5 p.m. Sunday.

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

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