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University of California scientists are hustling to contain a deadly plant disease that is creeping northward in Humboldt County, killing swaths of tanoaks like a forest fire in slow motion.

Their techniques for fighting sudden oak death (SOD) are experimental and have been met with both skepticism and support by residents, however, project leader Yana Valachovic, the UC Cooperative Extension forest advisor for Humboldt County, believes studying methods of controlling the disease is better than standing by as it marches steadily toward the Oregon border.

SOD first appeared Humboldt County’s treasured redwood forests in 2002, infecting bay laurel trees near homes in the small communities of Redway and Garberville. A coalition of agencies worked with landowners to quickly pull out as many infected backyard and wildland bay trees as possible.

However, as a burning ember might drift off to ignite another hotspot, scientists believe spores alighted on bay laurel, tanoaks and other susceptible plants in Humboldt Redwoods State Park. In late 2005, a utility line pre-inspector, trained by UCCE to be on the lookout for SOD symptoms, was the first to see the disease in trees six miles north of the Redway-Garberville disease area.

The worker reported the sickly bay laurel and tanoak to Valachovic, who brought together representatives and scientists from UC Davis, California State Parks, California Department of Forestry, the U.S. Forest Service and others to quickly fund and implement a disease suppression program. Treatment began within two months.

David Rizzo, the UC Davis plant pathologist who first identified the cause of SOD, was part of the team.

“If we did nothing, the disease would continue to spread,” Rizzo said. “Our goal is to slow that inevitability down as much as we can. We are still learning about the disease, but in the meantime, we can’t wait till we know everything to start managing it.”

The project had to be completed within a tight timetable to comply with regulations that protect nesting habitat in the state park for the marbled murrelet, a threatened sea bird. Crews were brought into a 50-acre area to cut down all bay laurel and tanoak trees showing symptoms of SOD infection. Twigs, branches and leaves were burned. Logs were left to decompose. After the protected birds’ nesting season ends Sept. 15, crews will return to underburn half the treated area to test fire’s effect on the diseased area. All parts of the treatment area will be carefully monitored to determine whether the efforts cleared out sudden oak death. Infestations on 70 acres of private land near Redway and Garberville that were treated in a similar manner are also part of the study.

“We know the bay laurel and tanoak will re-sprout,” Valachovic said. “We hope that the new growth will not be infected with the pathogen. By comparing burned areas with those that aren’t burned, we’ll learn more about what works to suppress the disease.”  

SOD was first discovered in California in 1995 in Marin County. Caused by the fungus-like pathogen Phytophthora ramorum, SOD has since killed hundreds of thousands of tanoak and coast live oak trees in 14 coastal counties. The Humboldt infestations mark the disease’s northern boundary in California.

“Many plants and trees in our area may be infected by the disease, but most don’t die from it,” Valachovic said. “On the leaves of two particular trees, tanoak and bay laurel, the pathogen readily produces spores, which help spread the disease. Our experimental treatment efforts are focusing on these two trees to slow reproduction of the pathogen.”

P. ramorum can also infect the North Coast’s distinctive conifer trees, including Douglas fir and coast redwood, but these trees do not die. SOD is still a major concern for the timber and nursery industries as its presence triggers countywide regulations that restrict movement of plant material to avoid spreading the disease elsewhere. Tanoak, which usually dies when infected, is used only occasionally by the lumber industry for flooring and wood working, but is an important part of the forest ecosystem. Its acorns are a traditional food source for Native Americans in the region and the acorns are food for many wildlife species.

Regrettably, as the efforts to suppress SOD in Humboldt County are being made, the notably wet winter, which helps spread the disease, was working against the scientists. At two spots along Highway 101, along the south fork of the Eel River, the brown leaves of dead tanoak stand in contrast to the lush green forest. On closer inspection, bay laurel leaves show tell-tale dieback of the leaf tips and darkening of tissue along the main vein.

“We just contacted CalTrans because the infected plants are in their right of way,” Valachovic said. “They’ll help us treat it. I think the spores may be moving up the air column that follows the Eel River.

“This story has not yet fully unraveled, but the worst thing we can do is nothing.”