El Nino Rains Could Boost Sudden Oak Death

by Daniel Mintz, May 25, 2016

Humboldt County’s winter and early spring rainfall could intensify the spread of the pathogen that causes Sudden Oak Death (SOD) but the outcome won’t be known for another year or two.

And while new areas of SOD infection weren’t identified in aerial surveys last year, the pathogen continues to spread in existing areas and it’s uncertain that drought has had an effect due to Humboldt County’s numerous, wet “micro-climates.”

Phytophthora ramorum, the pathogen that causes SOD, thrives in moist conditions. While it’s possible that drought has staunched its spread in some parts of California, the effect might not apply to Humboldt County.

Dan Stark, the North Coast SOD outreach coordinator for the University of California Cooperative Extension said that Humboldt has many “micro-climates” that include streams and other watercourses.

“So even during a drought, there’s the potential for spread,” he continued. Although he noted that there’s conflicting information on the current degree of spreading, Stark said Humboldt isn’t seeing a reduction in tree mortality.

“Drought might have slowed the spread but our micro-climates stayed damp, moist and dark even through the summer up here,” he added.

Stark said the notion of drought slowing SOD hasn’t been backed up with scientific data and “the measurements we’ve taken show that (drought) didn’t slow it down at all.”

Recent rains may boost the pathogen’s spread but signs of it won’t be noticed soon. Stark said there’s a “lag time” between rain periods and SOD activity.

“It might actually be spreading now but we won’t see dead trees for another one to two years,” he continued.

In Humboldt County, the pathogen primarily affects tan oaks. Spread of the pathogen is noted by observing dead trees and Stark said researchers rely heavily on aerial fly-overs by the U.S. Forest Service to track SOD.

New fly-overs will be done in June, July and August. “We do know that SOD expands in areas that are currently affected,” Stark said, adding that Southern Humboldt is “by far the most heavily-affected area.”

The county’s Sudden Oak Death outbreak was originally detected outside of Redway and is now present in Redwood Valley and as far north as Redwood National Park, where it’s “actively managed and, we think, contained,” said Stark.

The pathogen’s spread hasn’t crossed the Humboldt border into Del Norte County.

Human activity and distribution of infected nursery plants can spread the pathogen but its primary means of travel is via water. Wind-blown water droplets from the leaves of infected trees can lead to new infections and the pathogen also moves through streams.

Stark noted that p.ramorum is “not sexually mating at this point – it’s just propagating.” But in Curry County, Oregon, a mating strain was recently detected on a single tan oak that was removed.

“It looks like it was successfully eradicated,” Stark said, but he added that “there is always that threat, that we could have the mating type on the landscape and the pathogen could become more or perhaps less virulent.”

Sudden Oak Death is a global phenomenon and researchers from around the world – including Humboldt County – will share information at the Sixth Annual Sudden Oak Death Science Symposium in San Francisco next month.

Stark said Humboldt researchers will deliver presentations on experimental treatments that were carried out 10 years ago and review how effective they’ve been.

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