Drought slows spread of sudden oak death in Sonoma County

Guy Kovner, THE PRESS DEMOCRAT

Four years of drought have slowed the spread of sudden oak death to its lowest level in a decade, but western Sonoma County remains one of the hot spots in the 15 infested counties from Monterey to Humboldt, and when rain comes again the tree-killer will resume its rampage through Northern and Central California woodlands.

UC Berkeley Forest Pathology and Mycology Lab, (http://nature.berkeley.edu/garbelottowp/?p=1596)
UC Berkeley Forest Pathology and Mycology Lab, (http://nature.berkeley.edu/garbelottowp/?p=1596)

Analysis of more than 2,100 bay laurel tree leaves sampled during an annual citizen-powered survey last spring found a 3.7 percent estimated rate of sudden oak death infection, down from 4.4 percent in 2014 and possibly the lowest level since the disease erupted in 1995.

“I think we’re at the bottom of the infection rate,” said Matteo Garbelotto of the UC Berkeley Forest Pathology and Mycology Lab.

Drought conditions thwart the spread of sudden oak death, which largely depends on wet, windy weather to blow infectious spores from bay laurel trees, which host the pathogen, to oak and tanoak trees that die within a few years of infection.

Predictions of a strong El Niño weather pattern this winter could mean heavy rains for the North Coast. And when rain starts falling again, the as-yet unstoppable tree-killer will renew its assault, Garbelotto said.

“We know the sudden oak death pathogen can respond readily to wet conditions,” he said.A relatively wet climate, even during the drought, explains why west Sonoma County had an estimated 12.6 percent infection rate this year, up from 7.1 percent in 2014, he said.

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