The North Coast’s wet spring was a widespread boon to living things, including the insidious pathogen that has killed more than 3 million trees in the past two decades, and another rainy season could spell doom to many more of the region’s treasured oak trees.
Four years of drought had slowed the spread of sudden oak death in Sonoma and 14 other counties from Humboldt south to Monterey, but the same rainfall that pumped up reservoirs and turned hillsides green this year also triggered a rebound in the spread of the disease that surprised the experts.
“It’s on the rise pretty much everywhere,” said Matteo Garbelotto, director of the forest pathology and mycology laboratory at UC Berkeley.
The volunteer survey that his lab conducts every spring found the rate of infection by Phytophthora ramorum, the pathogen responsible for sudden oak death, had doubled — on average — throughout the region, the biggest increase in 10 years.
In Sonoma County north of Santa Rosa, the infection rate nearly tripled, to 4.4 percent this year up from 1.5 percent in 2015. The infection rate east of Highway 101, from Santa Rosa south, nearly doubled, from 3.8 percent last year to 7.4 percent this year, while the rate west of the freeway held virtually steady at 12.5 percent.
Experts had anticipated a weather-driven rebound, but the results of the 2016 SOD blitz were “much more significant than we expected,” Garbelotto said.
If the current rainy season also delivers plenty of precipitation, sudden oak death could become the “primary killer of oaks” in the coastal woodlands, he said. The infection takes about two years to kill an oak, so a wet season extending into 2017 would produce lethal results in 2019, Garbelotto said.
Infected bay trees were found this year in areas where the disease’s spread had subsided during the drought, including northern and central Sonoma County and Napa Valley, while new outbreaks were detected near Ukiah and in southern coastal Mendocino County. Bay laurel trees carry the disease but do not suffer from it.
The spread of sudden oak death is monitored in bay trees because they are considered reservoirs of the pathogen, a fungus-like microbe that produces an obvious discoloration of the bay leaves but does not kill the trees.
Among the surprises in the 2016 blitz was the discovery of the pathogen for the first time in San Luis Obispo County — the 16th infected county — and on Mount Diablo in Contra Costa County.
About 500 volunteers, who attended training sessions, participated in the blitz last spring, surveying more than 14,000 trees and submitting more than 2,000 leaves to Garbelotto’s lab for genetic testing.
In Sonoma County, 61 volunteers surveyed 1,926 trees and submitted 290 samples, of which 73 tested positive for the pathogen.
Based on the findings from the blitz, the UC Berkeley lab calculates an “estimated true infection rate” of the oak death pathogen in bay laurel trees. Sudden oak death spreads, largely by wind-blown water droplets, from bays to four species of oak trees and tanoak trees.
Bruce McConnell, a resident of the Fountaingrove II subdivision, and his granddaughter, Caitlyn Oliver, collected leaves from about 30 trees in the subdivision’s 225-acre open space area in May, and about six were infected.