Doug Schmidt takes samples from coastal live oak believed to have been killed by sudden oak death in Golden Gate Park. Photo: Brant Ward/The Chronicle SF
‘The number of oak trees in California that died from the virulent forest disease known as sudden oak death has increased tenfold in just a year’s time as the pathogen spread into several new parts of the Bay Area, including San Francisco’s Golden Gate Park, biologists revealed this week.
Aerial and ground surveys conducted by the U.S. Forest Service documented 375,700 new cases of dead live oak and tan oak trees over 54,400 acres of California where the pathogen is known to exist. That’s compared to 38,000 dead trees covering 8,000 acres a year ago.
The sudden increase in deaths is believed to have been caused by two years of abnormally high rainfall followed by this year’s dry weather.
The pattern is one that scientists at the UC Berkeley Forest Pathology and Mycology Laboratory foresaw last year when the deadly microbe was detected in woodlands and residential areas throughout the Bay Area. The area of infection expanded even more this year, said Matteo Garbelotto, the forest pathologist who heads the lab.
Sudden oak death, discovered in Mill Valley in 1995, exists in forests and wildlands in 14 California counties and in Curry County, Ore. It kills big oak trees and the smaller understory tan oaks, which have been ravaged in portions of Big Sur, Jack London State Park in Sonoma County, China Camp State Park in Marin County and the Marin Municipal Water District watershed lands near Mount Tamalpais. Scientists fear the pathogen could one day wipe out all of the state’s live and tan oaks.
The disease, known scientifically as Phytophthora ramorum, has 107 susceptible host plants. Infected California bay laurels are the most effective spreaders of the deadly microbe, but such common garden ornamentals as camellias and rhododendrons can also spread the pathogen to oaks.
Samples from California bay laurels and other host plants taken by volunteers around the Bay Area show that the pathogen’s spores are virtually everywhere in the hills of Burlingame and other parts of the Peninsula, and the infection has spread farther into residential areas of the East Bay hills, and Napa and Sonoma counties.
The microbe was even found in neighborhoods on the outskirts of Santa Cruz and, most surprising of all, in a cluster of three trees near Middle Lake, in the southwest corner of Golden Gate Park, near Sunset Boulevard.
The source of the San Francisco infection, which has yet to kill any trees, is a mystery given that there are no nurseries nearby spreading the disease like there were several years ago when the pathogen was last detected in the park near the AIDS Memorial Grove, Garbelotto said.
“The area doesn’t have a lot of oaks, so I’m not worried about oaks there, but I’m worried about other plants being infected and, of course, people carrying it on their shoes,” Garbelotto said. “It’s puzzling that we found it there because it’s a totally urban environment, and I really didn’t expect it. It shows how complicated and adaptable this organism is.”‘
via Explosive growth in sudden oak death – SFGate.