New survey detects rapid growth in sudden oak tree death syndrome

An annual survey of the spread of sudden oak death (SOD) throughout the state has detected that oak trees surveyed in towns on the Peninsula between Redwood City and Los Altos Hills have a 21.6% infection rate, with significant new outbreaks reported.

The syndrome was detected locally in oaks in northern Woodside, Portola Valley, Emerald Hills, and Palomar Park, according to the report from the University of California at Berkeley Forest Pathology and Mycology Lab.

At the same time, the infection rate has almost doubled throughout the state in the past year and in some locations increased up to 10 times, ominous news for researchers who have been following the infection that kills tan oak, coast live oak, Shreve’s oak, California black oak and canyon live oak species.

The oak trees get infected in the trunk and die slowly over two to three years, according to Debbie Mendelson, who heads the Portola Valley and Woodside version of the survey known as the SOD Blitz.

“It’s sad that our numbers are higher this year,” Mendelson said. “I would hope that a lot of people will come out for next year’s Blitz so that we can make every effort to protect and save these species of native trees.”

The SOD Blitz is held annually at 25 locations throughout the state. During the Blitz, volunteers go out into the oak forests and test bay laurel trees, which carry the disease and can infect oak trees growing nearby. Leaf samples from the bay laurel trees are then sent to the forest pathology lab for testing.

The SOD Blitz in Woodside and Portola Valley was done on April 17, and the results were reported at a meeting in Woodside on Nov. 9.

Statewide, teams surveyed 16,227 trees across 16 counties, collecting approximately 9,000 leaves from 1,732 trees, according the forest pathology lab.

Results from the data collection are available at SODBlitz.org and SODMap.org.

The infection rate for oaks within 15 feet of a bay laurel is about 75%, according to Matteo Garbelotto, the lab’s director. After a distance of about 20 yards, the infection rate drops dramatically.

The infected oaks can fall down when they are still green, potentially on top of a house or a car, Garbelotto said.

“If an infected oak is near your house, you want to take it out because if it burns in a fire it will spread to your home,” he said.

In San Mateo County, the SOD infections, carried by bay laurel and tan oaks, move through the ridge top of the Santa Cruz Mountains and then east down the hill through Woodside and Portola Valley to urban areas, where the temperature is warmer and the severity of the infections decreases, said Garbelotto, who was on the team that first isolated the microbe that that causes SOD.

The team found that the pathogen originated in ornamental plants that had arrived from Asia, including rhododendrons and camilla, and escaped into the larger environment, he said.

The phenomenon happened first in California, but there have also been recent SOD outbreaks in England, Belgium, Holland, Ireland, France and southern Oregon, he said.

Forest managers can use sanitation, chemical treatments, and the targeted removal of bay trees, but these tools are only useful before oaks and tan oaks are infected, so early detection is critical to slowing the epidemic, according to the University of California Division of Agriculture and Natural Resources.

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