Chimney Sweep

‘Scariest tree pathogen on the earth’ spreading quickly in California

Sudden Oak Death (SOD), a deadly disease affecting oak trees, is on the rise in California. According to a survey by scientists at UC Berkeley, the number of trees infected has almost doubled since 2018.

Matteo Garbelotto, director of the UC Berkeley Forest Pathology and Mycology Laboratory, has been involved in conducting the survey in 14 California counties (from Humboldt to Monterey) for the past 12 years. This year he noticed two aspects of the results.

“We saw the biggest increase in the number of trees affected this year,” said Garbelotto. However, this was expected due to the wet winters we’ve had in California over the past two years – the spores spread faster with significant rainfall.

What was really unexpected was the extent to which they found the breakouts.

“I’ve seen a lot of outbreaks that we had seen before in the 12 years of our program, but I saw all of the outbreaks come out at once this year,” he said. In recent years, some outbreaks would recede while others flare up again – every outbreak flared this year. “This pattern shows me that the organism has really spread into the Coastal California ecosystem. Now it is already established everywhere and flares up when the weather is good. “

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Other findings from the survey include a recent outbreak in Del Norte County, which was the only county so far between the massive California infestation and the southern Oregon outbreak that was free of the disease. San Luis Obispo remains free from the disease, but there is a high risk of contagion. All parks in San Francisco remain completely uninfected.

While analyzed by scientists at UC Berkeley, 100 percent of the survey data is collected from volunteers. UC Berkeley goes to local communities where SOD is a problem and trains local volunteers to identify the symptoms. This year’s results are particularly alarming, but Garbelotto sees a positive finding: “It’s kind of a testament to what you can achieve by working with volunteers.”

With the data available through a mobile app, people can identify the risk of trees around them and take precautions before it’s too late. The pathogen often spreads on the leaves of infected California bay laurel and tanoak, which can be easily identified.

“Leaves are always attacked before oaks are attacked,” said Garbelotto. “Something can be done, but for it to be effective we have to do it before the oaks are hit.”

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Proactive methods of combating the disease include sanitation, chemical treatments, and targeted removal of bay trees.

It’s important to keep track of SOD, as Garbelotto calls it “the scariest tree pathogen in the world” for two reasons: “First, its ability to reproduce and, second, its ability to affect other plants is locally very high.”

And remember that the more dead trees that remain, the more forest fire lighting there is.

Madeline Wells is an SFGATE digital reporter. Email: madeline.wells@sfgate.com | Twitter: @ madwells22

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