A pathogen causing a phenomenon known as ‘sudden oak death’ has killed millions of oak and tanoak trees in California’s coastal forests since 1995.

It doesn’t affect all trees, but it has a devastating effect on those it does. Traveling through water and wind, the pathogen (Phytophthora ramorum) is a mold that causes tree trunks to crack open and bleed out sap.

“Millions of acres of land have been affected in coastal California,” said Richard Cobb, a postdoc at the University of California, Davis, in an interview with the Washington Post. “It spreads via wind and rain, and it’s made some really big jumps to different parts of the state and into Oregon. It probably spread into California via the nursery trade. And it has been moved around the country a lot, also within the nursery trade.”

New research, published in the Proceedings of the National Academy of Sciences by Cobb and his colleagues, shows that the opportunity to suppress the disease’s spread has passed.

Killing trees since 1995, the pathogen’s mass is too great to be contained, said Cobb. And the cost is too high.

Cobb’s research says the disease is now in a “phase 3” epidemiological state, and eradication is no longer possible. The study also found that the disease will grow to affect nearly 10 times the current area, from around 600 square miles today to 5,400 square miles by 2030. Removing infected trees would cost around $100 million annually, the study said.

California must live with the disease’s impacts, said Cobb, which include increase risk of forest fires and carbon sequestration.

Forest managers and state agencies can make a difference by trying to contain the disease, Cobb said.

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