This disease has killed a million trees in California, and scientists say it’s basically unstoppable

Healthy forests are especially important at a time of climate change — they’re an incredible tool to remove carbon dioxide from the atmosphere. Dead forests, on the other hand, can light the spark for wildfires, which are already showing a long-predicted uptick in activity.

In California’s coastal forests, health is anything but good. Since 1995, a fungal pathogen that causes a phenomenon dubbed ‘sudden oak death’ (a far catchier name than that of the pathogen itself, Phytophthora ramorum) has taken out millions of oak and tanoak trees, particularly along the coast extending northward from Monterey County. That includes areas of Marin County, Sonoma County and Big Sur.

The pathogen is a fungus that affects different trees differently, and not all are susceptible. It will tear through a forest and kill some trees while leaving others standing.

But in some trees, the pathogen causes tree trunks to crack open a ‘canker’ and literally bleed out sap. The disease is actually related to the pathogen that caused the Irish potato famine in the 1800s.

“Millions of acres of land have been affected in coastal California,” says Richard Cobb, a postdoc at the University of California, Davis, who studies the disease. “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.”

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