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.”

Unfortunately, new research on this invasive disease, published in the Proceedings of the National Academy of Sciences Monday by Cobb and a group of colleagues, finds that while there may once have been a chance to stop the spread of sudden oak death — around the year 2002 — that opportunity has since passed. Forces didn’t mobilize fast enough or spend enough money, and the disease model employed in the new research (a model not so dissimilar from those used to study how various diseases can spread among humans) suggests the continual spread of the disease.

“Slowing the spread of P. ramorum is now not possible, and has been impossible for a number of years,” the study states bluntly. The research was led by Nik Cunniffe of the University of Cambridge, in collaboration with researchers from the University of California, Davis and North Carolina State University.

To stop sudden oak death, you have to go in and remove infected trees across a large area. And there’s so much pathogen mass now in California forests that the model finds that it will just spread, and spread. As pathogen biomass increases, says Cobb, “the rates of spread accelerate, and so does cost.”

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