Tree Deaths Spike as Sudden Oak Death Spreads Across California

The U.S. Forest Service says hundreds of thousands of oak trees have died over the last year due to the plant disease known as Sudden Oak Death. Scientists still don’t have a reliable way to control the epidemic.

The spike in tree deaths, more than 300,000 in California this past year, is due to the wet spring and dry summer – ideal conditions for Sudden Oak Death to spread. The disease is appearing in new areas, including the East Bay, Big Sur and Mendocino.

Susan Frankel of the U.S. Forest Service says they expect it to get worse in years ahead. “Our models and observations predict that millions and millions and millions of tan oaks will die over the next 30 to 40 years,” she says.

“The frustrating thing is that this is an invasive pathogen,” she says. It was introduced to the Bay Area 15 years ago through nursery plants. The spores are spread by native bay laurel trees, as well as rhododendrons, camellias and other ornamental trees.

The widespread impact of the disease is easy to see in Marin County. “Around Mt. Tamalpais, we’ve had an entire change of the ecology,” says Frankel. “The tan oak used to be the dominant species there and now it’s at a low level. All the wildlife that feeds on acorns has to find other food sources.” Dead trees also pose a fire risk.

Slowing the spread of Sudden Oak Death has proved challenging. “We are trying to develop resistant oak trees, but it will take some time to develop,” says Frankel.

Preventive pesticide treatments for uninfected trees show promise, but Frankel says they’re most likely too expensive to use on a widespread basis, since trees must be treated repeatedly. (More about home treatment and what symptoms to look for).

Scientists have also tried to get the word out to plant nurseries, where the disease is easily spread. “There is progress,” says Frankel. “There are quarantines and nurserymen who are starting to control how they use water. The infestation level is going down but it is still a major concern, particularly for long-distance spread to other parts of the U.S.”

Citizen scientists are also tracking the spread of Sudden Oak Death through “SOD blitzes.” The effort is organized by a group at U.C. Berkeley, which is holding public meetings this fall about the symptoms and spread of the disease.