Sudden Oak Death pathogen found in coast redwood, Douglas fir
Researchers unable to predict long-term impact on valued forests

By Sarah Yang, Media Relations

11 September 2002 | Two of California’s most highly prized trees — coast redwood and Douglas fir — are susceptible to Phytophthora ramorum, the pathogen that causes Sudden Oak Death, University of California researchers have confirmed. Over the past seven years, Sudden Oak Death, a highly contagious fungus-like disease, has killed tens of thousands of oaks and tanoaks along the northern coast of the state.

Researchers from Berkeley and UC Davis have isolated living cultures of P. ramorum from the branches and needles of coast redwood and Douglas fir saplings that had shown symptoms of infection. The researchers first announced the discovery of P. ramorum DNA in the trees earlier this year, but couldn’t confirm that the pathogen was causing infection until living cultures were successfully grown from the field samples.

It is not yet clear how seriously the disease will impact California’s coast redwood and Douglas fir trees, which are ecologically and economically vital to the state, particularly to the timber, nursery, landscape, and construction industries.

The infected redwood saplings were found at Jack London State Park in Sonoma County and Henry Cowell State Park in Santa Cruz County. The infected Douglas firs were found at another site in Sonoma County.

The researchers also conducted DNA tests on diseased sprouts growing from the base of mature redwood trees in Marin, Alameda, and Monterey counties. The presence of the pathogen in the sampled trees has been strongly suggested by repeated positive DNA identification.

The number of identified species susceptible to the Sudden Oak Death pathogen has steadily grown since the disease was first reported in Marin County in 1995. With the addition of coast redwood and Doug-las fir, there are now 17 known species worldwide susceptible to P. ramorum. Sixteen of them are found in California, including the madrone, bay laurel, and buckeye.

“It seems that some species are able to tolerate the pathogen better than others,” says David Rizzo, associate professor of plant pathology at UC Davis. The research was a collaboration between the laboratories of Rizzo and Matteo Garbelotto, adjunct assistant professor of ecosystem science and a cooperative extension specialist at Berkeley’s College of Natural Resources.

“We see a whole range of symptoms in the field, from nasty cankers on the trunks of oaks to minor spots on the leaves of the buckeye,” says Rizzo.

It is unclear what the new findings mean for the health of redwoods and Douglas firs in the long run, say the researchers. “Since we have not seen evidence of disease symptoms or death from the pathogen in large, mature trees, we cannot say what the effects of the infection will be long-term,” says Garbelotto, noting that symptoms have been detected only on the needles and very small branches of redwoods.

In addition to checking diseased trees in the field, the researchers conducted a battery of lab tests to see how P. ramorum would affect healthy trees and to confirm that the pathogen was the cause of the symptoms observed in nature. “We essentially confirmed in the lab what we observed out in the field,” says Rizzo.

The discovery of P. ramorum in the redwood — one of California’s most treasured symbols — hits a sensitive chord for many in the state. The majestic trees can reach heights of more than 350 feet and live to be 600 to 2,000 years old.

“It may take years before we can start answering questions about the ecological impacts of the disease on coast redwood and Douglas fir,” says Rizzo. The researchers emphasize the need for further study, noting that they have been studying the biology of P. ramorum in redwoods and Douglas firs for only a short time.