From Mendocino to Santa Barbara counties, California's oak trees are dying at an alarming rate. The cause of the massive mortality is unknown.

UC plant pathologists are studying two unidentified fungi species that were isolated from tanoaks and coast live oaks to see what, if any, role they play in the oak deaths. “I think it's likely that we won't find a single organism is causing these trees to die,” says Tom Gordon, UC Davis plant pathologist. He notes that many fungi live on the oaks without causing a problem, but under stress, trees may become vulnerable.

Since 1995, tanoak (Lithocarpus densiflorus) and coast live oak (Quercus agrifolia) have been dying in large numbers in Marin, Santa Clara, Santa Cruz, and Monterey counties. The mysterious disease has spread to black oak species.

Pavel Svihra, Marin County horticultural advisor, flew over 30 plots of coast live oak and estimated the loss at about 40%. Marin Municipal Water District estimates 10,000 tanoak trees have been killed by the enigmatic pathogen since 1995.

Not only does the loss of these highly valued trees from gardens and forests cause concern, but the thousands of dead trees also pose a serious fire hazard. The rate of live oak death in Marin County prompted county supervisors to ask the state for $3.1 million for research and to help manage the epidemic.

"Never before have we experienced such a rapid death of oaks,” Svihra says. “When symptoms start to manifest, it will last no more than 6-8 weeks, then the tree is gone.”

The first noticeable sign is wilted shoots, then older leaves turn pale green and 2 to 3 weeks later, the foliage turns brown. The inner bark of affected trees “bleeds” burgundy-red sap.

Ambrosia beetles and oak bark beetles attack the distressed oaks. Svihra recommends spraying diseased trees with the insecticide Astro to prevent beetles from boring inside the tree and hastening its death. By applying the insecticide between June and August last year, he was able to prevent beetle attack until mid-November.