Dead wood. An oak-tree plague is creeping through California.

Culprit Named in "Sudden Oak Death"

A dread disease is striking California's coast live oaks with the ferocity of an oak-tree Ebola virus, causing the trees to sprout sores, hemorrhage sap, and become infested with beetles and various fungi. The trees die within a few weeks of their first symptoms.

To find the cause of "Sudden Oak Death," state officials earlier this year assembled a team of researchers from the University of California (UC). It didn't take them long. Last week plant pathologist David Rizzo of UC Davis announced that his team had found the fungus that causes the disease.

The tree slayer is a new member of the genus Phytophthora, whose name means "plant destroyer." Its kin include pathogens responsible for the Irish potato famine and for die-offs in Australian eucalyptus forests and European oak groves. "We don't know if [the new species] was just recently introduced, or if it has always been here and something else has changed that has allowed it to go crazy," Rizzo says.

The first trees to succumb to the plague 5 years ago were tanoaks, which often grow in the understory of redwood forests. Last year the disease began hitting large numbers of coast live oaks, the signature species in scenic coastal woodlands. As many as 40% of oaks in some patches died, creating a fire hazard and disrupting habitat. Alarmingly, another species, the black oak, has also begun succumbing.

Knowing the culprit doesn't make the outlook much brighter. Fungicides, applied early enough, can save individual oaks, says Rizzo, but "we can't go to Mount Tamalpais and spray 10,000 trees." And prevention is largely limited to warning homeowners not to overwater trees--the pathogen thrives in damp conditions--and asking people not to carry oak firewood to uninfected areas. The beloved Sierra Nevada hosts black oaks, and Rizzo fears the deadly spores could wind up blighting groves in Yosemite valley.

Related sites

Press release by the University of California, Davis

Monitoring sudden oak death in coastal California