California's Live Oaks Face New Fungus Disease Threat

California's emblematic coast live oaks are facing a new threat: an irreversible and potentially fatal fungal disease spread by a formerly innocuous beetle. The western bark beetle, which was once considered a minor pest of the evergreen oaks, was recently found to be spreading a disease that researchers call "foamy bark canker disease" to trees in half a dozen California counties.

"We have found declining coast live oak trees throughout urban landscapes in Los Angeles, Orange, Riverside, Santa Barbara, Ventura and Monterey counties," said Akif Eskalen, of the Department of Plant Pathology and Microbiology at the University of California, Riverside.

Eskalen says that the disease, which the beetles spread when they bore into the tree's vascular tissue beneath the bark, causes the tree to exude reddish sap and abundant foamy liquid from the beetle holes. Infected branches then die back, and advanced infections can kill the entire tree.

The fungus that causes the disease, Geosmithia pallida, has long been known from other parts of the world but it's new to the state. Other species of Geosmithia also cause tree canker diseases and are associated with bark beetles as a vector for spreading them to new trees.

The western oak bark beetle, whose taxonomic name Pseudopityophthorus pubipennis is a whole lot longer than the beetle's two-millimeter body, is native to California and has been burrowing through the bark of coast live oaks since time immemorial. The beetles are not known to burrow into trees of other species. That could mean that other species of oaks such as the canyon and interior live oaks are safe from the fungus for the time being. It could also mean that scientists have just not documented the beetles using those other species.

There's no treatment for the fungal infection at present. Eskalen suggests that tree owners who fear their oak may have been infected call their relevant county agency or email his lab at akif.eskalen@ucr.edu.

A couple of common-sense measures do suggest themselves. Western bark beetles tend to be attracted to oak trees under stress, and Eskalen mentions finding diseased trees in urban areas. Many urban live oaks are under continual stress, usually from getting too much water in the summer. You should almost never water plants growing under the drip line of a coast live oak. If you've been doing so, now's a good time to stop.

And if you have an oak tree that has succumbed to unexplained ailments, do not move the resulting firewood to another location lest bark beetles emerging from the wood infect healthy trees.

(For oak lovers seeking more information on the trees' care and feeding, he California Oaks Foundation has an excellent care sheet for live oaks in a garden setting.)

By the way, foamy bark canker disease shouldn't be confused with sudden oak death, the infection that's been killing live oaks and other trees in coastal Northern and Central California. Sudden oak death, caused by the pathogen Phytophthora ramorum, hasn't yet been confirmed in trees south of Paso Robles. It's been thought that Southern California's drier climate has limited the spread of Phytophthora, an odd pathogen once thought to be a fungus but now considered to be more closely related to kelp than it is to chanterelles.

Sadly, now it looks like SoCal has its own infectious live oak disease to worry about.