• California's 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" (Geosmithia pallida) to trees in half a dozen California counties.

    The western oak bark beetle is a brown colored small beetle native to California and measures about 2 millimeters long that burrows through the bark of the coast live oak tree, usually drought stressed or recently wounded oaks, excavating shallow tunnels under the bark across the grain of wood while female beetles lay their eggs in the tunnels. Symptoms include wet discoloration around the beetle entry holes, followed later by a reddish sap, then a white to very light brown foam. They are usually seen first on the trunk and primary branches. Infected trees can produce a prodigious amount of foamy liquid that can run up to 2 feet down the trunk. There may be dozens of infection sites on one tree, on the branches or the trunk. The beetles are not known to burrow into trees of other species.

    "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. http://ucrtoday.ucr.edu/22096

    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.  Currently, no control methods are in place to control the fungus or the beetle.

  • 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.

    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.