Our native coast live oaks (Quercus agrifolia) are under a new threat.
In 2015, a new fungal disease of live oaks was noticed in southern California. That disease was recently spotted in Los Gatos and that disease is called foamy bark canker.
Foamy bark canker is, like most canker diseases, fungal. This particular fungal disease is caused by the Geosmithia pallida fungi, which is carried by the western oak bark beetle (Pseudopityophthorus pubipennis).
Drought has weakened the majority of large trees in California, making them susceptible to countless pests and diseases. Physical injuries, other diseases, and feeding by other pests all leave holes in the bark. Healthy trees are able to protect themselves, but drought-stressed trees are not.
Symptoms of foamy bark canker
Red, oozing discharges can be seen at beetle feeding injuries along the trunk and from primary branches. As the disease progresses, that ooze becomes foamy and prolific. Drips may run down the trunk as much as 2 feet. If you peel back the bark, you will see that the phloem has died around the entry point. You may also see several entry holes with similar symptoms. Foamy bark canker is easily mistaken for fusarium wilt or shot hole borer, but the entry holes are smaller.
Western oak bark beetle
By itself, this small (2 mm) bark beetle is more of a pest than a threat. Bark beetles burrow under the bark, creating a gallery of tunnels that cross the wood grain. Female beetles lay their eggs in tiny pockets that are perpendicular to these tunnels and the larva feed and tunnel their way to adulthood. These pests can certainly weaken a tree, but healthy trees are generally able to survive. However, just as elm bark beetles carry the devastating Dutch elm disease, which killed 75-90% of the global elm tree population, this fungal disease may prove to be a serious threat to coastal live oaks in the Bay Area.
Management of foamy bark canker
Unfortunately, there are no cures for foamy bark canker. The best thing you can do is to keep trees healthy in the first place, when proper selection, placement, irrigation, pruning, and feeding. Large trees are a big investment and they are difficult (and expensive) to replace. In times of drought, large trees should be watered to a depth of at least one foot, two or three times a month.
If your tree develops symptoms, please contact your local Cooperative Extension Office office right away. This will help them document the spread of the disease and they can provide helpful information.
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