The reproductive host list for tree species affected by the polyphagous shot hole borer has topped 35 species as of March 2015. This list includes species that are capable of successfully growing the fungi that cause Fusarium dieback, and supporting reproduction of the polyphagous shot hole borer beetle, as confirmed by UC Extension Plant Pathologist Dr. Akif Eskalen.

Why is that important?  Even though another 200+ species may be susceptible to Fusarium dieback, and may even be killed by it, these species are the ones that are capable to spread the pest to new areas, and managing this disease complex relies on stemming that spread. It's extremely important to monitor any of trees of these species that have an active infestation, and make sure to remove wood that is an active source of the beetle, whether by removing branches, or the entire tree.  

The list includes 13 species native to California, including all of the major riparian tree species found in the southern region - this is where it's most likely to cause significant ecological damage.  It also includes many of our most common street and residential trees, as well as a high value agricultural product, avocado.

There are also two common and problematic invasive species on the list, castor bean, and tree of heaven. This may seem like a silver lining, but because these species are so widespread, they may be active agents in vectoring the pest to new areas. 

 Below, UC Extension Plant Pathologist Akif Eskalen trains crew for PSHB identification in the lower Arroyo Seco. 

Re-"printed" from UCANR's Topics in Subtropics Blog - to see the original post, visit http://ucanr.edu/blogs/blogcore/postdetail.cfm?postnum=11722

 

Concern about the polyphagous shot hole borer is spreading up the west coast, with this article from the Seattle Times. The author does a great job of laying out the known history of PSHB research, and concisely stating why everyone should be concerned: 

The consequences of a wide-ranging infestation could be enormous. Common city trees, such as American sweet gum and maple, would become public branch-dropping hazards. Native trees such as the California sycamore and the coast live oak have started to succumb, creating a fire risk in the form of dead, dry tinder. Avocados and other crops could face huge financial losses.

 

UCR's Plant Pathology and Entomology team just issued a new alert for a pest affecting Coast Live Oak (Quercus agrifolia) throughout Southern California and up the Central Coast (LA, Orange, Riverside, Ventura, Santa Barbara, and Monterey counties). Foamy Bark Canker Disease is caused by a fungus, Geosmithia pallida, that's spread by the Western Oak Bark Beetle (Pseudopityophthorus pubipennis). 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. You can tell the difference between this disease complex and the polyphagous shot hole borer as the entry hole is even smaller than the >0.1 inches shot hole borers make. You can read the complete pest alert here. 

The continuing drought has made native oaks more susceptible to insects and disease. In most years, its advisable not to water oaks in natural settings, but in areas that haven't received any spring rain, it may be advisable. For more information on managing landscape trees during the drought, check here.