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Concerned citizens and scientists have joined forces to combat a major threat to walnut trees: Thousand Cankers disease, a fungal infection, spread by the tiny, native walnut twig beetle. Susceptible trees include our magnificent Northern California black walnuts, Paradox walnuts and English walnuts, which are still grown commercially here. The disease was confirmed in Napa County late last year.

On Feb. 9, I attended a seminar presented by the Napa County Walnut TCD Task Force, an opportunity to learn the latest information on this tree pest/disease complex.

Presented by the Napa County agricultural commissioner’s office, the program included Dr. Richard Bostock, professor of plant pathology at UC Davis; Steve Seybold, research entomologist, UC Davis and U.S. Forest Service; Gretchen Hayes of Tessera Sciences; and well-informed people who have been following the problem.

Here is a brief rundown:

The disease is caused by a fungus, Geosmithia morbida, which hitches a ride on the bodies of walnut twig beetles (Pityophthorus juglandis). The beetles are only about 1.5 mm in length. They make pin-size entry and exit holes in the bark of the trunk and branches. Of themselves, the wounds and the egg galleries would probably not be life-threatening to a vigorous tree, but the fungus causes cankers, dead patches that enlarge and eventually cut off the supply of nutrients. There may be thousands of these in a single tree, hence the name.

The effects are seen as oozing patches, flagging of the foliage, dieback of branches and eventual death of the tree.

The beetle was discovered in New Mexico around 1896 but its association with the fungus at that time is not known. The fungus was new to science in 2011, Dr. Bostock said.

The disease was confirmed in walnuts in Utah and Oregon in the 1990s, then New Mexico, Colorado, Washington State, California, Nevada, Idaho, Tennessee, Pennsylvania, Ohio, Virginia and North Carolina. And now it has been confirmed in English walnuts growing in northeast Italy.

It is believed that an infestation in Bucks County, Pennsylvania, originated from a black walnut log transported there from Chico.

The extent of the disease in California includes at least 15 counties from Sutter in the north, to Los Angeles in the south. In September 2015, California’s Department of Food and Agriculture confirmed the first report of Thousand Cankers disease in Napa County.

Paradox walnut, Luther Burbank’s hybrid of English and Northern California black walnut, which is the rootstock for about 70 percent of commercially grown English walnuts, is most susceptible to the disease. Northern California black walnut and English walnuts are somewhat less susceptible.

Of particular interest to me is the similarity of this pest/disease complex to that of foamy canker in oaks, caused by Geosmithia pallida and spread by another tiny beetle, the western oak bark beetle (Pseudopityophthorus pubipennis). Interestingly, infestations of that beetle in live oaks, in my experience, have not been fatal except in recent years, when foamy canker disease began to be confirmed in our region.

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Literature distributed at the seminar, and available online from UC IPM, states, “There are currently no known control measures for diseased trees.”

Monitoring, by trapping and identifying beetles as described in “Detecting and Identifying WTB” (UC IPM, March 2013), is being employed “to detect an incipient population of WTB or delimit a known population where it has been recently discovered,” the publication says.

Management guidelines include:

— Infected trees should be removed and destroyed by grinding or burning immediately to kill the beetles. Beware that coarse wood chips could harbor live beetles carrying the disease.

— Do not move infested walnut chips, firewood or stock for woodworking to new areas, even among the counties of California. Live beetles may lurk in tiny crevices in the bark.

— Report possible detection to your agricultural commissioner’s office or local UC Cooperative Extension office.

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Bill Pramuk is a registered consulting arborist. Visit his website,, email questions to, or call him at 707-226-2884.