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Bill Pramuk, Trees and People: New pest and disease complex killing upvalley oaks
Trees and People

Bill Pramuk, Trees and People: New pest and disease complex killing upvalley oaks

  • Updated

On Feb. 24, I attended a workshop in St. Helena covering a newly reported pest/disease problem killing oaks in and around upper Napa Valley. It is new in North America, and Calistoga appears to have the only outbreak.

Presented by the California Department of Food and Agriculture (CDFA), the workshop was attended by about 50 local arborists, UC Master Gardeners, and interested professionals. It began with a presentation on the current status of Sudden Oak Death and recommended management practices that can help stem the spread of the disease. I will get to that in my next column.

The main attraction was this newly confirmed pest and disease complex killing mature valley oaks, and now some blue oaks, mainly in Calistoga, but with some finds in St. Helena and one in Middletown. It started with mature valley oaks in Calistoga and, hopefully, has not spread beyond a small geographical area.

I got wind of this in 2017, when I met with an ad hoc group including professors from the UC Berkeley Forest Pathology and Mycology Lab and some local arborists investigating dying mature valley oaks. At that time, we noted signs typical of ambrosia beetle infestation, a tiny beetle commonly infesting stressed and dying oaks. The outward signs being powdery, whitish frass accumulated on the bark. Cutting into a dying tree revealed numerous borer tunnels and dark staining in sapwood that is normally light tan to nearly white.

The lab took samples, cultured them and found a previously unreported fungus. They ran a field trial, inoculating small trees on a volunteered private property. The field study results were not conclusive with respect to the fungus as a confirmed tree killer, but they reported their work to CDFA, which is now working to develop a better understanding of it and guidelines for prevention and management.

This particular beetle is called Xyleborus monographus, a kind of ambrosia beetle. They bore into trees and introduce a fungus that turns wood into food for the beetle larvae. The fungus is Raffaelea montetyi.

That strategy is typical for ambrosia beetles in general. They infested stressed and dying trees and use the fungus to make food out of wood that is not otherwise digestible. The worrisome part is that this beetle is new to the region and, though not mentioned in the presentation, Raffaelea is a genus of fungi that includes Japanese oak wilt, a devastating oak forest disease in Japan and Korea.

Referencing a study mentioned in a USDA research forum in 2006, the closely related fungus “is the first example of an ambrosia beetle fungus that kills vigorous trees.” This is of serious concern with respect to our valley oaks, but so far, the Calistoga finds seem to be limited to mature trees stressed by years of drought and it is not the same species of Raffaelea.

This Xyleborus species is native to “Mediterranean” type climates, including parts of Europe to the Middle East and North Africa and it infests various trees including white oaks, maples, walnuts, beech, elm, chestnut, cherry and hornbeams. Locally, another tree species at risk is cork oak, of which we have some remarkable specimens here in the valley.

At present, Dr. Akif Eskalen, plant pathologist at UC. Davis, is studying the pathogenicity of the disease: How bad is it— an opportunist or an aggressive pest/disease complex?

CDFA is working on management guidelines. Dr. Tom Smith, speaking on the latter, said we do not have them yet. He said we do not know how this pest got here and we do not have a formal zone of infestation, but “we hope we are in the early phase of infestation.” He added, “Maybe we can do something; maybe we have a chance for control.”

At the moment, lacking formal guidelines and regulations, the consensus I took away from the meeting is:

— Moving wood moves pests. Keep it on site.

— Solarize small stacks of wood with clear plastic sheeting, snugged tightly to the ground for six months in cooler weather or six weeks if the tree is taken down in hot weather.

— The pest can survive the woodchipper. If equipment is capable, chipping to less than ½-inch pieces will probably kill the beetles. In my estimation, if the chips are big, it makes sense to solarize the chip pile, same as the logs.

No formal guidelines are available yet for insecticide treatments.

I will share more news when it is available.

Note: The 2020 Napa Sudden Oak Death Blitz will be held on April 11, at 9:30 a.m., at the UCCE classroom, 1710 Soscol Ave. This free event explains SOD and gives participants the opportunity for free SOD testing using California bay laurel leaves from their own property. There is no charge and no pre-registration.

Bill Pramuk is a registered consulting arborist. Visit his website, email questions to or call him at 707-363-0114.

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