Steve Seybold

DAVIS — A newly discovered disease that kills black walnut trees is becoming a "significant problem" in California and seven other western states and could very well spread throughout the United States, Davis-based researchers say.

Thousand cankers disease, caused by a newly described fungus spread by the tiny walnut beetle, has been detected in 15 California counties from Sutter County to Los Angeles," said research entomologist Steve Seybold of the Pacific Southwest Research Station, USDA Forest Service, Davis, and an affiliate of the Department of Entomology, University of California, Davis.

Seybold will offer an update on the disease at a departmental-sponsored seminar from 12:10 to 1 p.m. on Wednesday (May 19) in 122 Briggs Hall, UC Davis. Interested persons can view the webcast live. The lecture will be archived at http://entomology.ucdavis.edu/news/webcastlinks.html.

The disease, caused by a fungus with the proposed name of Geosmithia morbida, is spread by the tiny walnut twig beetle, Pityophthorus juglandis. Often the first symptoms of the disease are flagging and yellowing leaves and branch dieback. Affected branches show sap staining and pinhole-sized beetle holes. Beneath the surface are dark stains caused by the fungus.

"The disease appears to be very abundant in Los Angeles, Solano, Sutter, Tulare, Ventura, and Yolo counties," said Seybold, a forest entomologist and chemical ecologist who has studied the chemical ecology and behavior of bark beetles for more than 25 years. "Besides one case reported from Lake County and several cases from Sutter County, it has not been detected further north in the state yet, but we believe the disease is still spreading."

Beginning this summer, a USDA/UC Davis research team will track the pathogen and the beetle throughout the state, particularly in commercial orchards.

Scientists believe that the disease occurs only on walnut, predominantly native black walnut, Juglans californica and J. hindsii, although the disease has been recorded on 10 species of walnuts or their hybrids in California. The fungus enters the tree when the beetle tunnels into the bark to prepare egg galleries. "The fungus is probably carried as conidia on the beetle's elytra or wing covers," Seybold said. 


Once inside the tree, the fungus spreads into the phloem, cambium and surface of the xylem, killing tissue and causing lesions or cankers beneath the bark.

A tree can survive the stress produced by a few cankers, but when high populations of the beetle enter the tree and the numerous small cankers coalesce, the disease girdles twigs and branches. Eventually the disease attacks the main stem of the weakened tree down to the soil line.

The walnut twig beetle, believed to be native to Arizona, California, New Mexico and Mexico, was first collected in Los Angeles County more than 50 years ago. By itself, the pest was not considered a problem, but in association with the newly described fungus, it is.

The disease was first noticed in canker-riddled black walnuts in Utah and Oregon in the early 1990s, but scientists attributed that to environmental stress. In 2006, plant pathologist Ned Tisserat and entomologist Whitney Cranshaw of Colorado State University identified the pathogen and characterized its association with the beetle in declining black walnut trees in central Colorado.

The disease since has been found in New Mexico, Colorado, Arizona, Idaho and Washington, as well as California.

"The potential spread of the disease to the eastern United States is now of great concern, especially through movement of infested/infected raw wood and firewood," Seybold said.

No control or management program prevents infection of thousand cankers disease or salvaging of infected trees, he said. However, scientists are testing pesticides targeting the walnut twig beetle and developing an aggregation pheromone for the beetle to aid in future detection and monitoring.

The "thousand cankers disease" team from Davis includes Seybold; professor Mary Louise Flint from the Department of Entomology; professor Rick Bostock and postdoctoral researcher Andrew Graves, both of the Department of Plant Pathology; and walnut specialist Charles Leslie of the Department of Plant Sciences. A team of key farm advisors from Lake, Solano/Yolo, Sutter/Yuba, and Tulare counties will provide assistance in the field.

To prevent spread, infected trees should be removed and destroyed immediately by grinding or burning to ensure that beetles are destroyed, Seybold said. Infested walnut for chips, firewood or woodworking should not be moved to new areas. Possible detections can be reported to the local agricultural commissioner's office or to the local UC Cooperative Extension office.

Related link: Beetle and fungus one-two punch threatens black walnut trees, scientists warn