The swing of the ax at state and federal budgets continues to chip away funds to battle agricultural pests and diseases in the West including thousand cankers disease (TCD) in walnut (Juglans) species in California.

At least 60 cases of the vector-transmitted disease are confirmed in California; about half each in commercial English walnut orchards and native black walnut trees in residential and riparian areas, says Richard Bostock, UC Davis plant pathologist.

“Thousand cankers is an example of new and emerging diseases which require diagnostic and research expertise to develop effective disease management options,” Bostock said.

Research dollars are limited to learn more about TCD in California walnut species. Bostock and his colleagues received funds from the USDA to evaluate various walnut (Juglans) species, including English walnut, for differences in susceptibility and resistance to TCD.

Bostock is collaborating with USDA Forest Service entomologist Steven Seybold of the Pacific Southwest Research Station in Davis, Calif., to understand the biology of TCD, particularly as the disease develops in California.

TCD occurs after the walnut twig beetle (WTB), Pityophthorus juglandis, carries spores of the fungus Geosmithia morbida on the insect’s body into the tree. The tiny insect bores holes in the tree bark to the phloem tissue. Numerous entry holes by beetles into the bark and subsequent fungal cankers are the reason for the term thousand cankers.

The phloem is the tree’s major pathway for carbohydrate movement. During colonization, the fungus kills the phloem and cankers form around the beetle galleries. Male beetles generate pheromone which attracts females that in turn create galleries to lay eggs. As the phloem degrades, bleeding cankers can become visible on the external bark.

Early TCD symptoms include yellowed leaves and thinning foliage in the tree’s upper crown followed by large branch dieback and tree collapse. Often there is bleeding on the branches and trunk. The disease has been confirmed on several black walnut species, English walnut, and Paradox and black walnut rootstocks.

There is no effective control for TCD. The disease is only found in walnuts. Still unknown is how the beetle acquired TCD.

“I don’t think there will be a magic bullet to solve the TCD issue,” Bostock said. “Insect-vectored canker diseases in trees are very difficult to manage. Our hope is TCD will not become a significant problem in English walnut orchards and California native black walnuts. However, the experience elsewhere in the West in plantings of eastern black walnut (Juglans nigra) is that the disease can aggressively kill trees.”

TCD and the association of the fungus and the insect vector were first identified though studies of dying eastern black walnut in Colorado by plant pathologist Ned Tisserat and entomologist Whitney Cranshaw of Colorado State University.

In California, native black walnuts (Juglans hindsii) infected with TCD were first confirmed in Yolo County in June 2008 and one month later in Solano County by the Colorado scientists, Seybold, and UC Davis researchers. Counties with confirmed cases in English walnut include Sutter, Yuba, Tulare, Yolo, Solano, Stanislaus, Fresno, and San Benito.