ShareThisDiners in the courtyard of Sophia's Thai Bar and Kitchen in Davis, Calif., will want to enjoy the shade while they can, because the black walnut tree providing the canopy is at risk.
"Thousand cankers disease," the coined name for a newly discovered pathogen, is infecting and slowly killing hundreds of black walnut trees in California and seven other Western states.
The disease is caused by a previously undescribed fungus hitchhiking on a tiny bark beetle native to California and widely distributed from San Diego to Shasta counties.
By itself, the walnut twig beetle does limited damage. But the aggressive fungus it carries can kill a walnut tree in several years, said Steven Seybold, a forest entomologist with the Davis-based Pacific Southwest Research Station and an affiliate of the University of California, Davis, entomology department.
"The beetle has been collected in California for a long time," said Charles Leslie, a staff research associate for the walnut improvement program in the UC Davis department of plant sciences. "But we don't know yet if the fungus is new to the trees."
Its high genetic diversity "suggests that it may have been in the U.S. for a long time, but it is behaving like an invasive pathogen," Seybold said.
The fungus was first identified in 2008 by Ned Tisserat, a plant pathologist at Colorado State University, who named the disease "thousand cankers." It also has been found in Colorado, Arizona, Idaho, New Mexico, Oregon, Utah and Washington.
Because the disease is already present throughout California, Seybold said, continued high levels of crown dieback and tree death are anticipated.
California black walnut -- different from the commercial variety commonly called English walnut that is grown in the Central Valley of California -- is known more as a shade tree than for its hard-to-crack nut. It provides food for wildlife, and its wood is used for furniture and musical instruments.
Seybold said the disease and the beetle can affect the English walnut, as well, but those trees seem to have a lower incidence of infection in the field.
"All reports at this point indicate it does not attack English walnut," said Steve Lyle, director of public affairs for the California Department of Food and Agriculture.
Yet, scientists and commercial growers are concerned. California's $754 million walnut industry involves more than 4,000 growers and more than 200,000 acres in the Central and coastal valleys, according to the California Walnut Commission. That accounts for 99 per cent of the U.S. commercial supply and two-thirds of the world's stock.
The primary disease symptom is dark-stained spots on the outer bark of branches, which -- when peeled off -- reveals dead tissue in the phloem and cambium layers of bark. Infected trees also show yellowing and thinning of the upper crown, wilting leaves, flagging branches, dieback and eventual death, sometimes all within three years.
Seybold said there are no tested methods for managing the disease. His lab is working on an attractant to monitor the beetle, but removing infected trees, and burning or grinding them, is probably the best practice, he said.
(Distributed by Scripps Howard News Service, www.scrippsnews.com.)
Must credit Sacramento Bee
