In 2008, Ned Tisserat, a plant pathologist at Colorado State University, first identified the fungus, Geosmithia sp., involved in the disease.
The fungal disease enters the black walnut trees with the walnut twig beetle, a tiny bark beetle native to North America that enters the tree to produce egg galleries.
Cankers on the branch of a black walnut tree in Colorado (Photo courtesy Colorado State U.)
The fungus aggressively colonizes, producing large dead areas, or cankers, at each point of attack. Together, the beetle and fungus is known as thousand cankers disease.
The cankers ultimately fuse to girdle limbs and the trunk, restricting a tree’s water and nutrient movement. Infected trees exhibit drought-like symptoms such as leaf yellowing, wilting and branch dieback.
Walnut trees infected with the disease may contain tens of thousands of walnut twig beetles.
If infested logs are transported, the disease can move into new geographic areas.
Currently, the eastern edge of thousand cankers disease exists in pocketed areas along the Colorado’s Front Range.
Whitney Cranshaw, a Colorado State University entomologist, says this serious situation may become "catastrophic" if infective walnut twig beetles colonize in areas east of Colorado where black walnut is a native forest species.
Based on patterns seen in the West, such a colonization is "likely to develop into an uncontrollable outbreak," he says.
To prevent such an outbreak, it is critical that foresters, arborists, woodworkers, mills and companies involved in lumber movement take every precaution to avoid transporting infected walnut logs with intact bark to areas further east, the Colorado State scientists advise.
To date, the walnut twig beetle has been found in Washington, Oregon and California, as well as in Idaho, Utah, Colorado, Arizona and New Mexico and across the border in the Mexican state of Chihuahua.
Wilting and dying, this black walnut is infested with thousand cankers. (Photo courtesy CSU)
The first published record of a cluster of black walnut mortality associated with the walnut twig beetle was in the Espanola Valley of northern New Mexico where large numbers of mature black walnut died in 2001.
However, this may have been preceded in Utah where undetermined black walnut mortality occurred in the early 1990s along the Wasatch Front; a Utah record of the beetle dates to 1988.
In the Boise-Meridian area of Idaho, where the twig beetle was first confirmed present in 2003, large numbers of black walnut trees have died.
Black walnut declines have been noted in some Front Range communities in Colorado since 2001 and the twig beetle has been confirmed present in the state since 2004. In those communities where the insect has been detected, the majority of black walnut trees have died.
Mapping the spread of thousand cankers disease, particularly along its eastern range, is a focus of research at Colorado State this year.
Controls for thousand cankers disease have not yet been identified and their development will require better understanding of the biology of the walnut twig beetle and the canker-producing Geosmithia fungus, according to a Pest Alert issued by the Colorado State scientists.
Because of the extended period when adult beetles are active and the extensive areas of the trees that are colonized, foliar insecticide spray applications likely have limited effectiveness, the scientists say.
They warn that colonization of the bark and cambium by the Geosmithia fungus may continue even if adult beetles or larvae are killed by insecticide.
Rapid detection and removal of infected trees currently remains the primary means of managing thousand cankers disease.
To report the location of a walnut tree suspected of thousand cankers disease, please contact: CAS_WalnutSurvey@ColoState.EDU.
Copyright Environment News Service, ENS, 2009. All rights reserved.