CHEYENNE – Officials have confirmed the presence of Thousand Cankers Disease (TCD) – an invasive, destructive pest – in a black walnut tree within Cheyenne.
A specimen collected on Aug. 4 of black walnut, Juglans nigra, located on the state Administration and Information Department grounds in Cheyenne, was confirmed by the Colorado State University Plant Diagnostic Clinic to have cankers caused by TCD. This finding represents the first time the pathogen has been officially identified in Wyoming.
Assistant State Forester Aaron Lumley noted “finding Thousand Cankers in Wyoming was not unexpected, but frustrating for our tree canopy. While not common in our rural landscape, walnuts can be found providing shade, reducing energy costs and beautifying our communities around the state.”
TCD is considered an “insect-disease complex” consisting of three factors: 1) walnut twig beetle, Pityopthorus juglandis, a native beetle that feeds on the inner bark, 2) fungal spores, Geosmithia morbida, carried on wings of the beetle and deposited around galleries, and 3) subsequent canker formation around galleries (TCD Pest Alert, 2010). TCD originated in the western United States and primarily impacts black walnut, Juglans nigra. Other walnut species and trees within the genus Juglans experience mild to no impact from TCD.
Although the walnut twig beetle (south-western native range) and black walnut trees (eastern native range) are both native to North America, neither are native to Wyoming. Initial reports of the walnut twig beetle were reported in Colorado as early as 2004. Along the Front Range of Colorado, thousands of black walnut trees have been removed due to TCD mortality.
According to the city of Cheyenne Urban Forestry Department online tree inventory, there are 13 city-owned black walnut trees. This does not account for private trees (i.e. backyard trees).
Black walnut trees found throughout Wyoming are grown as residential or park landscape trees and hold little commercial value.
The detection of TCD is difficult during early stages of the disease. Generally, detection is only possible when there are external symptoms, after the tree is already infested. Typical symptoms include early yellowing foliage and thinning canopies. Twig and branch dieback follows, with larger areas wilting. The progression of tree mortality typically occurs within two to three years for mature trees. However, symptoms may not present for 5-10 or more years after initial infestation. Round exit holes (>1/16th inch) in fading branches (<3/4rd inch) may indicate walnut twig beetle activity and subsequently TCD.
Management options for TCD are limited. While the beetles are susceptible to bark beetle insecticides, effective control with spraying have not been able to prevent the spread of TCD.
Wood infected with TCD should be removed as soon as possible and sanitized to prevent further spread of the walnut twig beetles. Sanitization for TCD requires one of the following processes: heat treating, double chipping, or burying of infested material.
Wyoming State Forestry Division is working with city, county, state, and federal partners on future management of TCD.