Thousand Cankers Disease of Walnut

Black walnut is ecologically and economically important to Missouri, and thousand cankers disease (TCD) represents a serious threat to this resource. TCD occurs when the walnut twig beetle (WTB), Pityophthorus juglandis, attacks walnut trees, spreading the fungus Geosmithia morbida which causes small cankers in the phloem tissue under the tree bark, eventually causing tree decline and mortality.

TCD has not been detected in Missouri; however there is concern that undetected TCD infestations could be present, or that spread may occur from western states or Indiana, Maryland, North Carolina, Ohio, Pennsylvania, Tennessee and Virginia where G. morbida or TCD has been detected. Potential long- distance spread of TCD through movement of infected walnut materials enhances this threat. Existing survey technology is not very sensitive, and TCD is unlikely to be detected until several years after introduction. Detection of well- established infestations makes eradication efforts difficult. It is important to conduct detection surveys for TCD, as well as inform citizens about the disease and the risk of wood movement from areas with known infestations.

In 2014, both the Missouri Department of Conservation (MDC) and the Missouri Department of Agriculture (MDA) conducted surveys for TCD using USDA Forest Service and USDA Farm Bill funding respectively. MDC placed 76 WTB pheromone-baited Lindgren funnel traps at high-risk locations with declining walnut trees (urban areas,campgrounds, sawmills) in central and south central Missouri. MDA placed 103 WTB traps in the St Louis and southwest Missouri metro areas. Visual surveys were conducted by both agencies in high-risk areas to identify potentially infested trees. Branches were examined from suspect trees. When suspicious symptoms were observed under the bark, sample branch sections were triple-bagged, placed in a cooler with ice and transported to diagnostic facilities for evaluation and culturing as needed. Survey efforts are rotated to different areas each year; 58 counties were sur- veyed in 2014. Since 2010, 1158 locations have been surveyed visually and 434 WTB traps deployed. No evidence of WTB or TCD has been detected in Missouri. Common walnut problems detected during surveys included drought stress, site-related issues, and infestation by several other wood- boring insects (primarily roundheaded and flatheaded borer larvae and ambrosia beetles).

Outreach efforts were continued to raise public awareness about TCD. Messages included the potential impact of TCD, the threat posed by movement of infected walnut materials, and identification of suspect trees. Several state agencies and institutions including MDC and MDA as well as stakeholder groups were involved in outreach.