Black Walnut Mortality in Colorado Caused by the Walnut Twig Beetle and Thousand Cankers Disease

Since 2001, widespread mortality of black walnut (Juglans nigra) has been reported in Colorado, USA. Affected trees initially show a yellowing and thinning of leaves in the upper crown, followed by twig and branch dieback and ultimately tree death. We report that this mortality is the result of a combination of an expanded geographic range of the walnut twig beetle (Pityophthorus juglandis), its aggressive feeding behavior on black walnut, and extensive cankering caused by an unnamed Geosmithia fungus associated with the beetle. Geosmithia was consistently recovered from the bodies of P. juglandis and this insect introduces the fungus into healthy trees during gallery formation. This is the first report of Geosmithia as a pathogen of black walnut. We propose the name Thousand Cankers to describe this disease because mortality is the result of bark necrosis caused by an enormous number of coalescing branch and trunk cankers. A second pathogen, Fusarium solani, was isolated from the margins of elongate trunk cankers during the final stages of decline, but not from cankers surrounding beetle galleries. Thousand Cankers Disease is eliminating black walnut along the Front Range of Colorado and poses a grave risk to this species in its native range in eastern North America should the insect/Geosmithia complex be introduced. Accepted for publication 26 May 2009. Published 11 August 2009.