For at least the past decade, an unusual decline of black walnut (Juglans nigra) has been observed in several western states. More recently, the decline which has been attributed to thousand cankers disease (TCD), has been found in the tree’s native range: Tennessee, Virginia and Pennsylvania.

The adult walnut twig beetle carries fungal spores into trees.
Steven Valley, Oregon Department of Agriculture, Bugwood.org

TCD is an insect-disease complex native to the western United States that primarily affects black walnut. This disease is the result of the combined activity of a fungus (Geosmithia morbida) and the walnut twig beetle (Pityophthorus juglandis). TCD currently threatens millions of black walnut trees, an important species with great economic and ecological value. Several quarantines have been established in an attempt to prevent the disease from spreading.

In July 2010, the walnut twig beetle and G. morbida were found in Knoxville, Tenn. This was a highly significant discovery because it was the first report of TCD appearing within the native range of black walnut. The outbreak in the Knoxville area indicates the disease was originally introduced with infective walnut twig beetles for a considerable period. Prior to these reports, walnut twig beetle had never been associated with Juglans mortality. In most areas where the die-offs have occurred, drought was originally suspected as the cause, with the beetle as a secondary pest. The widespread area across which Juglans spp. die-off have been reported, the documented presence of an associated canker-producing fungal pathogen carried by the twig beetle, and the occurrence of black walnut death in irrigated sites not sustaining drought, all suggested an alternate underlying cause – thousand cankers disease.