Thousand canker disease (TCD) is an example of a native pest that expanded its geographic range and discovered new host species with no coevolved resistance.
The vector is the Walnut Twig Beetle, Pityophthorus juglandis, native to Arizona, New Mexico, and northern Mexico, where it attacks Arizona walnut.
Since the 1990s, black walnut, a tree native to the eastern U.S. but planted widely in the west as an ornamental, have been under attack.
In 2004, widespread mortality in the Front Range of Colorado was linked to the presence of the beetle, and the fungus it carries, Geosmithia morbida.
The fungus causes small cankers, but many of these together kill the tree. A Colorado State University pest alert offers comprehensive information.
The disease has recently been found in Knoxville, Tennessee. Eastern forest managers should be on the lookout for TCD. The Northern Research Station has produced a booklet to help identify walnut problems.