Thousand canker disease (TCD) ) is an example of a forest health condition caused by native pests that have expanded their geographic ranges and switched to new host species with no coevolved resistance. The vector is the Walnut Twig Beetle, Pityophthorus juglandis, which historically fed on Arizona walnut in its native Arizona, New Mexico, and northern Mexico, and perhaps on southern California black walnut in southern California. The beetle carries a fungus, Geosmithia morbida, which causes small cankers beneath the bark of the branches and stem of walnut trees. The presence of numerous cankers, for which the disease is named, will lead to the death of affected trees.
Since the 1990s, black walnut, a tree native to the eastern U.S. but planted widely in the west as an ornamental, has 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.
In 2010, the first case of TCD in the native distribution of black walnut was reported in Knoxville, Tennessee. In 2011, TCD was confirmed in Virginia and Pennsylvania. This was followed by reports in 2013 of TCD in Ohio and of G. morbida (but not walnut twig beetle) from North Carolina. Eastern forest managers should be on the lookout for TCD.
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