Bob Polomski, Ph.D., Horticulture and Urban Forestry
School of Agricultural, Forestry, and Environmental Science
Clemson University
Since the mid-1990s I’ve been following Thousand Cankers Disease or TCD and the demise of black walnuts (Juglans nigra) in the western U.S.1, 2 Then the Tennessee Department of Agriculture in August 2010 found TCD in Knoxville, the first state east of the Mississippi that detected its presence in the black walnut’s native range.3 Nearly a year later the Virginia Department of Agriculture and Consumer Affairs issued a press release that documented the arrival of TCD in Chesterfield and Henrico counties.4
TCD is an insidious disease that requires the complicity of two vectors: a walnut twig beetle and a fungus, Geosmitha morbida. The walnut twig beetle, native to Arizona, California, and New Mexico, is associated with Arizona walnut (J. major) and perhaps other native southwestern species such as J. californica and J. hindsii.5 The walnut twig beetle was originally described in 1928, but only recently was the disease complex recognized and described in Colorado.6
Alone, the bark beetle and fungus do not kill trees; together, they prove to be a lethal combination. The tunnels and egg galleries created by the larvae and adults, respectively, provide colonizing sites for the fungus. Infections lead to the formation of quarter-sized cankers in cambial tissue. These cankers eventually coalesce and lead to twig, branch, and trunk dieback. TCD-afflicted trees, riddled with a multitude of cankers, eventually succumb within 3 to 4 years of the initial attack.2, 7
Currently, there are no preventive or curative measures against TCD. Officials in quarantined areas recommend rapid detection and removal of infected trees. To curb its spread, officials prohibit the movement of all walnut plants and plant parts of walnut, including logs, stumps, firewood, roots, branches, mulch and chips from quarantined areas. Unfortunately, the actions and movements of the walnut twig beetles and accompanying fungus are unknown, as well as the impact of TCD on native black walnut ecosystems.
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