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Don't aid spread of tree disease
Thousand cankers disease poses threat to black walnut trees.

 Beneath the bark are cankers caused by the beetles that spread thousand cankers disease.
Elizabeth Bush/Virginia tech
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Date published: 8/12/2011

AMALADY known as thousand cankers disease which infects black walnut (Juglans nigra) trees was found in Virginia earlier this summer, less than a year after it was first discovered in the Eastern U.S. Experts suspect it is widespread.

While the name may conjure up pictures of a tree riddled with big cankers, which are typically areas of sunken wood tissue sometimes resembling a target, the cankers from this disease are not obvious to the casual observer. They are small, as cankers go, but they are many. Over the course of many years, they coalesce and begin to cut off the flow of water and nutrients throughout the tree. This leads to a gradual decline in vigor and eventual death.

The fungus (Geosmithia morbida) that causes the cankers is introduced into the tree by a 2 millimeter "twig beetle" (Pityophthorus juglandis). This Latin name tells us that the beetle is specific to trees in the Juglandaceae, or walnut family. Fortunately, pecan seems to be resistant to thousand cankers disease, as is the main host of the beetle, Arizona walnut (Juglans major).

This beetle and disease have been well known for over a decade in the West. In fact, both the beetle and the fungal pathogen are believed to be native to North America. The problem seems to have begun when black walnut (an Eastern species) was introduced to the West. When it became apparent that black walnut was not resistant to thousand cankers disease in the West, suspicions were high that the same would be true if the disease ever found its way east.

It did. Last August, the first Eastern occurrence of thousand cankers disease was confirmed in Knox County, Tenn. A subsequent confirmation in Chesterfield County near Richmond less than a year later suggests its presence throughout the Eastern U.S. There's a good chance that black walnut trees in poor health may have had some other disease or condition that masked the presence of thousand cankers disease.

At first glance, the symptoms tend to mimic such issues as as compaction, root damage, Phytophthora collar rot, etc. The cankers are not obvious. The first symptoms may be sudden leaf wilting in the spring and dying branches.

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Adam Downing is an agent in Virginia Cooperative Extension's Madison County office, specializing in forestry and natural resources. Phone 540/948-6881; fax 540/948-6883; email adowning@vt.edu.