Thousand Cankers Disease

thousand canker
Walnut leaves that have collapsed and wilted. Photo Curtis Utley, Colorado State University Extension, bugwood.org

Originally found only in western states, a recent report from Knoxville Tennessee confirmed that Thousand cankers disease of walnut has become established in that area. This new finding suggests that the walnut twig beetle was introduced and established in Tennesee a very long time ago. How it got there is still a mystery, but hopes of keeping this disease confined to western states is now not possible. Infested wood may have been transported out of Tennessee to other eastern states for years. Alerts are being sent out and several states are now considering initiating surveys for initial symptoms.

thousand canker
Relative size of a Walnut twig beetle compared to a cat flea (flea top photo)
Photo: D. L. Clement, UME, Home and Garden

The walnut twig beetle is reddish-brown and 1.5 to 1.9 millimeters long. The disease is spread by this beetle when it makes galleries under the bark. The fungus causes the tissue to die disrupting the trees’ ability to take up water. The beetles entrance holes into the black walnut aren’t much bigger than a pin hole. While it’s unlikely you will see the insect, its damage is more obvious. To scout for symptoms look first in black walnut trees with existing crown dieback. Then look for individual branches that show flagging with either yellowing leaves remaining attached or leaves that have collapsed and wilted (Fig. 2). The latter is a somewhat stronger possible symptom. Then try to collect a dead or dying limb and look for the minute exit holes.

thousand canker
Walnut twig beetle galleries under the bark of an infested tree. Photo Whitney Cranshaw, Colorado State University

Links: Thousand Cankers

         Maryland Department of Agriculture Quarantine

         Maryland Department of Agriculture walnut twig beetle and thousand cankers disease

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