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Posted 27 September 2010. PMN Crop News.

Scout Your Black Walnuts for a New Disease - Thousand Cankers Disease

Source: University of Maryland Press Release.

College Park, Maryland (September 10, 2010)--Emerald ash borer, sudden oak death... just when you think it canít get any worse, another disease appears on the horizon, this time threatening our black walnut trees!


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. 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 treesí ability to take up water. The beetles entrance holes into the black walnut arenít much bigger than a pin hole. While itís 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. The latter is a somewhat stronger possible symptom. Then try to collect a dead or dying limb and look for the minute exit holes.

When branches are peeled carefully, the exit holes will have associated larval galleries. This can be considered confirmation of this disease since the damage caused by the walnut twig beetle, Pityophthorus juglandis will be closely associated with the fungus Geosmithia morbida which causes thousand cankers disease. The best size for looking for evidence of colonization is walnut branches 1-2 inches in diameter.

If you find black walnut trees with suspicious symptoms please call the Home and Garden Information Center (HGIC) at 1-800-342-2507 or e-mail the location, description and photos to HGIC at

David Clement