BRANCHING OUT Thousand Cankers Disease


Article contributed by International Society of Arboriculture


One of the newest and most alarming threats to our forests is the thousand cankers disease (TCD) affecting black walnut (Juglans nigra) trees. TCD has been found primarily in the western part of the United States; however, this summer it was discovered in eastern Tennessee. This marked the first time that an outbreak of TCD was found east of the Mississippi River and in the black walnut’s native range.


Why is TCD a concern?


Black walnut is a valuable source of both timber and nuts. Other species of walnut have been found to be susceptible to the disease in varying degrees, as well. Trees are often killed within three years of symptoms becoming visible; however, infection usually begins years before it is detected.


TCD is associated with a fungus (Geosmithia sp.) which appears to develop in conjunction with the presence of the walnut twig beetle (Pityophthorus juglandis). When the walnut twig beetle, a very small .06 – .08 inches (1.5 – 1.99 mm) insect native to North America, attacks a black walnut, the fungus (Geosmithia sp.) tags along, forming cankers around the gallery tunnels created by the beetle. A second fungus (Fusarium solani) has also been found in the trunk and larger limbs in association with canker formation there.


The first signs of TCD are yellowing and thinning in the upper crown of the tree, followed by death of progressively larger branches. Eventually, in the final stages, there may be a rapid wilting of foliage. The cankers may or may not be visible, though there may be an amber stain on the bark. And the cankers expand quickly, eventually girdling branches and producing dieback.


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Currently, there is no cure or control for TCD. Chipping the wood does not necessarily kill all walnut twig beetles due to their size. Even if the beetles are controlled using systemic insecticides, once a tree has started to produce the cankers, the fungus may continue to spread. Right now the diligent detection and removal of trees is the best means of preventing the spread to neighboring walnuts, and there have been many quarantines on moving wood put in place in states such as Missouri, Kansas, Michigan, Nebraska, Oklahoma, Indiana, North Carolina and Wisconsin. It is uncertain how effective insecticides may be, though drenching insecticides applied in late summer before the beetle overwinters in the tree may help.


Trees are often killed within three years of symptoms becoming visible; however, infection usually begins years before it is detected.


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