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A health black walnut (Juglans nigra)
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A black walnut infected with Thousand Cankers disease.
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Thousand canker disease has been sweeping through black walnuts in the northwestern suburbs toward Denver for several years now, and now is here in full force. First observed in the Front Range in 2003, there are now known infestations in eighteen Denver neighborhoods, from Regis to Harvey Park and east to Cole and Whittier.
This disease girdles trees as tens of thousands of walnut twig beetles (Pityophthorus juglandis) infect trees with a small canker-producing fungus (Geosmithia sp.). Infected trees generally die within a couple of months to a couple of years after they first show symptoms of leaf yellowing and branch dieback. The leaves typically flag on the tree: first yellowing, then drying and remaining on the tree for an extended period. Usually the upper branches are the first to decline. This is followed by a progressive dieback that over time causes death of larger limbs and, ultimately, the entire tree. Although the fungus creates cankers within the bark, they are only occasionally visible as a dark stain on the trunk.
The production of leaf yellowing/flagging of black walnuts can have many other - and far less serious - causes. However, in areas where the disease is suspected to occur there should be additional examination to determine if thousand cankers disease is the cause.
If you have a black walnut on your property that is showing symptoms of this disease or if you would like to schedule an inspection of your tree, please call the City Forester's Office at 720-913-0651 or contact the Arboreal Inspector in your area.
The long-term prognosis for black walnuts as either an ornamental or a fruit-producing tree in Colorado is not good. Some companies report limited success when pre-treating trees with an insecticide to prevent colonization by the twig beetles, but reports are inconsistent.
While eventually walnuts may be completely lost in Denver, it is important to note that this tree is not native to this area and makes up a very small percentage of Denver's trees (as of 2008, there were approximately 4000 black walnuts in the city). One of our primary concerns is preventing further spread of the disease. Do not transport infected walnut wood, especially to the east. Black walnut is a common and valued forest tree throughout the eastern United States, with its range extending from the eastern seaboard to central Kansas, Oklahoma, and Texas. Walnut wood must not be transported nearer to the black walnut's native range because if this disease takes hold within the walnut's home range, we may lose this tree completely.
For more information, please call the Denver City Forester's Office to talk to a tree care expert or view the Pest Alert issued by Colorado State University.
UPDATE: DON'T MOVE WALNUT WOOD.
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