Ongoing die-off of black walnuts in the Denver metro area are due to a tree disease called Thousand Cankers.
No consistently effective cure is known, but we are are working with state and local entities on treatment options. If you’re worried about your black walnut, and live in Denver metro or Boulder, it’s best to contact us for a free assessment.
How can you tell if your walnut is susceptible or already infected?
It looks like your black walnut is dying.
Premature yellowing of leaves or thinning at the crown is an early symptom – called thinning and dieback. From there, whole branches progressively die from the top down.
If you see dead areas (cankers) on twigs and branches, this could mean a fungus has been introduced.
Cankers may also be visible on the trunk in the latter stages. these dead spots gradually coalesce and girdle the tree, killing it.
Is my tree a black walnut?
Most literature focuses on thousand canker targeting black walnut, which is highly susceptible. In Colorado, almost all walnuts are black walnuts (according to anecdotal stories from the arborist community).
Arizona walnut and English walnut is thought to be highly resistant, California walnut and pecan moderately.
Location, location, location
Be alert. If you see evidence of a black walnut that is sick near your house, clearly the walnut twig beetle – the disease vector – is in the neighborhood.
Thousand Cankers is similar to the new threat Emerald Ash Borer in that the disease frequently isn’t noticed until it’s fairly well-advanced – at which point it’s already killing your tree – and that treatments so far have been difficult to determine based on this factor.
The good news is that scientists are confident that the the fungus is not systemic in the same way dutch elm disease is. Since the fungus attacks a limited area there is potential to isolate and control the fungus. The bad news is that there is no known way (yet) to do that. The most comprehensive treatment is removal of the tree before it can pass the WTB infestation to any neighbors.
Preventive measures that eliminate the carrier twig beetle itself before it hits the tree will probably be the first line of defense just as it is in controlling mountain pine beetle. Unfortunately tests using similar treatment sprays have not prevented decline.