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Thousand Canker Infecting Denver's Walnut Trees

Recent die-off of black walnuts in the Denver metro area are being attributed to a new tree disease called thousand canker. No consistent cure is known but we are are working with state and local entities on treatment options. If you are worried about your black walnut, and live in Denver metro or Boulder,  it's best to contact us for a free evaluation.

How can you tell if you're walnut is susceptible or already infected?

It looks like you're black walnut is dying.

Premature yellowing of leaves or thinning at the crown is an early symptom.  If you see dead areas (cankers) on twigs and branches that could mean a fungus has been introduced.  Cankers may also be visible on the trunk in the latter stages. The walnut will die from the top down as thousand canker progresses.

By the way, do I have 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 you're house, clearly the walnut twig beetle, which carries the disease, is in the neighborhood.  

Treatment

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. 

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 beetles. Unfortunately tests using similar treatment sprays have not prevented decline.