Thousand Cankers Disease hitting Northern Colorado black walnut trees

Take a good look at your black walnut trees this summer. If they are yellowing, wilting and showing dieback in the upper crown area, you will want to read this article. If they are healthy, you probably still want to read this article. The walnut twig beetle (Pityophthorus juglandis) is spreading thousand cankers disease to black walnut trees (Juglans nigra) in northern Colorado and few trees are surviving.

By Beth Thiret,
Colorado State University Extension Master Gardener in Larimer County

The decline of black walnut trees in Colorado was initially noted in 2001, and it was suspected to have been drought-related. Thanks to leading research by Colorado State University professors Ned Tisserat and Whitney Cranshaw in the Department of Bioagricultural Sciences and Pest Management, in 2009, the cause was determined to be a fungus (Geosmithia morbida) introduced into the branches and stems of black walnut trees by the walnut twig beetle.

As the miniscule beetle (1.5-1.9 mm) bores into black walnut trees it carries the canker-producing fungus on its body. When it tunnels through the live portions of the bark down to the cambium layer, the fungus colonizes the surrounding tissue leaving small dead areas, or cankers, in the bark. While the beetle is small, thousands of them can be found in even small branches. The resulting “thousands of cankers” stop the flow of nutrients to the tree and the tree eventually dies.

In Fort Collins, the number of infected trees reported increased to 30 in 2012 from 3 the year before. Ralph Zentz of the Fort Collins City Forestry Division estimates there are approximately 1,100 black walnut trees in the area. If the disease spreads at the same rate as last year, it won’t be long before all of these trees are infected.

To date, there are no methods proven effective in saving a tree once thousand cankers disease is present. Insecticides and irrigation controls have failed. However, the City of Fort Collins is putting sanitation efforts into action earlier than other communities and it’s hoped these efforts will help control the spread of the disease.

Zentz is working closely with CSU and following their recommended sanitation approach. “That means detection as early as possible and the quick removal and proper disposal of the walnut wood,” he says.

Owners of black walnut trees can also help by observing their own trees. In addition to the symptoms mentioned earlier, the presence of beetle holes in the bark and excessive staining of the bark surface may be observed.

If a tree owner suspects their black walnut may be infected, Zentz asks that they contact the City Forestry Division for confirmation (970-221-6660; Unfortunately once the disease is confirmed, the trees have already been infected for at least a year and usually die within another two or three years.

The city will work with homeowners to haul usable logs at no charge. Upon delivery to the mill logs will be quickly processed ensuring the beetles will not spread to other trees. Any parts of the tree not milled must be buried in a landfill to also prevent further spread of the beetle.
Beetles continue to live in trees killed by Thousand Cankers Disease and an outbreak can start in a new location when a single log is brought in from an infected area. Thus, one of the most important things to remember when removing infected black walnut trees is that the wood must not be transported to uninfected areas.

To protect their own trees and help stop the spread of the disease, many states have initiated quarantines of all unfinished black walnut material originating from states where thousand cankers sisease has been detected. Black walnut trees are valued for their wood and nut production in their native range of the eastern United States. Losing the trees in that area would have an economic as well as ecological impact. As Cranshaw advises, “Keep it local.”

CSU remains the leading research organization working to better understand and manage this disease. More information can be found at their thousand cankers web site or at

The impact of Thousand Cankers Disease is clear. If you have a black walnut tree, a watchful eye is always helpful to prevent further spreading of the disease. Again, any suspicions should be reported to your local forester. In Fort Collins, emails may be sent to [email protected].

Thousand cankers disease is not specific to only Fort Collins, as it has been positively identified in Loveland, Berthoud, LaPorte and Greeley.

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