Tracing Black Walnut Tree Decline

Black walnut trees are in decline along Colorado’s Front Range and the results could be devastating.

black-walnut-blogThe news is bad for Front Range homeowners with black walnut trees in their yards.

Boulder officials are telling residents that their trees will have to be cut down. This may comes as a surprise for some, but as early as 2004, CSU researchers had identified a decline in black walnut tree populations that was spreading in several western states and had appeared along the Front Range.

In October 2007, CSU plant pathologists were warning of the walnut twig beetle (Pityophthorus juglandis) causing major decline of Juglans nigra in Boulder, Westminster and Arvada.

The report found that multiple trunk and branch cankers were caused by a fungus carried by the beetle, but it wasn’t clear whether the insect came first or the canker.

In August 2008, CSU reported increased walnut tree decline in Boulder and the spread of the disease throughout Arvada, Wheat Ridge and northwest Denver. In Colorado Springs the disease had nearly eliminated all of the walnut trees there.

CSU explained that the tree decline is the result of  a canker caused by a fungus carried by the walnut twig beetle. Multiple cankers caused by multiple beetles led to the name “thousand cankers” to describe the disease. It was discovered that the fungus carried by the beetle grows in advance of bark beetle tunneling.

At that time, no controls for thousand cankers disease had been identified, but CSU entomologists were trying to get a better understanding of the walnut twig beetle and plant pathologists were trying to get a handle on the canker-causing fungus.  Click on this link for a complete Pest Alert from CSU called “Walnut Twig Beetle and Thousand Cankers Disease of Black Walnut.”

This report is especially troubling if the disease continues its march east where there are forests of black walnut trees. That’s why it’s important to control the spread of the diease by finding and removing disease trees in Boulder and other cities along the Front Range.

If you have a black walnut tree in your yard, like I do in mine, be vigilant to the initial symptoms of the disease which includes yellowing and thinning of the upper crown. Trees can die within three years.

Whitney Cranshaw, CSU entomologist, is involved in a project to identify the locations of black walnut trees in eastern Colorado. Contact him by email at whitney.cranshaw@ColoState.edu for more information on this project.


 

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