Thousand cankers disease is a fungal disease primarily affecting walnut trees, spread by the walnut twig beetle (Pityophthorus juglandis). While impacts vary greatly, diagnostic tools and an updated integrated pest management program are urgently needed. A new paper in the Journal of Integrated Pest Management highlights strategies that can be effective in managing TCD in commercial walnut plantations. (Photo by Steven Valley, Oregon Department of Agriculture, Bugwood.org)

By David Coyle, Ph.D.

David Coyle, Ph.D.

It is a bit nutty to think about all the uses for black walnut (Juglans nigra) trees. While folks not intimately involved in the forestry realm may only know black walnut as “that tree where nothing grows underneath it” (it’s allelopathy), black walnut is actually one of the most valuable tree species in the United States. The lumber is prized for its deep color and is used for everything from furniture to gun stocks. The nuts are nutritious for both people and wildlife. It tends to grow with a beautiful form. All in all, it’s an awesome tree.

But, alas, like most trees, black walnut has pest problems, specifically thousand cankers disease (TCD). The fungus Geosmithia morbida is spread by the walnut twig beetle (Pityophthorus juglandis). Native to the western U.S., TCD was first found near Knoxville, Tennessee, in 2011 and in several other states since then, and it poses a threat to black walnuts throughout the eastern U.S. Stressed trees are most susceptible to this disease, which causes lesions in the phloem, eventually hindering (and cutting off) the tree’s ability to transport nutrients.

Stressed trees are most susceptible to thousand cankers disease (TCD), which causes lesions in the phloem, eventually hindering (and cutting off) an infected tree’s ability to transport nutrients. Here, a tree in Walla Walla, Washington, shows advanced stages of TCD infection. (Photo originally published in Smallwood et al 2022, Journal of Integrated Pest Management)

Stressed trees are most susceptible to thousand cankers disease, which causes lesions in the phloem, eventually hindering (and cutting off) an infected tree’s ability to transport nutrients. Here, a canker injury is seen on a tree in Walla Walla, Washington. (Photo originally published in Smallwood et al 2022, Journal of Integrated Pest Management)

Because of black walnut’s value, effective management strategies are needed. And while several strategies have been evaluated so far, nearly all have occurred in cut bolts (logs) or controlled settings. Field evaluations of realistic control methods are sorely needed, especially in natural, standing trees in high-productivity plantations.

A new study published in February in the Journal of Integrated Pest Management details thousand cankers disease management approaches in commercial walnut plantations in Washington state and serves as a field test to the efficacy of these methods on an operational level. The authors—Christopher Smallwood, Ph.D., Matthew Ethington, Ph.D., and Matthew Ginzel, Ph.D., of Purdue University—compared different insecticide, fungicide, and trap tree methods for TCD control.

The walnut twig beetle (Pityophthorus juglandis) is quite small in size but packs a punch, spreading a fungal pathogen that causes thousand cankers disease in walnut trees. (Photo by Eric R. Day, Virginia Polytechnic Institute and State University, Bugwood.org)

Monitoring traps in tree plantations can detect the presence of walnut twig beetles (Pityophthorus juglandis), vectors of the a fungal pathogen that causes thousand cankers disease. (Photo originally published in Smallwood et al 2022, Journal of Integrated Pest Management)

They found quite a bit of variation in walnut twig beetle populations year to year, but they also showed how effective beetle monitoring can be to determine populations. High doses of some fungicides did elicit a phytotoxic response in trees (e.g., leaves looked yellowish), but overall the combination of insecticide treatments and girdled trap trees did succeed in keeping walnut twig beetle populations very low in the treatment areas. Unfortunately, drought stress also played a part in tree susceptibility to TCD—but, let’s be honest, unless you have a major irrigation system installed, there isn’t a lot you can do about a lack of rain.

Thousand cankers disease, while certainly alarming and potentially deadly to black walnut trees, isn’t necessarily the death sentence we once thought it was. There are several options for management, and this paper shows they can work on an operational scale. In the world of forest pest management, that’s about the best news you can get.

David Coyle, Ph.D., is an assistant professor in the Department of Forestry and Environmental Conservation at Clemson University. Twitter/Instagram/TikTok: @drdavecoyle. Email: dcoyle@clemson.edu.