New Guide Details IPM for Thousand Cankers Disease in Black Walnut
By 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.
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.
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.
Journal of Integrated Pest Management