Black walnut trees and Thousand Cankers Disease by Nellie Brown
I’m interested in fruits and nuts that are native to North America, especially those in our region. One of these native nuts is the black walnut, Juglans nigra. The nut of this tree is more flavorful than any other walnut, but the wood is the most valuable part. Challenging mahogany in its beauty, it is dense and strong, much desired by wood workers and furniture makers. The trees are tough and slow growing, tolerating sporadic flooding and accumulating value as a veneer log for future generations in rural areas. It is one of the most valuable woods in North America, and in Missouri. Unfortunately, this natural resource is at risk.
The original native range of the black walnut is the eastern United States. However, as settlers moved west, they brought walnuts with them. The expanded range of black walnut eventually overlapped the range of the Arizona walnut (J. major), and the diseases and pests of that species. A native fungal pathogen (Geosmithia morbida) causes small cankers on dead and dying twigs of the Arizona walnut, and it is spread by the (also native) walnut twig beetle (Pityophthoris juglandis). On the Arizona walnut, this insect and the disease it spreads are minor problems. The disease is slightly more damaging on the geographically-close southern California walnut (J. californica) and little walnut (J. microcarpa).
In the 1990’s and early 2000’s, numerous black walnut trees in western states were observed to be in decline and dying. Walnut twig beetles were collected from logs after the trees died, but this was considered coincidental, since the beetle was thought to cause only minimal damage. The decline was initially attributed to drought. However, city foresters in Colorado Springs and Boulder noticed that irrigated trees were also affected, and the end of the drought did not slow the mortality of black walnuts. It turned out that when walnut twig beetles began feeding on black walnuts in the western United States, they introduced the fungal pathogen to a brand new host with no resistance. In black walnut, the walnut twig beetle buries into the bark of the tree in multiple locations to find the best possible egg-laying site. In the process, it deposits the fungus, which causes numerous small cankers, infecting the trees’ phloem (circulatory system) and disrupting the movement of nutrients. These cankers grow and often merge into larger dead patches on the stem.
The sheer number of the fungal infections in an infected tree led to the name of the disease: Thousand Cankers Disease (TCD). Infected black walnut trees have a thin upper canopy as the leaves turn yellow and die, and the branches follow suit. Within two to three years of showing symptoms, an infected tree will die. The problem gets worse if the diseased log is sold and moved to a woodworkers’ yard in another state. The walnut twig beetles emerge, carrying with them the fungus that causes thousand cankers disease and infecting more trees in the vicinity.
Humans have played several roles in the story of this disease. Introduction of black walnuts into the native range of the walnut twig beetle created the opportunity for this disease to jump to a new species. Humans have exacerbated the problem by moving the wood (with the fungus and walnut twig beetles) across state lines, introducing the disease into eastern states in the original geographic range of the black walnut. Ohio, Pennsylvania, Tennessee, Virginia, and North Carolina all have confirmed cases of TCD. It was recently found in northeastern Italy in plantations of English walnut, which may have profound consequences for the export market for black walnut lumber.
Numerous states have implemented quarantines limiting the movement of walnut logs and lumber in an effort to limit the spread of this disease and its economic impact. Most state quarantines allow processed lumber (100% bark free, kiln dried, with square edges) and finished wood products (walnut furniture, instruments, and gunstocks). Following the regulations when moving black walnut logs and wood will help protect this valuable resource as long as possible for future generations.
Nellie Brown has worked in the green industry for over fifteen years, first as a nursery inspector for the Missouri Department of Agriculture, now as an independent consultant and certified arborist. She helps people solve their insect and disease problems with their trees, garden and home. Her website is: www.nelliebrownconsulting.com.
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