Thousand Cankers Black Walnut Disease
Originally found to be killing trees in Colorado as early as 2003, this is a newly recognized disease (2008) of primarily Black walnut (Juglans Nigra) and caused by a fungus (Geosmithia Morbida) that is vectored into the tree by the walnut twig beetle (Pityophthorous Juglandis). Thousand Cankers Disease has produced widespread death of black walnuts in many western states during the past decade. Other species of walnut such as Arizona walnut, English walnut, and California walnut have all shown varying degrees of susceptibility to this fungus.
A newly recognized fungus, Geosmithia, kills a localized area in the phloem just under the bark in smaller than 2cm wood after an introduction by the walnut twig beetle. These dead areas often overlap or coalesce from numerous strikes (35 insects per square inch of wood) causing nutrient disruption to foliage and thus leading to branch dieback.
The cankers rarely show any of the external symptoms that are associated with most canker producing fungi that affect trees. The affected area is shallow and confined to the phloem of the tree so that it can easily be missed if inspection cuts are made too deeply into the sapwood. Minor weeping may occur at points where walnut twig beetles enter the bark but often no symptoms are associated with the beetle attacks aside from minute entry wounds or star shaped cracks.
Early symptoms are yellowing of leaves and foliage thinning of the upper crown of the tree. As the disease progresses larger limbs are killed which may have died, flagging leaves associated with them. In the end stages, the fungus may be introduced into the trunk and large cankered areas develop in the trunk. In susceptible hosts, such as black walnut (Julgans Nigra), trees usually die within three years after initial symptoms are observed in the crown of the tree.
The beetles prefer wood larger than 2 centimeters and feed on young branch tissue in the upper canopy early in the spring. At these tunneling sites, cankers are diffuse, brown to black, and are often not visible until the outer bark is lightly shaved. Later in the summer, adults move into the lower trunk to overwinter and continue to inoculate the phloem tissue with the fungus. As the disease progresses, these cankers coalesce as well and can elongate to 2 meters in length. In some cases, a brown or black stain will appear on the surface of these large trunk cankers.
Thousand Cankers kill trees from the cumulative effects of numerous coalescing cankers that develop around individual entry wounds made by walnut twig beetles. Although the fungus does grow within the tree, the area infected is limited; it does not move systemically in the plant as do some other insect vectored fungi, such as the species involved in Dutch Elm disease (Ophiostoma Novo-ulmi). Instead, tree death results from disruption of phloem tissues transporting nutrients resulting in a progressive depletion of energy.
We do not currently know how long it takes to kill a tree once it has been initially colonized by walnut twig beetles and the Geosmithia associate. It is possible it may take many years - possibly sometimes a decade or more to kill even a highly susceptible black walnut. However, observations of black walnut in the western states indicate that Thousand Cankers is ultimately fatal to essentially all trees of this species.
Trees that are well-sited and grow vigorously may resist, in part, the effects of Thousand Cankers Disease. Furthermore, some Juglans species and hybrids appear to be more resistant to Thousand Cankers than is Juglans nigra (black walnut). The course of the disease may be substantially slowed in such trees. Theoretically, methods that can prevent tunneling by walnut twig beetles (e.g., certain insecticides) can prevent further spread of this disease. However, to date, effective spraying techniques to control the walnut twig beetle have not been identified.
Thousand Cankers Now
Confirmed populations are scattered throughout western states (Washington, Oregon, California, Idaho, Utah, Colorado, Arizona, New Mexico, and now, Tennessee) and the disease is thought to be widespread. Cooperators from the impacted states believe that Thousand Canker Disease may be present wherever susceptible walnut species grow.
The most likely pathway for movement is raw wood (logs, burls, stumps, firewood, wood packaging material - WPM). Other potential pathways include nursery stock, scion wood for grafting, and natural spread. The beetle/pathogen complex is likely to enter the east with each entrance event, as follows:
- Movement of untreated walnut (logs, burls, stumps, firewood) across the country from the west into eastern states appears limited but it does occur and it is rarely documented. Low-grade walnut may be utilized if the bark is attached this could be an important pathway. Raw wood is the most critical pathway. Campsites and sawmills in the Great Plains states may facilitate the eastern movement of Thousand Cankers Disease.
- To date there have been no reports of infected trees in walnut production nurseries; however, if nurseries do become infected, this could become an important pathway. Natural spread along riparian corridors is likely to occur.
We considered potential economic and environmental damage that could occur if the vector and pathogen were to become established in the eastern United States. Juglans Nigra is a valuable timber and nut species in the east; production sectors that face negative impact include timber, furniture, nut, and nursery stock. Exports could be affected. Homeowners may face the cost of tree removal. Additionally, because Juglans Nigra is a hard mast producer, wildlife may be negatively impacted.
Currently, there are no known insecticide sprays that reliably control this disease. Some techniques directed at the vector ultimately may prove to be useful in suppressing the rate of disease spread. However, it may be unlikely that effective treatments will be found that can control walnut twig beetles once tree attacks have begun. Control of walnut twig beetle by use of drenching trunk/branch sprays of insecticides (permethrin, bifenthrin) is a technique used successfully against some other bark beetles (e.g., mountain pine beetle, Ips beetles). However, infested black walnut trees that have received repeated insecticide spray treatments by arborists in Colorado are observed to continue to decline and die.
We would like to express our appreciation to the following individuals. For GIS-related data:
- Dan Borchert of USDA APHIS.
- Manuel Colunga, Michigan State University
- Frank Koch, United States Department of Agriculture (USDA) Forest Service
- Bill Smith, USDA Forest Service
We would like to express our appreciation for the assistance in data collection and updates on the current status of Thousand Cankers Disease:
- Marion Murray and Diane Alston, USU
- Malli Aradhya, USDA ARS NCGR
- Tom Culliney
- Tim Ford, IPPFB
- Jim Hafferty
- Bruce Moltzan, USDA FS
- Todd Morgan, UMT
- Mitch Nelson
- Carolyn Pizzo
- Jay Pscheidt, OSU
- Chuck Leslie and Steve Seybold, UCA-Davis
- Mikell Tanner, USDA APHIS
- Mark Stirling, CDFA
- Ned Tisserat, CSU
We thank Bob Rabaglia and Bruce Moltzan of the USDA Forest Service and Heike Meissner, Andrea Lemay and Jim Smith, USDA APHIS for their willingness to serve as reviewers.