What Is 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.
What are symptoms of the disease?
A newly recognized fungus, Geosmithia, kills a localized area in the phloem just under the bark in >2cm wood after 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 area 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 dead, flagging leaves associated with them. In 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.
*Photo by Kathy Keatley Garvey
*Photo by Jim LaBonte, Oregon Department of Agriculture.
The beetles prefer wood larger than 2cm and feed on young branch tissue in the upper canopy early in the spring. At these tunneling sites, cankers are diffus, 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.
Walnut twig beetle and associated staining around tunnel.
Coalescing branch cankers produced by Geosmithia.
Adult twig beetle tunneling on excised stem after 24 hours.
Large trunk cankers of black walnut.
*Photos courtesy of Colorado State University
Can Trees Survive This Disease?
Thousand cankers kills 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 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.
?Does It Look Like??
Where is 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.
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 bark is attached this could be an important pathway. Raw wood is the most
Campsites and sawmills in the Great Plains states may facilitate the eastern movement of Thousand Canker 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 J. nigra is a hard mast producer, wildlife may be negatively impacted.
Map depicting TCD Distribution as of 11/2009
Native / Natural range extends throughout eastern U.S. and into Kansas and Nebraska; planted
throughout U.S. Timber, nut and ornamental tree; used as rootstock for English walnut grafts
Highly susceptible to Thousand Canker Disease
What Treatments are Available for Thousand Cankers Disease?
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. This method appears to have only limited effectiveness, at most.
The use of soil applied systemic neonicotinoid insecticides (e.g., imidacloprid, dinotefuran, clothianidin) is a possibility for bark beetle control and there has been some attempts to use imidacloprid (e.g., Merit) for this purpose. Anecdotally these treatments seem to have had negligible effectiveness. Researchers report that the disease may be suppressed by soil applications of imidacloprid if they are applied before the disease has become well established in the tree; treatments made after symptoms begin to appear are ineffective.
Trunk injected fungicides combined with insecticides may be the most effective way to eliminate the beetle and the fungus. Additionally, injected fertilizers will assist in restoring the nutrients to the tree.
Injection of fungicides and insecticides in combination with injected fertilizers may save the tree.
We would like to express our appreciation to the following individuals: For GIS-related data: Manuel
Colunga, Michigan State University; Frank Koch and Bill Smith, USDA Forest Service; and Dan
Borchert of USDA APHIS. For their assistance in data collection and updates on the current status of
TCD: Ned Tisserat, CSU; Mitch Nelson, Carolyn Pizzo, Tom Culliney, Mikell Tanner, USDA APHIS;
Bruce Moltzan, USDA FS; Mark Stirling, CDFA; Chuck Leslie and Steve Seybold, UCA-Davis; Malli
Aradhya, USDA ARS NCGR; Tim Ford, IPPFB; Marion Murray and Diane Alston, USU; Jim Hafferty;
Todd Morgan, UMT; Jay Pscheidt, OSU. 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.