Physical Description
Thousand cankers disease is the result of an insect-fungus complex caused by a beetle, Pityophthorus juglandis, and a canker fungus, Geosmithia morbida.
Pityophthorus juglandis is a dark reddish-brown beetle about 1.5-2.0 mm long as an adult. It is characterized by four to six concentric ridges on the upper surface of the shield-like cover behind and over the head (known as the pronotum). The adult beetle also has yellowish hairs on its head and a sharp angle to its posterior (NIFA, n.d.). The larvae of this beetle are white and C-shaped. The galleries under the bark created by the adult beetles run horizontally (across the grain of the wood), while the galleries created by the larvae tend to be vertical (along the grain of the wood) (USDA, 2013).
The adult beetles carry fungal spores of the pathogen Geosmithia morbida, which are then introduced into the phloem of eastern black walnut trees. Once the tree has been infested by this insect-fungus complex, numerous cankers develop around the galleries and complete girdling of the tree is possible (USDA, 2013).
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| Image: Whitney Cranshaw, Colorado State University, Bugwood.org |
Life Cycle
Adult beetles fly to black walnut trees, where they tunnel galleries beneath the bark of twigs larger than 2 cm in diameter and in the trunk. The adult beetles overwinter within these cavities. In late-April, the adults resume activity when they fly to branches to mate, excavate new tunnels beneath the bark for egg galleries, and lay eggs. After emerging from the eggs, the larvae feed for 4-6 weeks underneath the bark and create tunnels that run perpendicular to the egg galleries. The larvae then pupate at the end of these tunnels. Adults emerge to produce a second generation of walnut twig beetles, after which they enter peak flight activity from mid-July to late August. After flying to new sites, the adult beetles enter hibernation in early fall. It is during the creation of egg galleries that the fungus Geosmithia morbida is introduced into the tree and subsequently grows to create cankers beneath the bark (Colorado State University, n.d.).
As walnut twig beetles move to new black walnut trees, they introduce fungal spores of Geosmithia morbida into the phloem tissue just beneath the bark of the tree. The fungus kills the tissue, creating a small darkened canker. Repeated feeding and egg laying of the twig beetle then allows the fungus to be introduced into multiple areas of the same tree, creating even more cankers. Thousands of cankers can develop, hindering the flow of nutrients through the tree and eventually killing it (NIFA, n.d.). A black walnut infested by TCD can die within 3 years of initial symptoms.
Thousand cankers disease can affect many walnut trees (Juglans spp.), but black walnut (Juglans nigra) is the most susceptible to the disease (NIFA, n.d.).
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| Image: Jennifer Juzwik, Research Plant Pathologist, USDA Forest Service |
Black walnut trees can be infected with TCD for many years before showing symptoms. Symptoms of TCD include:
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wilting and yellowing of leaves mid-summer
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twig and branch dieback leading to a thinning of the canopy
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numerous distinctive circular oblong cankers growing on the phloem underneath the bark
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development of thousands of cankers that kill the phloem and cambium, girdling the tree and eventually killing it (NIFA, n.d.; USDA, 2013)
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the bark surface itself may have no symptoms, or can develop a dark-amber to black stain, as well as cracked bark directly above the canker
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tiny entrance and exit holes created by the walnut twig beetles may also be visible on the dead or dying branches and trunk.
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bark beetle galleries are also typically found within the cankers (USDA, 2013)
If a black walnut tree is showing symptoms of thousand cankers disease, it is important to fully evaluate the tree, as some of these symptoms are similar to other common diseases of walnut or could be signs of decline from other factors, such as environmental stress (NIFA, n.d.).
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| Image: Jennifer Juzwik, Research Plant Pathologist, USDA Forest Service |
The full extent of TCD is not known, as symptoms of this disease may be confused with other forms of natural mortality of black walnut, however confirmed distribution of TCD in the United States exists (OFAH/OMNR Invading Species Awareness Program, 2012).
The walnut twig beetle is native to Arizona, California, and New Mexico, where it originally was associated as a native pest of Arizona walnut. The beetle has invaded Colorado, Idaho, Nevada, Oregon, Utah, and Washington, where eastern black walnut has been extensively planted. As a result, thousand cankers disease has been found in all nine of these western states. Since 2010, TCD has also been confirmed east of the Great Plains, in Pennsylvania, Tennessee, and Virginia. Populations of either the beetle alone or the pathogen alone have also been found in Ohio and North Carolina, making TCD likely in these states as well (USDA, 2013). Other individual pockets of TCD have been reported in New England, Indiana, and Maryland, as well as in Texas and into Florida (Kansas Department of Agriculture, 2012-2016; OFAH/OMNR Invading Species Awareness Program, 2012).
The disease is expected to move throughout the range of eastern black walnut. This is because of the widespread distribution of black walnut, the high susceptibility of the tree to the disease, and the capacity of the fungus and beetle to survive within a wide range of climatic conditions (USDA, 2013). TCD can also be spread by moving walnut wood for woodworking or as firewood (NIFA, n.d.).
The spread of thousand cankers disease can greatly hinder the survival of eastern black walnut trees, causing economic, environmental, and social impacts. As black walnut plays an important ecological role in forest ecosystems and is highly valued for lumber and veneer, the spread of TCD can have major consequences (NIFA, n.d.)
Economic Impacts
Since beetle tunneling and canker growth only occurs in the phloem just beneath the bark, TCD does not diminish the quality of black walnut wood (Kansas Forest Service, 2014). However, it can kill trees within 3 years of symptoms emerging, so supply of black walnut to the forest industry could be reduced if the trees are not harvested in time. Since the black walnut tree is a highly-valued timber species in North America, being used for high-end cabinetry, furniture, musical instruments, and other veneer products, the reduced supply of black walnut trees could have a negative impact on the hardwood forest industry (OFAH/OMNR Invading Species Awareness Program, 2012).
Ecological Impacts
Black walnut trees are valuable nut-producing trees, providing a source of nutrition for humans and many forest-dwelling animals. TCD does not affect the nuts of infected trees, so they are still safe to eat (Conservation Commission of Missouri, 2015). However, the loss of black walnut trees from the landscape would mean a lost food source for a wide range of wildlife.
Further, the roots of black walnut trees stabilize soil, the foliage provides food for insects and birds, and the trees slow water runoff. Black walnut trees also act as a carbon sink and reduce atmospheric carbon dioxide levels (Conservation Commission of Missouri, 2015). The loss of black walnut trees would therefore mean a loss of these important ecological services.
Since the extent of TCD is not fully known, and it is still moving throughout North America, the long-term ecological impacts of TCD are not yet fully understood.
Social Impacts
Healthy black walnut trees can grow to become beautiful, large hardwood trees that add to the biodiversity of the rural and urban forest, as well as its aesthetic value. Black walnut trees are also commonly used as landscape trees and provide shade within urban areas, offering social and economic value to urban citizens. The loss of black walnut trees could mean a decrease in the aesthetic and social value of hardwood and urban forests.
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| Image: Curtis Utley, CSUE, Bugwood.org |
In order to detect the disease, visually inspecting walnut trees for dieback is currently the best survey tool. To detect the beetle, a pheromone-baited trap is available and should be placed near (but never on) walnut trees (USDA, 2013).
Beyond detection of the disease, there is currently no effective management strategy available to prevent or control thousand cankers disease, or the walnut twig beetle. Because the beetles are so small, it is not feasible to spray insecticides to control them. Further, there is no approved fungicide available for injection into black walnuts since this could affect the nut produced by the tree and thus be harmful to humans or animals. Some states have imposed strict sanitation methods by removing all diseased trees, which has proven successful for early and small infestations of TCD (UF/IFAS, 2015).
Quarantines on the movement of non-treated or unfinished walnut lumber and wood products from infested areas should also be imposed to avoid the spread of TCD into new areas.
As the beetle and fungus do not diminish the wood quality of black walnuts, infected trees can still be salvaged for their commercial value. If this is done, the bark, phloem, and cambium should be removed and disposed of properly to avoid movement of the disease to other areas (TCD, 2014).