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Thousand cankers disease
Thousand cankers disease is caused when the walnut twig beetle Pityophthorus juglandis and a fungus (Geosmithia morbida) interact. Eastern black walnuts (Juglans nigra) can easily get this disease. They are also native to southern Wisconsin forests, are grown in plantations here and are in urban areas throughout the state.
Thousand cankers disease in North America
To date, thousand cankers disease has not been found in Wisconsin, although it has been seen in walnut species in several western states (see map). The first confirmation of thousand cankers disease within the natural range of black walnut was made in Tennessee in July 2010. Since then, the disease was confirmed in other eastern states such as Pennsylvania (2011), Virginia (2011), North Carolina (2013) and Ohio (2013). In addition to black walnut, butternut is also at risk of thousand cankers disease.
National distribution of thousand cankers disease (February 2014)
The potential impact of thousand cankers disease in Wisconsin forests
In the western United States, thousand cankers disease has killed numerous walnut species, including eastern black walnut (Juglans nigra) which is native to Wisconsin.
Black walnuts are considered highly susceptible to this disease. In Wisconsin, black walnut grows mainly in southern forests and is grown in plantations. Black walnut is also found in urban areas throughout the state. It is valued for high quality wood as well as an important food source for wildlife and people.
The potential damage of this disease to Wisconsin’s forests could be great, due to the financial value of the wood, how easy walnut gets the disease and the ability for the beetle and the fungus to survive in the climate of Wisconsin.
The walnut twig beetle Pityophthorus juglandis carries spores of a canker-causing fungus Geosmithia morbida as it tunnels through the bark and into the phloem of a tree. The beetle then continues to transfer the spores beneath the bark surface as it constructs galleries in the wood and lays its eggs. The walnut twig beetle is native to Arizona, California and New Mexico, but has spread throughout nearby western states and several eastern states such as Tennessee, Pennsylvania, Virginia and North Carolina.
As the fungus grows, it creates many small cankers beneath the bark, resulting in crown dieback. An infected tree often dies within three years of developing symptoms. However, the length of time a beetle infestation is present before symptoms are visible in the tree crown is unknown. Currently, experts believe that it may take 3-10 years from initial beetle colonization before the onset of visible crown dieback.
Signs and Symptoms
Tiny reddish-brown bark beetles (1/16” long). Photos by Steve Valley, Oregon Department of Agriculture.
Crown (canopy) thinning and branch flagging
- Thinning of crown. Photo by Dr. Jenny Juzwik, USDA Forest Service.
- Flagging in crown. Photo by Dr. Whitney Cranshaw, Colorado State University.
Yellowing/wilting in the crown in late June to late August
- Yellowing and wilting leaves. Photo by Dr. Manfred Mielke, USDA Forest Service.
- Wilting leaves. Photo by Dr. Jenny Juzwik, USDA Forest Service.
Branch mortality/crown dieback
Numerous tiny, pin-sized holes on branches larger than one inch in diameter
- Numerous tiny, pin-sized holes caused by the walnut twig beetle. Photo by Dr. Whitney Cranshaw, Colorado State University.
Meandering tunnels and galleries
- Meandering tunnels and galleries caused by the walnut twig beetle. Photo by Dr. Whitney Cranshaw, Colorado State University.
- Beetle gallery with fungus evident (white area in center of canker). Photo by Ned Tisserat, Colorado State University.
Numerous small cankers (dark dead areas) beneath the bark
Walnut in Wisconsin
Finding thousand cankers disease
It is unknown how long the beetle infestation is present before visible crown symptoms appear. Experts believe that it may take 3-10 years before crown symptoms become visible. Therefore, for early detection, it is recommended that branches be examined for tiny beetle emergence holes during routine pruning and harvesting activities.
Despite the name, the walnut twig beetle does not attack small twigs. When conducting detection efforts, branches that are larger than one inch in diameter need to be examined.
The Wisconsin DNR has conducted detection surveys for TCD in black walnut plantations and forests with black walnut since 2011. To date, all surveys have been negative for detecting TCD or the walnut twig beetle. If you find walnut trees that are showing symptoms of TCD, please contact DNR regional forest health specialists.
Walnut resources in Wisconsin
There are nearly 19 million black walnut trees in Wisconsin. The majority are located in southwestern Wisconsin. The Wisconsin DNR is in the process of developing a walnut resource map . The 2011 map reflects information from DNR’s MFL (Managed Forest Land) database, input from DNR foresters and reports from the public.
To make a report, please fill out the Black Walnut Resource Data Report form and send it to Kyoko Scanlon, Forest Health Protection - Wisconsin DNR, 3911 Fish Hatchery Rd., Fitchburg, WI 53711.