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Pest: Major Pests |
Thousand Cankers Disease and the Walnut Twig Beetle
Introduction
Walnut twig beetle exit wounds made by adults.
credit: Whitney Cranshaw, Colorado State University, Bugwood.org
The native origin of thousand canker disease (Geosmithia morbida) is unknown at this point but the disease has been present in the western states since the late 90’s. The walnut twig beetle (Pityophthorus juglandis), however, is native to Arizona, California, and New Mexico and has moved to Colorado, Oregon, Idaho, and Washington. The walnut twig beetle (WTB) carries the spores of the thousand canker disease (TCD) when it tunnels into the trunks or branches of black walnuts and the combined activity of the fungus and the beetle lead to tree mortality.
Current Situation
Upon discovery in Knoxville in July 2010, several counties were placed under state government regulation. As of November 2013, the quarantined counties are: Anderson, Blount, Jefferson, Knox, Loudon, Morgan, Rhea, Sevier and Union. The buffer counties surrounding the quarantined area are: Bledsoe, Bradley, Campbell, Claiborne, Cocke, Cumberland, Fentress, Grainger, Hamblen, Hamilton, McMinn, Meigs, Monroe, Polk, Roane, Scott and Sequatchie. The movement of any walnut products including logs, lumber, firewood, and mulch are under regulation. Walnut trees are a highly valued timber species in addition to providing a nut crop for market.
Visual canker caused by geosmithia morbida.
The Tennessee Department of Agriculture and several research institutes install walnut twig beetle traps each year across the entire state. The traps are used for both detection and to help professionals and researchers understand the behavior of beetle. Visual inspections of walnuts for signs of dieback are ongoing.
Identification
Thousand cankers disease can only be identified by culturing on agar media. The visual symptoms are the “thousand” cankers that develop around each of the walnut twig beetle galleries. The cankers are found just under the living bark and are dark in color and can be anywhere from a less than 1 inch to over 1 yard long. A shallow tunnel made by the walnut twig beetle will typically be at the center of the canker. The walnut twig beetle itself is very small (1/16th inch long) and is reddish brown. The larvae are white, C shaped and can be found under the bark in the phloem of the tree.
A black walnut tree can be infected for several years before showing signs of disease. The first symptom of infection is called flagging (leaves wilting and yellowing in mid-summer). The crown then begins to thin due to twig and branch dieback. Epicormic branches (sprouts along the lower trunk and base) may appear as well as “thousands” of cankers under the bark. Once a walnut tree begins to show symptoms, the tree rapidly declines and will die within a few years. The fungus requires multiple introductions by the beetle in order to infect and kill the walnut tree.
Management Options
Canker under the bark caused by geosmithia morbida.
Since black walnut trees produce edible nuts, chemical control options are limited. Keep you walnut trees healthy by watering, fertilizing, and pruning. If your walnut tree becomes infected, immediately destroy the tree on site. Do not transport any walnut wood as further infestations may arise. If you own a forest woodlot, contact a professional forester or your local TDF office for more information on black walnuts and TCD management. Management decisions should consider your current forest management objectives, the amount of black walnut in your forest, current black walnut markets, and your proximity to a known TCD infestation. Natural TCD spread is thought to be slow. Monitor your trees for signs and symptoms. If your trees are healthy, continue managing your forest according to your forest management plan. If your trees become infected, you may consider a salvage harvest. Become familiar with the TCD quarantine before you conduct harvest operations.
A Thousand Cankers Disease Action Team has been established to prevent spread, reduce risk, minimize impact, and to maintain overall health and sustainability of Tennessee’s forests. This action team is made up of Tennessee Department of Agriculture (Division of Forestry & Division of Regulatory Services), the University of Tennessee, and the Tennessee Department of Environment and Conservation Division of Parks, and the US Forest Service.
What Can You Do?
Don’t Move Firewood! Firewood is a very likely transportation method for TCD and WTB. The beetle is also known to travel in unprocessed black walnut logs, black walnut nursery stock, and other black walnut commodities. It is very important to know where the TCD quarantines are if you are traveling between infested states or between counties that are known to be infested.
Learn to recognize what the signs and symptoms are of an infestation and immediately contact your county agent, professional forester, or the Division of Forestry at (615) 289-7373 if you suspect your black walnut tree is infected.
Publications
Identifying Hickory and Walnut Trees Native to Tennessee (UT Extension)
Walnut Twig Beetle and Thousand Cankers Disease: Symptom Checklist
Tennessee Department of Agriculture TCD web page
Tennessee Thousand Cankers Disease Action Plan
Other resources
TN Thousand Cankers Disease: FAQ
http://www.thousandcankerdisease.com/
http://www.tcdgroundzero.com/
