Thousand Cankers Disease Update
By Jerry Van Sambeek *
If the new information about the status of the Thousand Cankers Disease (TCD) changes as rapidly as it has the last few weeks, this update will be outdated before your read it. Having said that, I will try to summarize the latest information I have picked up from the group working on the USDA Forest Health Protection National Response Plan, a workshop held in Colorado, and reports by Whitney Cranshaw and by Bruce Moltzan on the recent find in Knoxville, Tennessee.
Yard tree in Denver with early signs of Thousand Cankers Disease - note yellow leaves on a small branch, flagging on a large terminal branch, and thin foliage on many of the terminal branches.
For members who still have not heard about the TCD on walnut, diseased trees decline and die after numerous cankers are produced in the inner bark and cambium on the branches and trunk. Cankers are caused by the fungus Geosmithia morbid that is carried by the walnut twig beetle, Pithyophthorus juglandis. The walnut twig beetle is a very small bark beetle (size of the letter 'i' or the President's ear on a dime) native to the southwest US and Mexico that normally attacks and kills small branches on Arizona walnut. When the adult beetle leaves its pupal chamber, it picks up spores of the fungus that had produced a small canker in the cambium and inner bark. Whether the fungus is necessary for the insect to complete its life cycle is unclear; however, neither the insect nor fungus has been found by itself in nature. The good news is that in laboratory tests, the fungus has not been shown to produce cankers on pecan or the other Carya species tested.
For several decades black walnut trees planted in the western US have been dying presumably in response to the droughts and other urban stresses. Upon the insistence of Kathleen Alexander, Boulder City Forester, Whitney Cranshaw, an entomologist, and Ned Tisserat, a plant pathologist, identified and described the causal agents and coined the name Thousand Cankers Disease (TCD) because it takes thousands of the small cankers to deplete the energy reserves and kill a black walnut tree.
Subsequently, it was determined that the insect-fungal complex has been killing walnut in nearly all the states in the western US. In response, Missouri, Kansas, Michigan, Indiana, Oklahoma, North Carolina and Nebraska have issued state exterior quarantines banning the movement of walnut products with bark on them from the western US and any other states where TCD is found unless kiln-dried in hopes of preventing the spread of TCD into the eastern US. At present, APHIS is unlikely to issue a national quarantine because this disease complex is thought to be caused by an insect and fungus native to the US (doesn't matter that the insect is native only to the southwestern states).
In late July we received news that TCD was confirmed on several street trees in Knoxville, Tennessee. Photographs of these trees show varying degrees of branch and crown dieback and included one dead tree. Because it normally takes 8 to 10 years from the time the insects initially attack a tree till it kills the tree, we assume it arrived nearly a decade ago. In early August, we heard TCD was confirmed at three urban sites in Knoxville, two of which were seven miles apart. Branches have been collected from suspect trees in Knox and surrounding counties and are currently being evaluated for the Geosmithia fungus to determine the extent of the disease.
Whitney Cranshaw and Ned Tisserat visited Knoxville in mid-August to assist with delimiting the area of infestation. The news was not good. Whitney reports infected trees were widely found in Knox County and it may have been in the county for as long as twenty years based on the number of stumps from walnuts already cut and removed. Whitney says we are well past the point of implementing an eradication program in Tennessee.
Yard tree in Knoxville showing late symptoms of Thousand Cankers Disease with evident flagging and epicormic sprouting from stem.
Because TCD has been found in the quarantined Tennessee counties of Knox, Blout, Union, and Anderson, it is quite likely it will be found in additional urban areas and possible managed walnut stands and plantings. The USDA Forest Service has posted a pest alert at http://www.na.fs.fed.us/pubs/palerts.shtm to aid growers in TCD identification. NNGA members need to survey their land and adjoining urban areas (most findings occur first in urban areas) for TCD symptoms. Symptoms to look for initially are thinning of foliage on some branches in the upper crown. In more advance stages some branches in spring and summer show flagging of the leaves as dead leaves wilt, turn brown, and do not fall off. Later in the summer, leaves on dying branches prematurely turn yellow and can be confused with anthracnose. Unlike with anthracnose, all leaflets on the leaves of infected branches turn yellow simultaneously rather than a gradual progression of premature yellowing and defoliation of leaflets on leaves from the base to tip of this year's shoots. In later stages of TCD, branches in the crown are dead and stems produce epicormic sprouts in an attempt to restore the crown.
If you see walnut trees with TCD symptoms and have a digital camera, we ask that you take photos of the entire tree with recognizable landmarks in the background (so we can relocate the tree in the winter) and close-ups of the foliage and symptoms. Submit photos to Jerry Van Sambeek at firstname.lastname@example.org. Alternatively submit photos to email@example.com so that Simeon Wright, MDC Forest Pathologist, and Rob Lawrence, MDC Forest Entomologist, can evaluate and arrange for a site visit if warranted.
Missouri continues to take the lead in the eastern US to manage this new pest problem. The Missouri Department of Conservation has published information in the Missouri Conservationist and is beginning to put on workshops to increase landowner awareness of the potential threat to their walnut. In addition, MDC has developed a list of frequently asked questions about TCD that is posted at http://extension.missouri.edu/emeraldashborer/pdf/thousandcankersfaq.pdf and more technical web-based resources can be found at the Missouri Department of Agriculture website (http://mda.mo.gov/plants/pests/thousandcankers.php). [The latter resource has links to the latest information about TCD in other states--Kansas, Michigan, Nebraska, Oklahoma and Indiana--at this time. Links to other states can be added in the future.] Other organizations like the Walnut Council are hosting teleconferences to discuss what needs to be done in addition to conducting surveys in the eastern US to determine where and how long TCD has been present. Missouri has been well represented at many of these conferences. If you have ideas or concerns, please let Missouri folks like Collin Wamsley, Doug LeDoux, Simeon Wright, Rob Lawrence, Mark Coggeshall, Harlan Palm, and Jerry Van Sambeek know.
* Jerry Van Sambeek is a a research plant physiologist at the USDA Forest Service in Columbia, MO and is a member of the Missouri Nut Growers Association (MNGA) and of the Northern Nut Growers Association (NNGA). He can be reached at firstname.lastname@example.org.