Consolidated report from NutGrowing.Org and UGA Center for Urban Agriculture
Another damaging disease that has the potential to kill trees has been identified. Before we delve into it, let's look at some of the present ones we are aware of.
Emerald ash borer is killing ash trees in the mid-western states and was recently found in Tennessee
Laurel wilt is killing red bay trees along the Georgia/Florida/South Carolina coast
For years we have been dealing with the repercussions of Phytophthora ramorum, cause of sudden oak death in California. (I believe I have seen exidence of same in Florida.)
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.
The disease that has researchers in Colorado concerned that black walnut (Juglans nigra) will succumb to disease just like the American elm to Dutch elm disease and the American chestnut to chestnut blight, is called Thousand Canker Disease (TCD) of black walnut and it was recently identified in Knoxville, TN. This recent discovery is significant because the disease had not been detected east of the Mississippi River and was originally thought to be limited to Colorado, Utah, Idaho, Washington, Oregon, and California. TCD is caused by a fungus, Geosmithia morbida sp. nov, that is introduced into the tree by the walnut twig beetle (Pityophthorus juglandis).
Infected trees die from multiple cankers that infect the cambial tissues of the black walnut trunk and branches. The trees are killed from a collective group of shallow cankers that interfere with water and nutrient transport.
On Arizona and California walnut, the disease causes minor damage affecting branch tips and acts as a natural thinner of foliage. However, the cankers produced on black walnut are larger and it affects larger branches and the trunk. Initial symptoms of infection are branch flagging (yellowing and browning) and crown decline, which may not be evident for several years after the beetles infest the tree.
Once infected with TCD, there is no cure and the tree will die. Once TCD symptoms are evident, the tree dies within 2 - 3 years. The twig beetles reproduce prolifically in black walnut. An infested walnut tree may contain tens of thousands of beetles that carry the fungus beneath their wings.
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.
In late July news that TCD was confirmed on several street trees in Knoxville, Tennessee was made public. 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, 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.
There is no control once the beetles infest the tree other than removal of infected trees and wood to reduce disease and beetle spread. Beetles can reproduce within cut logs and it is believed that transport of infested wood (logs with bark still attached) can spread the beetle and disease to new areas. Currently, the source of the beetle infestation and the disease in Tennessee is not known, but transport of beetle infested wood may be a possibility.
What makes this disease important and something to be on the lookout for in southern states adjacent to Tennesee is that based upon the severity of the disease on the affected trees in Tennessee, the beetle and disease has probably been in Tennessee for years before someone took a closer look as to why the trees were dying. Foresters initially believed the declining walnut trees were dying due to drought stress.
For more information and for images of the disease and beetle, please see the Tennessee Department of Agriculture website (http://tn.gov/agriculture/regulatory/tcd.html) and the TCD Research and Education website for Colorado State University (http://www.colostate.edu/Dept/bspm/extension%20and%20outreach/thousand%20cankers.html).
For those of you in the affected areas, if you see walnut trees with what you believe to be TCD symptoms and have a digital camera, take photos of the entire tree with recognizable landmarks in the background (so the tree can be identified in the winter) and close-ups of the foliage and symptoms. Submit photos to Jerry Van Sambeek at jvansambeek@fs.fed.us. Alternatively submit photos to forest.health@mdc.mo.gov so that Simeon Wright, MDC Forest Pathologist, and Rob Lawrence, MDC Forest Entomologist, can evaluate and arrange for a site visit if warranted.
