Thousand Canker Disease may be new threat to black walnut trees in Georgia

Are you ready for another damaging disease that has the potential to kill trees?  It seems there are new pest threats all the time:

• 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. 

Well not to sound alarming, but another disease 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. 

The disease 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. This is similar to how Seiridium canker affects and kills Leyland cypress branches and trees. 

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.

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 Georgia 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.  The effect of environmental stress is evident throughout Georgia and on many tree species.

TCD has not been identified in Georgia.  However, the native and planted range of black walnut encompasses almost all of Georgia, except the southeastern counties.  The best control for this disease is early detection.  If you see, have or receive calls from arborists, foresters or homeowners with declining black walnuts showing flagging and crown decline, take a closer look for beetle exit holes in the trunk and/or branches and the darkly, discolored cankers beneath the bark.  Samples should be taken to local county extension offices for submission to the Extension Plant Pathology Plant Disease Clinic in Athens for confirmation. 

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). 

If you have any questions, please contact me (706-542-9140 office; 706-540-7738 cell; jwoodwar@uga.edu)

Photo credit: Curtis Utley, CSUE, Bugwood.org   

TCD Factsheet - http://tinyurl.com/TCDfacts

Walnut Twig Beetle and Thousand Cankers Disease of Black Walnut - http://tinyurl.com/WTB-TCD

TCD Symptoms Checklist - http://tn.gov/agriculture/regulatory/tcdchecklist.html