The Plant Clinic has received several calls and a few samples of honey locust problems that did not involve insects. The complaint has been yellowing and wilting of the foliage, premature leaf drop, and stem dieback. (In many of the cases we see at the clinic, we are not able to identify the cause of the problem initially. With a bit of follow-up, it often becomes clear that the sample provided was not adequate for diagnosis.)
These symptoms can be caused by the disease Thyronectria canker. Look closely for these cankers. The wood is often slightly sunken; the canker is cracked and has a yellow-orange color. The cankers are elongated and can occur on young or old wood. If in doubt as to the presence of a canker, do a bit of investigating, trying not to do too much tissue damage. Use a knife to peel back some of the bark in the suspect area. The sapwood beneath the canker will be discolored reddish brown. Healthy wood should be white or tan or slightly green.
This canker disease is fairly common on stressed honey locust, although the canker can be easily overlooked. The disease has been linked to drought stress, so you'd think that we would not see this disease in 1998. However, some areas of the state were very dry early in the season and have seen the disease. Pest scouts at The Morton Arboretum reported this disease on some of their locust trees this summer.
As with most canker diseases, there is no rescue treatment that can be sprayed on the tree. Prune out dead wood in dry weather, water the trees when two weeks of drought occur, and avoid physical damage to the trees.