The major disease problems on honeylocust include root collar rot and stem or trunk cankers. Cankers are visible now as dead areas on the stems and trunk. Look for sunken, dead areas on the stems. Often these areas are a different color than the rest of the branch. They may be bumpy where fruiting bodies of a fungus break through the bark. When a branch is girdled by a canker, dieback occurs. Sparse foliage, yellow leaves, and premature leaf drop often help pinpoint the cankered areas.
Fungal organisms are blamed for cankers, but in most cases the fungus can infect only a weakened plant, such as one growing under stress. Wounds are ideal sites for canker fungi to invade. Several common canker fungi invading honeylocust are Cytospora, Nectria, Thyronectria, and Tubercularia. A disease report discussing cankers and dieback of woody plants is available online at http://www.ag.uiuc.edu/~vista/abstracts/a636.html.
Honeylocust trees with yellowing and wilting of the foliage, premature leaf drop, and stem dieback are usually infected with one of these canker fungi. 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 cause too much tissue damage. Use a knife to peel 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. The image shows a fungal canker on honeylocust, with fruiting bodies of the fungus clearly visible.
Canker diseases are fairly common on stressed honey-locust trees. Research by two Illinois Natural History Survey researchers in the late 1980s tested earlier theories by others and showed that fall infections resulted in more cankers than spring infections by canker fungi. They also showed that honeylocust cultivars vary in susceptibility to the canker fungi. ‘Imperial’, ‘Holka’, or ‘Shademaster’ honeylocust cultivars were more tolerant than some others. Look for resistance ratings when purchasing new honeylocust trees. The disease has been linked to drought stress in many cases. Where tree selection is not a choice, avoid injury, provide water in periods of drought stress, and help tree vitality by removing dead wood and fertilizing in fall or spring. Honeylocust trees can be easily bruised when young, especially with mowers and trimmers. This is one injury that can be avoided by mulching to keep equipment away from the trunk.
As with most canker diseases, there is no rescue treatment that can be sprayed on the tree. When you see a canker problem, try to determine the cause of stress and take measures to alleviate that stress.