Cankers are dead areas on stems, usually on trees, but they are also present on herbaceous plants. Fungal organisms are blamed for the cankers but, in most cases, the fungus can only infect a weakened plant, such as one growing under stress. Wounds are ideal sites for canker fungi to invade. If the canker girdles the stem, wood beyond that point dies. At this time of year, such dead wood is obvious because gardeners are looking closely at their plants and because it stands out clearly against the new green growth of spring.
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. 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 you are in doubt about 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, appearing reddish brown. Healthy wood should be white, tan, or slightly green.
This canker disease is fairly common on stressed honey locust, although the canker can be easily overlooked. It has been linked to drought stress in many cases. Keep in mind that drought stress may have initiated the problem a year ago or even longer. The canker did not pop up overnight. Some areas of the state, including the Chicago area, were very dry early in the season last year. The Morton Arboretum has already reported Thyronectria in 1999.
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. When you see a canker problem, try to determine the cause of stress and take measures to alleviate that stress.