Honeylocusts in Illinois are tolerant of many adverse conditions, or they would not be able to survive. They can tolerate drought, high pH, and salt, three factors that can limit growth of other trees in the same area. Mike Dirr, in Manual of Woody Landscape Plants, says that it is one of the more adaptable native trees. It is also able to tolerate many insect problems. Canker diseases and root rots round out the disease troubles. |
Cankers are dead areas on stems. 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. Honeylocust trees are particularly prone to Thyronectria canker, Cytospora canker, and Kaskaskia canker; but the actual organism involved does not make a great deal of difference. The fungi invade stressed trees, so management involves pinpointing the source of stress and trying to correct it. There are many insect and mite problems on this host (borer, plant bug, webworm, spider mites), some of which have been discussed in this newsletter. Such problems certainly stress a tree. Soil compaction also stresses this species. Roots are shallow, so look for the source of root injury. Last year, most areas of Illinois experienced about 6 to 8 weeks of drought. That is particularly stressful to shallow-rooted trees such as honeylocust. Winter was also dry in most of the state. Frequent applications of some lawn herbicides can also be harmful. Check label restrictions of all herbicides carefully before use.
The most aggressive canker disease of honeylocust is Thyronectria canker. Symptoms include yellowing and wilting of the foliage, premature leaf drop, and stem dieback. Look closely for 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 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 honeylocust trees; and the disease has been linked to drought stress in many cases. Still, the canker can be easily overlooked. 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 2 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. Canker diseases are discussed in Report on Plant Disease (RPD), no. 636, “Canker and Dieback Disease of Woody Plants,” available in Extension offices or on the Extension VISTA Web site.
Ganoderma root rot is another problem we see frequently on honeylocust. This root rot pathogen forms reddish brown fruiting bodies (also called conks or shelf fungus) on the trunk, usually near the soil line. They are large (5 to 10 inches in width is common) and appear to have been varnished. We see problems with Ganoderma where rooting is restricted, where soil is compacted, or where other major soil changes have occurred. I witnessed results of infection by this fungus in some large planters that had been accidentally flooded earlier in the year. The University of Illinois quad was planted with honey-locust to replace DED-infected elms in the 1950s. Most of these honeylocust trees have been removed because of Ganoderma root rot. Information about Ganoderma can be found in the RPD, no. 642, “Wood Rots and Decays.” You will also find a picture of the fruiting body on the front page of that publication.