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Honeylocust Disease Problems

June 6, 2001

This species is tolerant of many adverse conditions. It can tolerate drought, high pH, and salt. Mike Dirr in Manual of Woody Landscape Plants says it is one of the more adaptable native trees. Still, we see many insect problems on this species, along with canker disease and root rot.

Cankers are dead areas on stems, usually referring to trees but 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, 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. This host has many insect and mite problems (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 spe-cies. Roots are shallow, so look for source of root injury.

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, red-dish 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 to spray on the tree. Prune out dead wood in dry weather, water the trees when 2 weeks of drought occur, and avoid phy-sical damage to the trees. When you see a canker problem, try to determine the cause of stress and take measures to alleviate that stress.

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 wide is common) and appear to be varnished with a shellac. We see problems with Ganoderma where rooting is restricted, soil is compacted, or 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 U of I quad was planted with honeylocust 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 RPD no. 642, “Wood Rots and Decays,” which includes a picture of the fruiting body.

Author: Nancy Pataky


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