Hackberry trees are “tough old birds” that rarely express many problems. In fact, the two most common are really not problems at all. These include witches’-brooms and island chlorosis. The third problem reported to the Plant Clinic is a fairly heavy leaf drop.
Witches’-brooms are a proliferation of shoots from one point—making the stem appear to have a broom-like growth at the end. Common hackberries (Celtis occidentalis) in Illinois have this condition so frequently that we tend to think this symptom is part of the normal growth habit. Many of the twigs in a broom die back in the winter. The overall appearance of the tree is many clumps of thick growth throughout the canopy, with dead twigs interspersed. The brooms may be unpleasant to see but do not kill the tree.
These brooms are caused by a powdery mildew fungus in association with an eriophyid mite. Still, control measures are usually ineffective, and most people accept the brooming as the character of this species. If you are going to plant a new hackberry, consider Jesso hackberry, Celtis jessoensis, or sugar hackberry, Celtis laevigata. Sugarberry is less frequently affected with brooms, and Jesso is considered resistant. Report on Plant Disease (RPD) no. 662, “Witches’ Broom of Hackberry,” discusses this condition.
Island chlorosis is a disease that appears on hackberry leaves as yellow spots. These are very blocky because they are delineated by veins. With green areas around the yellow spots, affected leaves appear as yellow islands in a sea of green, thus the name island chlorosis. The disease is thought to be caused by a virus. As far as we have been able to determine, the disease is a curiosity but does not cause any damage.
The Plant Clinic has also received a few calls this year complaining about massive leaf drop from hackberry trees. This condition was also reported in 1998, but I have no idea if the same trees are involved. In 1998, a few samples were submitted, and staff found no infectious disease or insect problem on the foliage. Contact chemical burn was suspect, as was cold injury when leaves were in the bud. Affected trees developed healthy new leaves. No callers this year have made a sample submission. If you have problems with hackberry leaf drop, give us a call.
One suspect is root rot. Two common root pathogens, Ganoderma and Armillaria, may infect hackberry. I would expect to see these diseases on stressed trees, so look for root or trunk injury, evidence of a grade change or construction nearby, or some change since symptoms appeared. A mature, healthy hackberry is not going to be seriously injured by cold snaps, drought stress, or lack of fertilizer. Look for major site changes. Report on Plant Disease RPD no. 641, “Decline and Dieback of Trees and Shrubs,” may present some possible causes of stress.