Many plants develop a brooming symptom called witches’ brooms. On woody plants, this appears as a proliferation of shoots from one point- like a broom. The cause of these symptoms varies with the host. Cherry and blackberry witches’ brooms are caused by fungi. Some witches’ brooms are the result of phytoplasmas (mycoplasmas). Common hackberry trees in Illinois usually have witches’ brooms throughout the canopy. These brooms are thought to be caused by a powdery mildew fungus in association with an eriophyid mite. This is so often found on Illinois common hackberry trees that witches’ brooms seem almost characteristic of the species. Many of the twigs in a broom die back in the winter. Buds on the surviving twigs are numerous, larger than normal, usually grayish, and with looser scales than normal buds. The overall effect of the tree is that it has many clumps of thick growth throughout the canopy. The brooms may be unpleasant in appearance, but they will not kill a hackberry tree.
As far as we know, there are no practical control measures for witches’ brooms on hackberry. If you need a tree without the brooms, do not plant a common hackberry. Sugarberry is less frequently affected, and both Chinese hackberry and Jesso hackberry are considered resistant. Report on Plant Disease No. 662 discusses this condition.