When many shoots seem to originate from a single point on a stem, the result is a mass of dense shoots that resemble a broom--thus, the name witches' brooms. Many plants develop this brooming symptom. The cause varies with the host, but essentially there is a lack of apical dominance, which means all shoots develop equally.
Cherry and blackberry witches' brooms are caused by fungi. Some witches' brooms, such as those on trees infected with elm yellows or ash yellows, are the result of phytoplasmas (formerly called mycoplasmas). Still other witches' brooms are caused by insects. An aphid that causes brooms on honeysuckle is known as the honeysuckle witches' broom aphid.
Witches' broom of common hackberry is thought to be caused by a powdery mildew fungus in association with an eriophyid mite. This is such a common condition on Illinois common hackberry trees that one would think that witches' brooms were a characteristic of the species. Many of the twigs in a broom will 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 on the tree is many clumps of thick growth throughout the canopy. The brooms may be unpleasant in appearance but will not kill a hackberry tree.
As far as we know, there are no practical control measures for witches' brooms on hackberry. If you want 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. For more information about witches' broom on hackberry, refer to Report on Plant Diseases No. 662.