Witches’ brooms are proliferations of shoots growing from one point and giving the appearance of a broom. We see them at the ends of stems on shrubs and trees. Witches’ brooms are easily visible now before foliage is fully expanded. The cause varies with the host. Cherry and blackberry witches’ brooms are caused by fungi; some witches’ brooms are the result of phytoplasmas (mycoplasmas); and common hackberry brooms are thought to be caused by a powdery mildew fungus in association with an eriophyid mite. There are even some brooms in conifers that occur as a genetic mutation. Such brooms are sought for propagation and use as dwarf types.
The most common witches’ broom in Illinois is found on common hackberry trees. It is so common that many homeowners think witches’ brooms are 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. As a result, the tree has many clumps of thick growth throughout the canopy. The brooms may be unpleasant in appearance but will not kill a hackberry tree. Branches with brooms will become less vigorous with time, but the overall effect on the tree is minimal. The broomed areas may be more susceptible to wind, ice, and snow breakage.
As far as we know, there are no practical control measures for witches’ brooms on hackberry. Because the pathogen does not progress down the stem, pruning is an option on small-specimen trees, but that would be a rare event because pruning would further disfigure the tree. Other than improving tree vitality through watering and fertilizing, there is no recommended treatment for brooms on hackberry. Just accept it. 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. See RPD No. 662 for more information on this condition.
In most cases, nothing can be done to control witches’ brooms, but there are a few exceptions. If brooms are occurring on a shrub host, consult reference books or your Extension office to find the cause of the brooms. Mites or insects may be causing the problem, and possibly curative treatments will be effective, as with honeysuckle brooms caused by an aphid.