Peggy Sellers, Purdue University
The latest issue of Down the Garden Path (Issue #155 in PDF) includes this article.
A witches' broom is a symptom in woody plants where many twigs are densely clustered together, resulting in a mass of shoots that resemble a broom. This symptom can develop on many types of plants and can be caused by various microorganisms or insects. Witches' brooms caused by fungi can be found on cherry and blackberry. Phytoplasmas that cause elm or ash yellows can cause this symptom. Honeysuckle witches' broom is caused by an aphid.
Perhaps the most frequently seen witches' broom is found on common hackberry (Celtis occidentalis). Some people think witches' brooms are a characteristic of hackberry because it is so common. The hackberry witches' brooms are thought to be caused by a complex of a powdery mildew fungus (Sphaerotheca phytophila) and an eriophyid mite (Aceria sp.). 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. There is no control for withches' brooms on hackberry. However, pruning out clusters of witches' brooms back to the healthy wood can improve a tree's appearance.
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