A number of hackberry samples showing symptoms of witches'- broom have been arriving in the Plant Disease Clinic. As the common names implies, witches'-broom is characterized by a dense clustering of twigs. Each "broom" consists of numerous short twigs that arise close together. The cause of witches'-broom is not known with certainty, but it is thought that it caused the interaction of an eriophyid mite and a powdery mildew fungus.
These brooms arise from infested buds, which then give rise to shoots with more infested buds. Over time, branches appear to show many tight clusters of twigs along the axis. The brooms are easy to spot when there are no leaves on the tree.
Witches'-broom on hackberry seldom seriously injures trees. Extensive brooming may cause branches to break more easily and can reduce tree vigor. Pruning out the affected twigs, if feasible, can help improve the tree's appearance. There are no measures known that will prevent the disorder.
This article originally appeared in the June 23, 1995 issue, p. 91.