Symptoms: Witches'-brooms are compact clusters of branches that grow densely from a common point on a major branch. The brooms have a clumped, bushlike or bird's nest appearance. They are best seen after leaf fall.
Cause: Witches'-brooms have different causes on various host plants -- for example, anthracnose on sycamore, road deicing salt on many species, powdery mildew associated with gall mites on hackberry, viruses on black locust, leaf spot fungi on hickory, leaf blister on birch, and mycoplasma bacteria on ash and honey locust.
Control: There is no practical control for this disease symptom, but unsightly brooms can be pruned off.