Causes of Witches' Broom on a Tree

Cause

• Unlike the majority of other tree disorders that have exact causes, there is not just one causal agent of the witches' broom disorder. The problem could result from pathogenic infections such as from phytoplasmas, bacteria-like organisms, or fungi. Trees infested with pests such as mites and aphids also are likely to develop witches' brooms. Environmental conditions such as drought stress that damage the natural growth of the tree also lead to the formation of witches' brooms. The use of herbicides sometimes causes witches' brooms in trees.

Identification

• The disorder is characterized by the growth of compact clusters of twigs growing from a single point in the tree. This dense mass of stems creates a broom-like look. Witches' broom growth could last on affected plants for many years. The new stems gradually thicken with time. The foliage and shoots on affected areas grow discolored, stunted and distorted in growth, standing out distinctly in contrast to the rest of the tree.

Damage

• The presence of witches' brooms on trees is more of an aesthetic disorder that ruins the overall look of the affected plants. The disorder is not a serious threat to plant health. It is only in rare cases that witches' broom results in serious injury. The disorder is most evident on bare deciduous plants and trees in the fall.

Control

• There is no chemical option for the control of witches' broom. The best preventative is to maintain plant health and ensure that growth is vigorous. Prune out and remove the affected areas, making the cut several inches below the point of the disorder's origin. If tree is infested with pests, use recommended insecticides to control the spread of these insects.