What Is a Witches' Broom?

Witches' broom rust (Chrysomysa arctostaphyli) of spruce.
scotnelson/Flickr/CC BY 2.0

If you've ever looked up into a tree—particularly a conifer or deciduous—and seen a cluster or ball of twigs, you've seen a witches' broom. This happens when many small branches start growing in the same location. Tree species that are often affected by witches' broom include oak, incense cedar, pecan, lilac, walnut, dogwood, pine and more.

Witches' broom can be rather small or quite large. There might be just one cluster of twigs in a tree, or there might be many. Witches' broom can be caused by several different factors, and some are disease- or pest-related. Therefore, the tree should be carefully inspected to determine the cause of the problem.

Causes of Witches' Broom

The causes of witches' broom range from infection to parasites. For example, one cause of a witches' broom is dwarf mistletoe. This parasitic plant attaches to the branches so it can share the tree's water and nutrients. The witches' broom will form near these mistletoe-infested branches.

Witches' broom might also be the effect of infection by fungi or phytoplasmas, which is a single-celled organism, or infestation by mites. Quite often, the cause of the witches' broom can be determined by the species of tree. Fungal infections typically appear in pine or cherry trees, as well as blackberry bushes, while witches' broom in peach and black locusts is likely the result of a virus. Mites are typically responsible for the condition in willow trees, while aphids can be to blame in honeysuckle shrubs.

Witches' broom isn't always caused by a pest or disease. Sometimes it forms because the tree is stressed from a branch that broke off by accident or because pruning was not done properly. It can also be caused by a genetic mutation or environmental conditions that led to the death of the terminal buds of shoots. If the witches' broom is caused by a genetic mutation, there will likely only be one cluster of twigs in the tree. Conifer trees, such as pine, fir, spruce and juniper, might be affected by a genetic mutation that causes witches' broom.

How to Manage Witches' Broom

The witches' broom itself isn't harmful to the tree—it's a symptom, not a pest or disease itself—so you can leave it be if you don't feel like taking care of it. However, if you don't like the way witches' broom looks in a tree, simply prune it out with care. Make sure to disinfect your pruning tools in between making cuts; otherwise, you could spread disease to another part of the tree.

If you suspect that the witches' broom is caused by a pest or disease, though, you will want to treat the underlying cause. The cluster of twigs could reappear in the future—or you might spot a more serious symptom of the pest or disease—if the issue isn't taken care of in good time.