Witches' Brooms on Trees


A willow branch was recently submitted to the Plant Disease Clinic that had a dense cluster of twigs, a symptom referred to by plant pathologists as a "witches' broom".

In medieval times, mysterious and unexplainable occurrences were often blamed on witchcraft. Brooms during this time were made of bundles of twigs. The term witches' broom comes from the German word Hexenbesen, which means to bewitch (hex) a bundle of twigs (besom).

Witches' brooms occur on many different woody plant species, including deciduous trees such as hackberry, maple, and willow, and conifers such as pine and spruce. There may be only one broom in a tree, or they may be many scattered throughout the tree. In some cases, the brooms are quite large in size and are easily spotted. In others, they are small and well-hidden.

A number of stresses, both biological and environmental, can lead to the formation of brooms. Organisms such as fungi, phytoplasmas (bacterial-like organisms), mites, aphids, and mistletoe plants can cause abnormal growth when they attack a host tree. Environmental stresses that injure the growing points of branches can also trigger the formation of brooms. Some brooms appear to be caused by genetic mutations in the buds of the branches. Unlike brooms caused by living organisms, there is usually just one broom per tree when the cause is a genetic mutation.

Pinpointing the cause of a witches' broom can be difficult, especially if the formation is related to an environmental factor. Analyzing the plant tissue for infectious agents such as phytoplasmas requires specialized testing that can be costly. The cause of the witches' broom on the willow sample sent to the Plant Disease Clinic was not identified with certainty.

Witches' Broom

Witches' broom, cause unknown, on willow.

This article originally appeared in the 2/23/2005 issue.



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