This week we begin a monthly Q & A post about various natural history topics written by CREW volunteer naturalist, Dick Brewer. This month he tackles a frequently asked question from Bird Rookery Swamp visitors.
Q: What made the large platform up in the tree, and does anything live in it? (This can be seen at the 0.5 mile marker at Bird Rookery Swamp)
A: It’s called a Witches’ Broom. It’s a dense cluster of twigs/needles growing from a central source, sort of resembling a broom. It is a symptom of stress found in woody plants, mainly trees but also shrubs. The stress results in a deformed mass of twigs and branches which often appear broom-like. It can be small or up to several feet across.
The term Witches’ Broom dates to medieval Europe when people looked up into trees and saw what looked like a mat of twigs woven together and believed that witches placed them high in the trees and even rested on them. As brooms were once fashioned together from bundles of twigs, and since witches were presumed to be responsible for anything unusual, the abnormalities gave rise to the common name.
Factors which may cause Witches’ Brooms include infestations of mites or aphids or parasitic plants like mistletoe, genetic mutations, infection by fungi or phytoplasmas (wall-less single celled organisms with unorganized nuclei), or adverse environmental conditions that kill the terminal bud of the shoots.
Those caused by genetic mutation may be stable, so people have been able to propagate them vegetatively as dwarf cultivars. Regardless of the cause, each one is the only one of its kind in the world and is genetically unique.
In Florida, many of the Witches’ Brooms are a result of a fungal infection from Sphaeropsis tumefaciens (reference: http://mrec.ifas.ufl.edu/jos/Sphaeropsis.htm). Plants in Florida known to be affected by this disease are oleander, holly, bottlebrush, citrus, crepe myrtle, ligustrum, and even Brazilian pepper.
On some of these plants, the symptoms produced are a knotty gall rather than the mass of twigs known as Witches’ Broom. Sometimes, Witches’ Broom can be caused when the tree is stressed from a branch that broke off by accident or was poorly pruned by a person.
Witches’ Broom can last for several months to several years, and while it may be unsightly to some people, it really poses no serious threat to a healthy tree or shrub.
Witches’ Brooms can be ecologically important. They tend to be inhabited by a wide variety of organisms apart from the causative one. Some species of moths rely on them exclusively for food and shelter for their larvae, and larger animals including many arboreal rodents such as flying squirrels may nest in them.