
An example of a witches' broom

Witches' broom on pine
Photo: Lesley Ingram, Bugwood.org
Key Points
- Witches' brooms are characterized by a proliferation of shoots growing close together. The shoots are usually shorter, stockier, and have an upright but more compact growth habit than normal.
- Witches' brooms may be caused by fungal, viral, or mycoplasma-like organisms (MLOs). Eriophyid mites, mistletoe, environmental damage, or a mutation in vegetative cells may also cause witches' brooming.
- In most cases, the causal agent kills a growing point and results in the prolific growth of side shoots.
- The growth around the witches' broom may become less vigorous, indicating that the witches broom may divert nutrients from other parts of the plant.
- When witches' brooms are caused by mutation, horticulturists sometimes propagate them for breeding of dwarf plants.
Causes of Witch's Broom
- Mycoplasma-like organisms (MLOs) are also called phytoplasmas. They are related to bacteria, lack a rigid cell wall, and have an amoeba-like shape.
- MLOs appear to colonize in the sap conducting tissue (phloem) and damage the tissue by interrupting the sap flow.
- Diseases caused by MLOs are elm yellows, ash yellows, and bunch diseases of walnut. Witches' brooming, chlorosis, and general decline are symptoms of these diseases.
- MLOs may also be responsible for witches' brooming in lilac, dogwood, willow, apple, black locust, honeylocust, papaya, peach, and sassafras.
- Witches' brooms can be a symptom of fungal or viral infection.
- The fungus Ascomycetes causes witches' broom of cherry. The powdery mildew fungus, Sphaerotheca lanestris, may cause witches' brooms on live oak, willow oak, and ninebark. The fungus, Gymnosporangium nidus-avis, causes juniper broom rust. Other fungi cause witches' brooming primarily in evergreen plants.
Management
- When witches' brooming is noticed, prune out the affected parts, if possible.
- When fungi, virus or mycoplasma-like organisms are responsible for witches' brooms, the disease may have spread throughout the tree, so that pruning may not provide control.