University of Maryland Extension

Witches' Broom - Trees

Witches' broom
An example of a witches' broom

witches broom on pine
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.
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