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 phytoplasmas. 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 the breeding of dwarf plants.
Causes of witches' broom
- Phytoplasmas are related to bacteria, lack a rigid cell wall, and have an amoeba-like shape.
- Phytoplasma diseases appear to colonize in the sap conducting tissue (phloem) and damage the tissue by interrupting the sap flow.
- Diseases caused by phytoplasmas are elm yellows, ash yellows, and bunch diseases of walnut. Witches' brooming, chlorosis, and general decline are symptoms of these diseases.
- Phytoplasma diseases 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 fungi Taphrina cerasi and Taphrina weisneri cause 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, viruses, or phytoplasmas are responsible for witches' brooms, the disease may have spread throughout the tree, so that pruning may not provide control.