Witch's broom

Witch's broom is a deformity in a woody plant, typically a tree or shrub, where the natural structure of the plant is significantly altered. A dense mass of shoots grows from a single point, with the resulting structure resembling a broom or a bird's nest. This condition can be caused by various factors, including pathogens (such as fungi, viruses, and bacteria), parasitic plants (like mistletoe), damage from insects or mites, or environmental stress. Witch's broom can affect many different species of plants and can have significant impacts on forestry, agriculture, and horticulture.

Causes

Witch's broom can be induced by several biotic and abiotic factors:

• Pathogens: Fungi, such as Taphrina betulina which affects birch trees, and Ustilago esculenta, which affects Zizania species, are common causes. Viruses and phytoplasmas (a type of bacteria lacking cell walls) can also induce the condition. For example, the witches'-broom disease of lime (WBDL) is caused by a phytoplasma.
• Parasitic plants: Certain plants, like mistletoe, attach to the host tree and extract water and nutrients, which can lead to the development of witch's brooms.
• Insects and mites: Damage from these organisms can stimulate the growth of witch's brooms. For instance, the eriophyid mite Aceria anthocoptes can cause witch's broom in Canada thistle.
• Environmental stress: Damage from wind, snow, or mechanical injury can sometimes result in the formation of witch's brooms as the plant attempts to recover.

Impact

The impact of witch's broom on plants varies widely depending on the species and the severity of the infestation or damage. In forestry and agriculture, significant infestations can reduce the growth and yield of affected plants. In ornamental horticulture, witch's broom can be seen as a desirable trait for creating unique plant forms, and some cultivars have been selectively bred for this characteristic.

Management

Management strategies for witch's broom depend on the underlying cause. For pathogen-induced conditions, removing and destroying affected parts of the plant can help reduce the spread of the disease. In cases where insects or mites are the cause, appropriate pesticides may be used. For environmental or mechanical damage, proper care and maintenance of the plant can help prevent the condition.

In Culture

Witch's broom has also found a place in folklore and mythology. In some cultures, these formations were believed to be the work of witches, hence the name. They were often associated with magical or supernatural occurrences.