Witch’s Broom in Trees: Dense Clump of Leaves and Branches

Caitlin Dempsey

Updated:

Walk among some species of trees, and you might notice a dense clump of bright green leaves and branches that stand out among the normal pattern of growth on a tree. While this odd-looking growth of dense clusters of leaves might look like it’s a bird’s nest or a squirrel’s drey, these abnormalities on trees are part of the growth of the tree.

This growth anomaly, often resembling a broom or a bird’s nest, is known as a “witch’s broom.” The term “witch’s broom” comes from folklore, where such growths were believed to be caused by witches’ spells or other supernatural forces. In reality, this peculiar growth on trees is the result of more natural causes.

What causes witches’ brooms in trees?

The formation of witch’s broom can be attributed to a range of causes, including pathogens, parasites, genetic mutations, and environmental stressors. Pathogens, such as fungi, bacteria, and viruses, are frequently the reason behind a growth of witch’s broom.

A view up a tree with a dense growth of leaves known as a witch's broom.
A witch’s broom in a tree on the lower La Honda Preserve in La Honda, California. Photo: Caitlin Dempsey.

Fungal pathogens like Taphrina and Erysiphales are known to induce this growth by infecting the host plant. For instance, Taphrina betulina is responsible for witch’s broom in birch trees. Parasitic plants, such as mistletoe, can also cause abnormal growth in their host trees by extracting nutrients and water, leading to a proliferation of shoots near the site of attachment.


Free weekly newsletter

Fill out your e-mail address to receive our newsletter!
Email:  

Insects and mites are another common cause of witch’s broom. Certain insects, like eriophyid mites and aphids, can inject chemicals into the plant while feeding, disrupting normal growth patterns and causing the plant to produce excessive shoots. Environmental factors, including physical damage from pruning, storms, or other mechanical injuries, can also lead to the formation of witch’s broom as the plant attempts to recover from the damage.

An abnormal growth of leaves in a conifer tree.
A witch’s broom in a conifer tree in Bear Creek Redwoods Preserve in the Santa Cruz Mountains, California. Photo: Caitlin Dempsey.

Which trees are commonly known to be susceptible to developing witch’s broom?

Witch’s broom can affect a wide variety of trees and shrubs, but certain species are more susceptible due to their specific interactions with pathogens, parasites, and environmental conditions. For example, conifer trees in the Southwester United States infected by Arceuthobium spp. produce witch’s brooms as a result.

Some of the most commonly affected trees include:

  1. Birch Trees (Betula spp.): Particularly susceptible to fungal pathogens like Taphrina betulina, which causes significant witch’s broom formation.
  2. Spruce Trees (Picea spp.): Often affected by spruce broom rust (caused by the fungus Chrysomyxa arctostaphyli) and infestations by aphids or mites.
  3. Pine Trees (Pinus spp.): Susceptible to rust fungi like Melampsorella caryophyllacearum, which causes broom-like growths.
  4. Maple Trees (Acer spp.): Can develop witch’s broom due to eriophyid mite infestations or bacterial infections.
  5. Willow Trees (Salix spp.): Often affected by fungi, mites, and aphids, leading to the characteristic broom growths.
  6. Apple Trees (Malus spp.): Susceptible to various pathogens and insect infestations that can induce witch’s broom.
  7. Cherry Trees (Prunus spp.): Can develop broom-like growths due to fungal infections and mite activity.
  8. Hackberry Trees (Celtis spp.): Frequently affected by the fungus Podosphaera phytophila, causing notable broom formations.

While these trees are particularly prone to developing witch’s broom, the condition can potentially affect many other species as well, depending on local environmental conditions and the presence of specific pathogens or pests.

References

Hawksworth, F. G., & Wiens, D. (1966). Observations on witches’-broom formation, autoparasitism, and new hosts in PhoradendronMadroño18(7), 218-224.

Tinnin, R. O., Hawksworth, F. G., & Knutson, D. M. (1982). Witches’ broom formation in conifers infected by Arceuthobium spp.: an example of parasitic impact upon community dynamicsAmerican Midland Naturalist, 351-359. DOI: 10.2307/2425385

Photo of author
About the author
Caitlin Dempsey
Caitlin Dempsey is the editor of Geography Realm and holds a master's degree in Geography from UCLA as well as a Master of Library and Information Science (MLIS) from SJSU.