Colorado Master Gardener: Witches’ broom
For Steamboat Pilot & Today
Witches’ broom is the name given to the strange looking knot of growth on trees and shrubs. It looks like a broom — a large number of small branches growing from one spot — and may be found on both deciduous woody plants and conifers. It is interesting to observe out in the forest but may be concerning if found close to home.
A tree or shrub may grow witches’ brooms when stressed by insects, like mites or aphids, or a plant pathogen like fungi, bacteria, viruses, phytoplasmas or by parasitic plants In Colorado conifers, the stressor is likely one of five parasitic dwarf mistletoes (Arceuthobium).
Dwarf mistletoe infects a tree by growing root-like structures under the bark and into the wood. It lives by pulling nutrients and water from the tree.
It is a slow growing organism: after several years it will develop inconspicuous flowers and produce fruit that when ripe, will explode and send a seed into the air. The seed will stick to any surface up to 60 feet away. If the surface is a susceptible tree branch, the seed will germinate and grow into the bark, spreading the infection to another tree.
Witches’ brooms caused by other stressors rarely kill the host plant and may be pruned out to improve the appearance of the tree or shrub. Dwarf mistletoe, however, can be deadly. Over time, infected trees fail to thrive and may have witches’ brooms, unhealthy looking foliage and dead branches. The trees are then susceptible to fatal problems, like pine beetle attacks.
While no viable treatment is available for infected conifers, proper management may slow or stop a dwarf mistletoe infestation. Severely affected trees should be removed and other trees pruned to remove infection from lower branches. As dwarf mistletoe is species specific, its spread may be halted by planting different tree species between infected trees.
A chemical spray may be warranted under special circumstances. Please see CSU Extension Fact Sheet No. 2.925 for more details.
If you find a witches’ broom in your conifer and seek pruning information, please see CSU Extension GardenNotes No. 633. If you suspect a dwarf mistletoe infestation and would like help with a management plan, please contact a professional forester, the Colorado State Forest Service or the Master Gardener program.
Vicky Barney gardens for wildlife and is a member of the Master Gardener Class of 2011.
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