Witch's broom
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A Witch's broom is a disease or deformity in a woody plant, typically a tree, where the natural structure of the plant is changed. A dense mass of shoots grow from a single point, with the resulting structure resembling a broom or a bird's nest.
One example of this would be cytokinin, a phytohormone, interfering with an auxin-regulated bud. Usually, auxin would keep the secondary, tertiary, and so on apices from growing too much, but cytokinin releases them from this control, and that is why these other apexes grow into witch brooms.
Witch's broom growth may last several years and can be caused by many different types of organisms, such as fungi, insects, mistletoe, mites, nematodes, and viruses. Human activity is sometimes the reason for the introduction of these organisms; for example, when a person prunes a tree improperly and leaves the tree susceptible to disease.
Witch's brooms occasionally result in desirable changes. Some cultivars of trees, such as Picea orientalis 'Tom Thumb Gold', were discovered as witch's brooms.
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