Witch's broom

A witch's broom or witches' 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 grows from a single point, with the resulting structure resembling a broom or a bird's nest.

Witch-broom disease caused by phytoplasmas is economically important in a number of crop plants, including the cocoa tree Theobroma cacao,jujube (Ziziphus jujuba) and the timber tree Melia azedarach.

Witch's broom can be caused by cytokinin, a phytohormone, interfering with an auxin-regulated bud. In normal plant function, an auxin would keep the secondary, tertiary, and so on apices from overgrowing, but cytokinin can sometimes interfere with this control, causing these apices to grow into witch's brooms.

Witch's broom may be caused by many different types of organisms, including fungi, oomycetes, insects, mistletoe, dwarf mistletoes, mites, nematodes, phytoplasmas, or viruses. The broom growths may last for many years, typically for the life of the host plant. Human activity is sometimes behind the introduction of these organisms, for example, by failing to observe hygienic practice and thereby infecting the tree with the causative organism, or by pruning a tree improperly, and thereby weakening it.

Witches' brooms occasionally result in desirable changes. Some cultivars of trees, such as Picea orientalis 'Tom Thumb Gold', were discovered as witch's brooms. If twigs of witches' brooms are grafted onto normal rootstocks, freak trees result, showing that the attacking organism has changed the inherited growth pattern of the twigs.

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