Witch’s broom – Wikipedia
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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 tips 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.[4] 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.
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Doesn’t seem to be related to somatic mosaicism
https://en.wikipedia.org/wiki/Witch%27s_broom