Witch's broom orwitches' broom is a deformity in awoody plant, typically a tree, where the natural structure of the plant is changed. A dense mass ofshoots grows from a single point, with the resulting structure resembling abroom or a bird's nest. It is sometimes caused by pathogens.
Diseases with symptoms of witches' broom, caused byphytoplasmas or basidiomycetes, are economically important in a number of crop plants, including the cocoa treeTheobroma cacao,[1]jujube (Ziziphus jujuba)[2] and the timber treeMelia azedarach.[3]
A tree's characteristic shape, or habit, is in part the product ofauxins, hormones which control the growth of secondaryapices. The growth of an offshoot is limited by the auxin, while that of the parent branch is not. In cases of witch's broom, the normal hierarchy of buds is interrupted, and apices grow indiscriminately. This can be caused bycytokinin, aphytohormone which interferes with growth regulation. The phenomenon can also be caused by other organisms, includingfungi,oomycetes, insects,mites,nematodes,phytoplasmas, and viruses.[4] The broom growths may last for many years, typically for the life of the host plant. If twigs of witch's brooms are grafted onto normal rootstocks, freak trees result,[clarification needed] showing that the attacking organism has changed the inherited growth pattern of the twigs.[4]Hemiparasitism by thedwarf mistletoes species in genusArceuthobium are noted to induce witch's broom formation in parasitized branches of pines and cypresses.[5]
Witches' brooms provide nesting habitat for birds and mammals, such as thenorthern flying squirrel, which nests in them.[6][7]
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