witches' broom

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witches' broom A dense, broom-like cluster of proliferating branches or twigs at a particular point in a tree or shrub; from a distance it may resemble a bird's nest. Witches' brooms may be caused by infection with any of a number of organisms, including certain rusts; Taphrina deformans and T. turgida cause witches' brooms in birch trees (Betula pubescens and B. pendula respectively).