witches broom

A dense mass of deformed twigs, often resembling a bird's nest, caused by the host response to infection by certain insects, mites, viruses, fungi, or parasitic plants. Witches' broom of cacao is caused by the fungus Marasmius perniciosus and can result in almost total loss of yield as pods are also infected. Witches' broom of birch (Betula) is caused by the fungus Taphrina betulina.