Learn more about this interesting infection in trees.
Witches’-brooms occur on a number of conifers and deciduous tree species. They are caused by a number of factors that result in a great proliferation of shoots with short internodes that can look like a bundle of twigs or witch’s broom. In other cases they appear as a ball-shaped dwarf plant growing in a tree. Propagation of these witches’-brooms in confers has been the source of many dwarf cultivars.
Witches’-brooms are caused by a number of factors, including infection by fungi or phytoplasmas (a wall-less single celled organism with an unorganized nucleus), infestation of mites or dwarf mistletoe, genetic mutations, or adverse environmental conditions that kill the terminal bud of shoots. Those caused by genetic mutation may be stable allowing for them to be propagated vegetativly as dwarf cultivars. Some common trees that develop witches'-brooms include oak (caused by powdery mildew), incense cedar (caused by a rust), hackberry (cause by powdery mildew and an eriophyid mite), and rose rosette (caused by a virus). Phytoplasmas cause witches’-brooms and bunch disorders on pecan, hickory, lilac, walnut, willow, dogwood, ash, honeylocust, peach, elm. Juniper, firs, hemlocks, and pines can also develop witches’-brooms in reaction to infestation from dwarf mistletoe.
Information from the Missouri Botanical Gardens website.