Witches' broom

Overview

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 witches broom. In other cases, they appear as ball-shaped dwarf plants growing in a tree. Propagation of these witches’-brooms in confers has been the source of many dwarf cultivars.

Life Cycle

Witches’-brooms are caused by several 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 vegetatively as dwarf cultivars. Some common trees that develop witches'-brooms include oak (caused by powdery mildew), incense cedar (caused by rust), hackberry (caused 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, and elm. Juniper, firs, hemlocks, and pines can also develop witches’-brooms in reaction to infestation from dwarf mistletoe.

Integrated Pest Management Strategies

1. Prune out infected branches if you find their appearance objectionable. If practicable, prune out branches that are infected or cut back shoots to the point of origination on larger branches and trunks. Shoots may regrow requiring ongoing removal every few years.

Organic Strategies

This strategy is a strictly organic approach.