The term “witches’ brooms” may bring up images of Halloween transportation devices, but it also describes a type of growth disorder in plants. Witches’ brooms are dense masses of shoots arising from a single point on an otherwise normal branch. The shoots typically display dwarfed characteristics: slow growth, very short internodes (the space between leaves or side branches), and much smaller than normal leaves.
Witches’ brooms can be caused by a number of factors that alter normal growth including pathogens (fungi, viruses, bacteria, phytoplasmas), parasitic plants (dwarf mistletoe), and arthropods such as mites and aphids. Other witches’ brooms result from genetic mutations that arise from unknown factors.
Common hackberry (Celtis occidentalis), an adaptable North American shade tree, is frequently affected by witches’ brooms that seem to have two contributing causal agents: powdery mildew fungus and a tiny eriophyid mite. The witches’ brooms typically don’t affect the tree’s health (unless there’s a severe infestation) but can detract from its appearance. Another unsightly witches’ broom that I noticed in the Arboretum recently is common to a number of shrub honeysuckles (Lonicera) and is caused by the feeding of honeysuckle aphids, sometimes called Russian or Tatarian aphids for their Eurasian native range.
Many conifer species, including pines (Pinus), firs (Abies), spruce (Picea), and junipers (Juniperus), are noted for producing witches’ brooms. These may be caused by diseases such as spruce broom rust (a fungus) or the parasitic dwarf mistletoe, while others are genetic mutations. The latter are of particular interest for their ornamental potential as dwarf conifers. As far back as the mid 1800s, plant propagators were selecting, propagating, and naming cultivars of dwarf conifers that arose from witches’ brooms. The slow, dense growth and proportionally petite leaves of these cultivars make them popular for rock gardens and other landscape sites where small plants are desired. Interestingly, when conifer witches’ brooms produce seeds a fairly high percentage of the resulting seedlings may also have dwarfed characteristics. For a fascinating look at the etymology (yes, witches are involved!), history, and propagation of conifer witches’ brooms, read this 1967 Arnoldia article [pdf] by longtime Arboretum propagator Al Fordham.