What Is Witches’-Broom in Trees and Is It Harmful?

What’s that? Apical dominance is when the central growing shoot is stronger than the side shoots. When a shoot loses apical dominance, those side shoots take over the lead.

Often the dominant leader dies and the side shoots take over.

We’re going to talk about what can cause this disruption in growth next.

Causes

A witches’-broom can be caused by anything that disturbs the growth in the young bud.

That could be insects, pathogens like viruses, phytoplasms, and bacteria, fungi and oomycetes, nematodes, eriophyd mites, and disruptive phytohormones like cytokinin.

Parasitic plant growth can also cause hormone disruption.

It can also be a result of genetic mutations or even environmental conditions. Solar radiation is thought to be the reason that conifers at higher elevations have more witches’-brooms, for example.

Places where herbicides are used tend to have more odd deformities, too.

It’s common to see them on oak trees and ninebark, usually caused by powdery mildew, and on incense cedars and firs, caused by rust.

Dwarf mistletoe infestations commonly cause them on conifers like Douglas firs, junipers, and pines. In walnuts, it’s often phytoplasms that cause bunching disease that results in witches’-brooms.

Horticultural Significance

Some growers and experts keep an eye out when walking in the woods or through parks and nurseries for witches’-brooms. That’s because these growths can be propagated to make new cultivars.

In fact, some of the most popular dwarf conifers were grown out of witches’-brooms. Collecting them has become increasingly common and profitable in recent years.

In the housing boom after World War II, more people found themselves with backyards, most of them somewhat petite, and gardeners were looking for compact versions of the plants they love.

That trend continues today. So breeders started looking for witches’-brooms because they produce plants that stay small.

Plant breeders look for the growths that are caused by genetic mutations rather than disease or damage because they’re more likely to be stable and reproducible.

Once they find a good specimen, they’ll grab a ladder or a tree saw and will snag the growth. Believe it or not, on taller conifers, people are known to use a rifle and shoot them out of trees!

Then, they’ll graft or root the growths to develop a new plant. Some of the most common evergreens available are from witches’-brooms. Dwarf mugo pines are one common example.

The perennial favorite eastern white pine ‘Sea Urchin’ was a growth discovered by horticulturalist Dr. Sidney Waxman.

North Carolina State University has even asked people to help them hunt for witches’-brooms on loblolly pines for their Tree Improvement Program.