Witches' Broom

Witches' brooms are a common sight in autumn landscapes, but there is one variety that you will want to watch out for.


Witches' brooms are a symptom of plant disease or tissue damage, most commonly affecting woody plants, such as trees.


Symptoms of witches' broom
Witches' broom is easy to recognize. Where there is normally a single twig or stem, a clustered riot of shoots emerge, pointing in every direction. Stems may be twisted, discolored, or dwarfed. From a distance, it may look like a squirrel's nest, or an area of especially dense foliage. Closer inspection reveals a distinct deformity.

Witches’ broom on downy birch (Wikipedia)

Causes of witches' broom

There are several conditions that can cause witches' broom. Some diseases, such as aster yellows and squash gourd mosaic can cause withes' broom. Sometimes it is a malfunction within a plant's hormone system. Auxin, a plant hormone that regulates plant cell growth, can be thwarted by a different plant hormone called cytokinen. Left to grow uncontrollably, stems grow in every direction, in every place possible. This burst of growth ends up looking something like a messy witch's broom, hence the name. Witches' broom can also be caused by bacterial plant parasites called phytoplasma. Phytoplasma were discovered in 1967 and no one has been able to recreate these pests in the laboratory. This makes studying them and developing control measures difficult. Fungi, mistletoe, mites, nematodes, and viruses may also cause this growth deformity. But sometimes deformities are not a bad thing.


The good witch

In some cases, the witches' broom that emerges is more of a dwarf version of the parent tree. Very often, that dwarf version can be removed and propagated. This is how many of our modern dwarf confers were first begun!


Bad for the kitchen garden

Witches' broom can wreck havoc in your vegetable garden. The same conditions that cause aboveground twigs to grow uncontrollably can make your carrot and other root crops inedible. There is no known cure for witches' broom and infected plants can spread the problem throughout your landscape. Your only recourse is to remove and dispose of affected plants in the trash. Do not compost them.

Carrot infected with aster yellows (MSU Extension)

Preventing witches' broom

Unless you have the time to create new plant species, your plants will be healthier if you can prevent this problem in the first place. Phytoplasma catch a ride to your plants using leafhoppers, in a behavior called phoresy. Controlling leafhoppers can reduce the occurrence of witches' broom. Also, proper pruning, to remove dead and diseased branches, crop rotation, and general garden sanitation, can reduce the likelihood of witches' broom occurring.


If witches' broom occurs in your landscape, remove the affected stems several inches below the distorted area. Be sure to sanitize cutting tools with one part bleach and nine parts water afterwards and dispose of the affected plant material in the trash.