Witches' broom on trees: what the sources say causes it

This page describes what the named sources say. It does not identify the cause of a broom on any particular tree, say whether a tree is healthy, or tell anyone to cut or keep one. Dwarf mistletoe, one of the causes the sources list, has its own page for the plant itself: mistletoe on trees.

What it looks like

The Missouri Botanical Garden (page updated September 2024) says witches' brooms occur on many conifers and deciduous trees and come from factors that make shoots multiply with short internodes, so the growth can look like a bundle of twigs or a broom. In other cases it looks like a ball-shaped dwarf plant growing in a tree. Oregon State University Extension (publication EM 9679) describes branch brooms as an abnormal proliferation of branches or twigs on a single branch, and lists them with bole swelling and cankers as signs that a conifer is infected with dwarf mistletoe.

Causes the sources list

Source	Causes it names
Missouri Botanical Garden (2024)	Fungi or phytoplasmas, mites, dwarf mistletoe, genetic mutations, or adverse conditions that kill the terminal bud of shoots. Examples it gives: oak (powdery mildew), incense cedar (rust), hackberry (powdery mildew and an eriophyid mite), and rose rosette (a virus). Phytoplasmas on pecan, hickory, lilac, walnut, willow, dogwood, ash, honeylocust, peach and elm. Dwarf mistletoe on juniper, firs, hemlocks and pines.
Northern Woodlands magazine, quoting a U.S. Forest Service plant physiologist	Fungi, insects, mistletoe and dwarf mistletoe, mites, viruses and phytoplasmas. The organism attacks the growing tips, upsetting the hormone auxin that normally slows buds away from the tip, and raising cytokinin, which makes shoots multiply.
UC IPM Pest Note on mistletoes (March 2022)	For dwarf mistletoe it says other factors can cause brooms too, including genetic mutations, mites, viruses and fungi, so a broom is not necessarily diagnostic and the mistletoe must be confirmed by finding its shoots, flowers or fruits.

Whether the sources call it harmful

The Northern Woodlands article (Joe Rankin, The Outside Story) quotes the Forest Service physiologist saying most witches' brooms do not appear to weaken their host, and that many trees live with them, sometimes dozens, for decades. The same article says some brooms have heavy economic effects, such as a fungal broom disease of cocoa. For dwarf mistletoe, UC IPM says the most obvious symptoms are brooms, and that infestation slows and often stunts growth; Oregon State lists growth loss, distortion, topkill and mortality as effects of dwarf mistletoe. So the sources attach the harm to the cause rather than to the broom as a shape.

Other points the sources make

The Missouri Botanical Garden says brooms caused by a genetic mutation can be stable and have been propagated as dwarf cultivars, and that many dwarf conifer cultivars started this way. Northern Woodlands adds that northern flying squirrels, some birds and some insects use brooms for shelter. The Garden lists pruning out affected branches as an integrated pest management strategy and says shoots may regrow; this page does not set out a method. Rules on nests and protected animals in tree work are covered in the guide on wildlife and the law.

The other growth forms are listed on the odd tree growth page.

Witches' broom on trees: what the sources say causes it

What it looks like

Causes the sources list

Whether the sources call it harmful

Other points the sources make

Sources