A witches’ broom is the dense, bird’s-nest-like tangle of twigs that sometimes appears on a single branch of a tree or shrub.1PEI Untamed. Witches’ Broom It is a symptom with many possible causes rather than one disease, ranging from fungi and tiny mites to viruses, parasitic plants and even weedkiller.2University of Maryland Extension. Witches’ Broom Disease: How to Identify and Manage It Spotting a broom is easy, but managing it means working out what made it, because the fix for one cause does little for another. Not every broom is bad news, either: some are harmless quirks that gardeners have turned into prized dwarf plants.3New Forests. Cone structure and seed development in grafted witches’ broom and normal crown clones from the same trees of Pinus sibirica
What a Witches’ Broom Looks Like and Why It Forms
Look for a tight cluster of thin twigs or stems radiating from one point on an otherwise normal branch. The needles or leaves on the broom are often smaller, paler or more crowded than elsewhere on the plant. On a conifer, the clump can look like a large bird or squirrel nest until you get close. On a rose, the signs are red, rubbery, very thorny new shoots and distorted leaves.
The shape differs by plant. On spruce and other conifers, a broom is often a dense ball of upright, crowded shoots with short needles, much denser than the rest of the tree. On broadleaf trees like hackberry or birch, it is a thicket of twigs, and its summer leaves tend to be small and bunched. Bare winter branches make brooms easiest to see.
A real nest is woven from loose leaves, twigs and grass, so you can pull it away from the branch. A broom is living shoots growing out of the wood itself. Tug gently on a suspect clump from a ladder or with a pole: a nest shifts or comes apart, while a broom stays put. Look at where the twigs join, since in a broom they all emerge from one swollen or knotted spot. A broom is also in the same place year after year and slowly gets bigger, while a nest is seasonal and often torn up by weather. Heavy regrowth after pruning is different, because it sprouts from the cut points rather than one swollen spot.
Inside the plant, a healthy branch keeps order through its leading shoot, which makes a hormone that slows the growth of the shoots below so they don’t overtake it.4Conifer Quarterly. What Is a Witches’ Broom? When a mutation disrupts that control, or when fungi, insects, phytoplasmas (tiny bacteria that live in a plant’s sap and are spread by sap-feeding insects such as leafhoppers) or viruses trigger growth hormones, many dormant buds start growing at once.5Conifer Quarterly. What Is a Witches’ Broom? Dozens of short, thin shoots then sprout from one spot instead of one strong branch.
Some causes actively hijack the plant. Certain phytoplasmas release proteins that break down the plant’s own growth-control switches, waking dormant buds.6PubMed Central. Jujube witches’ broom phytoplasmas inhibit ZjBRC1-mediated abscisic acid metabolism to induce shoot proliferation These changes can also make infected plants more attractive to the leafhoppers that carry the infection to new plants.7PubMed Central. Extended phenotype of phytoplasmas in eukaryotic systems: mechanisms and ecological implications
The Main Causes of Witches’ Broom
Brooms can be caused by fungi, viruses, phytoplasmas, eriophyid mites or mistletoe.8University of Maryland Extension. Witches’ Broom Disease: How to Identify and Manage It On spruce and fir, they are usually caused by either broom rust fungi or dwarf mistletoe, a small parasitic plant on conifers.9PEI Untamed. Witches’ Broom A soil bacterium can also cause brooms on both flowering plants and conifers.10Great Lakes Phragmites Collaborative. Witches’ Broom Disease: How to Identify and Manage It Rose rosette virus, spread by a tiny mite, causes broom-like growth on roses across the United States.11Great Lakes Phragmites Collaborative. Witches’ Broom Disease: How to Identify and Manage It
Environmental damage can also trigger brooming.12University of Maryland Extension. Witches’ Broom Disease: How to Identify and Manage It In the Florida Keys, most brooms on woody plants since 2019 have been traced to misuse of a class of weedkillers.13UF/IFAS Extension Monroe County. What is Witch’s Broom – UF/IFAS Extension Monroe County If a broom shows up near where weedkiller was applied, herbicide drift is worth suspecting. A mutation in a single bud can also produce a broom with no infection at all.14University of Maryland Extension. Witches’ Broom Disease: How to Identify and Manage It
The plant type narrows the field. Spruce, fir and pine brooms point first to rust fungi or dwarf mistletoe. Rose brooms with red, thorny shoots strongly suggest rose rosette virus. Hackberry, lilac and blueberry brooms are often linked to mites, fungi or phytoplasmas. Several plants brooming at once in one yard suggest herbicide or a spreading cause rather than mutation.
Whether a cause spreads matters just as much. A mutation broom stays confined to one spot and cannot infect other plants. Herbicide damage doesn’t spread either, though new growth may stay distorted for a season. Fungi, mistletoe, mites, viruses and phytoplasmas can all move to other branches or plants.
Broom rust fungi often need a second host plant to complete their life cycle, so a spruce broom may be tied to nearby plants that carry the other stage of the fungus. The fungus lives inside the branch tissue and sends signals that make the buds sprout, which is why the broom keeps coming back on the same branch. On spruce, infected needles on the broom often turn yellowish and drop in autumn, which is one clue among several pointing toward rust.
Dwarf mistletoe lives inside the branch and sends out tiny shoots, so the plant itself is easy to miss even when the broom is obvious. Its small, leafless, yellowish or brownish shoots come out of the bark around the broom, and finding them is one of the best ways to confirm it. The mistletoe forces seeds out of ripe fruits, and they land on nearby branches or trees, which is why infestations spread outward from one tree.
Eriophyid mites are too small to see without magnification, so you will usually notice their effects, such as distorted buds and leaves, rather than the mites. They feed in the buds and can trigger them to sprout in tight clumps. The mite that carries rose rosette virus travels on the wind, so the virus can reach roses far from any infected plant.
Is a Witches’ Broom Harmful to Your Tree
In some fungal infections, the broom starts green and living, then dies within one to three months to form a dry, brown broom.15ResearchGate. Time for Chocolate: Current Understanding and New Perspectives on Cacao Witches’ Broom Disease Research That dead broom can release spores after alternating wet and dry spells, spreading the fungus further.16ResearchGate. Time for Chocolate: Current Understanding and New Perspectives on Cacao Witches’ Broom Disease Research
Severity varies a lot. Dwarf mistletoe draws water and nutrients from its host conifer, and heavy infestations weaken trees over years. Even so, a mistletoe broom can stay alive and produce viable seed cones. Rose rosette virus is generally fatal to infected roses within a few years.
A single broom uses only a small share of a tree’s energy, and a vigorous tree can grow around it for years without obvious decline. The most useful question is whether the broom is changing, so photograph it now and compare a year later to see if it has grown or multiplied. Several signs suggest a bigger problem: multiple brooms appearing across the tree over a few years, thinning, yellowing or dieback in the rest of the crown, or brooms spreading to neighboring plants of the same kind.
When things worsen, mistletoe weakens a conifer gradually, with thin crowns, slow growth and branch dieback often showing before the tree dies. Weakened trees are more vulnerable to bark beetles and drought, so mistletoe can contribute to a death even when it isn’t the direct cause. A rose with rosette disease usually looks worse each season, with more of the abnormal red shoots. Catching it early matters because the plant will not recover.
Large brooms also add weight and catch snow, which can break branches. Check any brooms over driveways, roofs or paths after storms.
How to Manage Witches’ Broom in Your Yard
Pruning off the affected branch is the first thing to do, and depending on the cause it may be all you need.17Organic Plant Care LLC. What Causes Witches’ Broom In Trees? Cut back to a healthy point on the branch below the broom, not through it. The fungus or mites may live in the wood beyond the visible broom, so cutting right at its base can leave infected tissue behind. Cutting through the middle of a broom tends to make more shoots sprout from the cut.
In practice, find where healthy tissue starts below the broom and cut just outside the branch collar or at a healthy side branch. Then bag the broom and put it in the trash. Throw the cuttings away rather than composting them, so fungal spores or insects don’t return.18Organic Plant Care LLC. What Causes Witches’ Broom In Trees? A home compost pile rarely gets hot enough to kill spores or mites. If you handle a dry brown broom, bag it where it stands, since shaking it can release spores onto the ground and other plants. Check local rules before burning, because open burning of yard waste is restricted in many places.
Disinfect pruning tools after each cut to avoid moving the infection to healthy branches.19Organic Plant Care LLC. What Causes Witches’ Broom In Trees? Rubbing alcohol or a diluted bleach solution both work, and the wiping should happen after every cut on a suspect branch, not just at the end of the job. Bleach corrodes metal, so rinse and oil the tools afterward. If you are pruning several plants, do the healthy ones first and the broom-affected ones last.
Timing helps. Dry weather lowers the chance of spreading spores. Remove dead brown brooms promptly since they may hold spores, and recheck the plant each season for new ones.
Heavy broom removal can also reduce how much a plant produces, so in fruiting plants it is balanced against yield.20Crop Protection. A novel, integrated method for management of witches’ broom disease in Cacao in Bahia, Brazil For a yard tree, remove brooms as soon as you notice them, while there are only a few.
Some causes need a different approach. Roses with rose rosette should be removed whole, including roots, because pruning doesn’t cure the virus. For mistletoe brooms on conifers, prune infected branches and remove badly infested trees so they don’t seed onto neighbors. For herbicide brooms, stop or correct the weedkiller use; the plant may recover with time. Copper fungicides and biological control fungi have reduced witches’ broom in crops like cacao,21Crop Protection. A novel, integrated method for management of witches’ broom disease in Cacao in Bahia, Brazil but these sprays are farm tools and are rarely needed for a single broom on a yard tree.
When to Get a Professional Diagnosis
Phytoplasmas and viruses can’t be seen by eye, and labs confirm them with DNA tests on plant samples.22PubMed. Specific PCR and real-time PCR assays for detection and quantitation of ‘Candidatus Phytoplasma phoenicium’ Such tests have found phytoplasma in brooming shrubs but not in healthy plants of the same species.23Chilean Journal of Agricultural Research. Presence of a Phytoplasma Associated with Witches’ Broom Disease in Ugni molinae Turcz. and Gaultheria phillyreifolia (Pers.) Sleumer Determined by DAPI, PCR, and DNA Sequencing A positive result tells you the broom comes from an infection, not a harmless mutation. Brooms from phytoplasmas, viruses and mutations can look almost identical, so no amount of looking will separate them. The lab searches for the organism’s DNA, so the sample must contain living tissue from the affected part. A negative test doesn’t always rule out infection, since the organism may be unevenly spread in the plant, so ask the lab how to read one.
Local extension offices and plant diagnostic clinics can identify brooms from photos or samples. Brooms can be hard to diagnose, especially when herbicide damage mimics disease.24UF/IFAS Extension Monroe County. What is Witch’s Broom – UF/IFAS Extension Monroe County
Start with clear photos of the whole plant and the broom up close. Cut a piece that includes the broom and some healthy-looking tissue next to it, since the lab compares the two. Seal it in a plastic bag, keep it cool but not frozen, and get it to the clinic quickly because fresh tissue gives better results. Write down the plant species, when you first noticed the broom, and any recent weedkiller use, which helps narrow down the cause.
Large trees, brooms high in the crown and brooms over structures call for an arborist, who can do the pruning safely. An arborist can also judge whether a mistletoe-infested tree should come down.
Brooms That Can Be Left Alone or Even Prized
Witches’ brooms provide nesting places for birds and mammals such as the northern flying squirrel.25PubMed. Witch’s broom Squirrels in pine forests use mistletoe brooms for nests and food storage.26ResearchGate. Use of Witches’ Brooms by Abert Squirrels in Ponderosa Pine Forests The dense twigs make a sheltered spot out of wind and rain. Leaving a broom on a tree far from your house costs little and can give wildlife a refuge. If the cause is a spreading one such as mistletoe, weigh the wildlife value against the risk to nearby conifers.
Keep only brooms on otherwise healthy trees with no signs of spreading. A broom on a conifer that stays green and unchanged for years may be a mutation, though only a lab test can rule out infection.
Mutation brooms are the source of most ornamental dwarf conifers with dense branching.27New Forests. Cone structure and seed development in grafted witches’ broom and normal crown clones from the same trees of Pinus sibirica A mutation broom grows slowly and compactly because its buds behave differently, and that is exactly what makes dwarf conifers appealing. Growers graft cuttings from the broom onto normal rootstock to make new compact varieties, and the cuttings keep the compact habit, which is why they are propagated rather than grown from seed. Many dwarf spruces and pines at nurseries began as a broom someone noticed on an ordinary tree.
To try it yourself, take cuttings or scion wood in late winter and graft onto a seedling of the same species. Expect slow growth and a compact shape.
Witches’ Broom Beyond the Backyard
A fungal witches’ broom disease of cacao, the chocolate plant, is among the most notorious forms.28Great Lakes Phragmites Collaborative. Witches’ Broom Disease: How to Identify and Manage It After it reached Brazil’s main cocoa region, production fell by 80% over a decade and about 250,000 workers lost jobs.29VoxDev. A blight on growth: The generational fallout of Brazil’s cocoa collapse In parts of Ecuador, cacao diseases including witches’ broom cost growers over a third of their 2023 harvest.30Frontiers in Sustainable Food Systems. Underestimation of the economic impact of moniliasis and witches’ broom on Theobroma cacao L.
Farms fight back with breeding: researchers are developing cacao varieties with natural resistance to the disease.31Plant Breeding. Sources of resistance to witches’ broom disease in cacao (Theobroma cacao L.): Progress update and perspectives An antibiotic injection restored normal growth in most mildly infected jujube trees, but this is a specialist farm treatment.32PubMed Central. Influences of Jujube Witches’ Broom (JWB) Phytoplasma Infection and Oxytetracycline Hydrochloride Treatment on the Gene Expression Profiling in Jujube Asking a nursery about disease resistance is an easy way to apply the resistant-variety idea at home.33PubMed Central. Assessing the Impact of Climate Change on the Distribution of Lime (16srii-B) and Alfalfa (16srii-D) Phytoplasma Disease Using MaxEnt