Witches’ Broom Disease: How to Identify and Manage It

Witches’ broom disease shows up as a dense, abnormal cluster of shoots sprouting from a single point on a branch, giving the plant a broom-like tuft that looks nothing like normal growth. It can affect dozens of plant species, from cacao and citrus to cassava and ornamental trees, and it is caused by very different pathogens depending on the host. Identifying the characteristic shoot proliferation is straightforward once you know what to look for, but managing the disease is another matter entirely, because the underlying cause varies and no single strategy works across all affected plants.

What Witches’ Broom Actually Looks Like

The hallmark symptom is a burst of thin, densely packed shoots growing from a single point on a stem or branch. Instead of one healthy branch extending outward, the plant pushes out many small, often stunted shoots that fan out in a tight cluster. The result genuinely resembles an old straw broom, which is where the name comes from. In trees, you often notice it first in winter after the leaves have dropped, because the bare broom-like clusters stand out starkly against the silhouette of normal branching.

Beyond the obvious shoot proliferation, infected plants commonly show a range of other symptoms. Leaves on broom growths tend to be smaller than normal and may yellow or curl. Internodes (the gaps between leaves along a stem) shorten dramatically, contributing to the bunchy, compressed look. In some hosts, flowers become deformed: petals turn leaf-like (a condition called phyllody) or develop abnormal green coloring where you would expect normal flower pigment. Stunted growth across the whole plant, twig dieback, and unseasonal color changes in the foliage are also common signs.¹ In cassava, for instance, the disease causes small-leaf proliferation and vascular damage inside the stems.²

Not every abnormal cluster of growth is witches’ broom disease. Aphid infestations, mistletoe, herbicide drift, and certain mite species can all cause dense, bushy growth that might look similar at a glance. The key differences: witches’ broom caused by pathogens usually involves shoots that are thinner and more uniform than normal, with reduced leaf size and progressive dieback of the cluster over time. Mistletoe is visibly a different plant growing on the host. Aphid damage tends to create curled, distorted leaves at branch tips rather than entirely new shoots. If you are unsure, look for whether the shoots originate from one swollen point and whether the plant shows systemic symptoms like yellowing elsewhere.

What Causes It

Witches’ broom is not one disease but a syndrome that different pathogens can trigger. The two main culprits are phytoplasmas and fungi, and which one is responsible depends heavily on the plant species involved.

Phytoplasmas are tiny, cell-wall-less bacteria that live inside a plant’s phloem, the tissue that transports sugars. They cannot be grown in a lab dish, which makes them notoriously difficult to study and diagnose. Phytoplasma-caused witches’ broom affects a remarkably wide range of hosts. In the Middle East alone, peanut witches’ broom phytoplasma strains have been found in over 80 host plant species, and diseases like almond witches’ broom, lime witches’ broom, and alfalfa witches’ broom rank among the most destructive in the region.³ Phytoplasma infections also cause witches’ broom in paulownia trees, hemp, sunnhemp, and many other plants worldwide.

On the fungal side, the most economically devastating example is Moniliophthora perniciosa, the fungus behind witches’ broom disease of cacao. This pathogen enters through young, actively growing tissues and manipulates the plant’s own hormone signaling. Specifically, it produces a plant hormone called isopentenyl adenine that pushes the host toward abnormal growth patterns, including the outgrowth of side shoots, swelling of stems, and even changes in fruit structure.⁴ The fungus essentially hijacks the plant’s growth program and redirects it to produce the dense broom structures that help the pathogen complete its life cycle.

Interestingly, for some plants the causal agent remains genuinely uncertain. Cassava witches’ broom disease was long assumed to be caused by phytoplasmas, but recent molecular work in the Philippines found very low or absent phytoplasma levels in symptomatic plants and instead detected a surprisingly high abundance of a fungus, Ceratobasidium, raising the possibility that a different pathogen may be responsible in some cases.² This is a reminder that “witches’ broom” describes a set of symptoms, not a single infection, and pinning down the actual pathogen in a given plant requires lab testing.

How It Spreads

Phytoplasma-type witches’ broom spreads through insect vectors, mainly leafhoppers and psyllids. These sap-sucking insects pick up the pathogen while feeding on an infected plant’s phloem, then carry it to healthy plants where it enters through feeding wounds. What makes this particularly insidious is that infected plants actively attract more vectors. Research on lime witches’ broom showed that the broom growths themselves draw in leafhoppers, creating a feedback loop: the disease makes the plant more appealing to the insects that spread the disease further.⁵

Separate research on phytoplasma effector proteins found that the attraction of leafhoppers to infected plants occurs independently of the visible flower deformities the disease causes. The pathogen produces a protein that makes the plant more appealing to the insect vector even before obvious symptoms develop, suggesting this is an evolved manipulation rather than just a side effect of abnormal growth.⁶ In practical terms, this means an infected plant in your garden or orchard is not just sick; it is a beacon drawing in the very insects that will spread the problem.

Fungal witches’ broom, such as the cacao disease, spreads differently. Moniliophthora perniciosa produces spores on the dried, dead broom structures that remain on the tree. When rain and humidity hit these dried brooms, they release basidiospores that are carried by wind or water to new growth points on nearby trees. This is why dead brooms left hanging on the tree are such a problem: they become a permanent spore factory.

Why It Matters Beyond Your Garden

Witches’ broom disease has caused some of the worst agricultural disasters in tropical farming. The most dramatic example is what happened to cacao production in the state of Bahia, Brazil. The fungal pathogen arrived in the region in 1989, and the consequences were staggering. Harvested cacao area fell by roughly 30%, total production dropped by about 65%, and yield per hectare was cut in half.⁷ The collapse was not just an agricultural problem; it was a social catastrophe. Planters desperate to avoid bankruptcy laid off hundreds of thousands of rural workers and resorted to selling off surviving tropical hardwood trees to stay solvent.⁸ An entire regional economy built around cacao effectively collapsed within a few years.

In the Middle East, lime witches’ broom has devastated acid-lime orchards across Oman, the United Arab Emirates, and Iran. Almond witches’ broom has similarly threatened almond production in parts of the region.³ These are not niche problems. When a witches’ broom pathogen establishes itself in a major crop region, the combination of difficult diagnosis, insect-mediated spread, and lack of effective cures can dismantle an industry.

How to Get a Proper Diagnosis

Visual identification can tell you that something looks like witches’ broom, but it cannot tell you which pathogen is causing it, and that matters for choosing the right management approach. The standard laboratory method for phytoplasma detection involves a technique called nested PCR, which amplifies a specific section of the pathogen’s DNA so it can be identified. In the case of hemp witches’ broom, for example, PCR targeting the pathogen’s ribosomal RNA gene confirmed the presence of a phytoplasma species and allowed researchers to match it to a known group.⁹

For home gardeners and small-scale growers, sending samples to a university plant disease diagnostic lab is the practical path. Most state cooperative extension services in the United States offer this through land-grant universities, and the cost is usually modest. When collecting a sample, include both symptomatic tissue (a section of the broom growth) and apparently healthy tissue from the same plant for comparison. Keep samples cool and get them to the lab quickly, since phytoplasmas degrade fast in dead tissue.

One complication worth knowing about: phytoplasmas can be unevenly distributed within a plant. Cassava witches’ broom research found that PCR detection rates from symptomatic tissue were surprisingly low, possibly because the pathogen exists at low concentrations or is patchy in its distribution through the plant.² A negative PCR result does not always mean the plant is clean. If symptoms strongly suggest witches’ broom and the first test comes back negative, a second sample from a different part of the plant may be warranted.

Management Strategies That Actually Work

There is no cure for witches’ broom disease in the way that an antibiotic cures a bacterial infection. Once a plant is systemically infected, especially with a phytoplasma, the pathogen is inside the plant’s vascular system and cannot be removed. Management is therefore about slowing spread, reducing damage, and where possible, replanting with resistant material.

Pruning and Sanitation

For both fungal and phytoplasma-type witches’ broom, removing infected material is a basic first step. In cacao, this means cutting off the broom growths and removing them from the field, since dried brooms left on trees or the ground produce spores for months. However, research in Brazil has shown that while phytosanitary broom removal reduces disease severity, it does not necessarily improve yields on its own. In fact, total pod production per tree was consistently low whenever broom removal was used as the sole strategy.¹⁰ The effort of constantly pruning seems to stress the tree without giving it enough advantage to outproduce the disease.

For ornamental trees and backyard fruit trees with phytoplasma infections, pruning out broom growths can improve the tree’s appearance and remove tissue that attracts insect vectors, but it is unlikely to save a heavily infected tree. In those cases, removing the entire tree to prevent spread to neighbors may be the better choice, particularly if the species is a valued crop like citrus or almond.

Biological and Chemical Control

The most promising results from cacao research have come from integrated approaches. A field study in Bahia found that alternating applications of copper hydroxide fungicide with a biocontrol fungus called Trichoderma stromaticum produced higher pod yields and lower disease losses than any single method used alone. The biocontrol fungus on its own did not reduce witches’ broom on pods, but it did suppress the formation of new vegetative brooms and increased the number of flowering sites on the tree.¹⁰ Importantly, this integrated approach eliminated the need for the expensive labor of manual broom removal while delivering better net economic returns.

The performance of Trichoderma stromaticum as a biocontrol agent depends on local environmental conditions. In shaded cacao plantations, canopy microclimate factors like humidity and temperature influence how effectively the biocontrol fungus colonizes and suppresses the pathogen.¹¹ This is a practical concern for growers: a biocontrol product that works well in one farm’s conditions may underperform in a plantation with different shade cover or drainage.

For phytoplasma-caused witches’ broom, antibiotics like tetracycline have been used experimentally and can suppress symptoms temporarily, but they do not eliminate the pathogen and are generally not practical or approved for field use in most countries. Insecticide applications targeting leafhopper vectors can slow the spread of phytoplasma diseases, but this is a preventive measure, not a treatment for already-infected plants.

Resistant Varieties

Breeding for resistance is the longest-term and most sustainable strategy. In cacao, researchers have identified genetic regions associated with witches’ broom resistance across multiple chromosomes, with a major resistance locus on chromosome IX traced back to a cacao clone called Scavina 6, which has long been recognized as a key source of disease resistance.¹² Breeding programs are working to combine this resistance with other desirable traits like flavor and yield.¹³

Some resistant cacao genotypes show both structural and biochemical defenses. One genotype studied for pod resistance exhibited delayed fungal colonization compared with a susceptible variety, along with higher activity of defense enzymes during the first 48 hours after inoculation.¹⁴ The plant essentially slows the fungus down at the gate, buying time for its immune response to kick in. For growers in affected regions, choosing resistant rootstock or cultivars, when available, is the single most impactful decision they can make.

Practical Advice for Home Gardeners

If you spot a broom-like growth on a tree in your yard, the first question is whether it matters. On many ornamental trees, a single witches’ broom is cosmetically unappealing but not life-threatening to the tree. Birches, hackberries, and honeysuckles commonly develop brooms from various causes, and the tree can live with them for decades. If the tree is otherwise healthy and the brooms are few, pruning them out during the dormant season is usually sufficient.

The calculus changes if you are growing fruit or nut trees, particularly citrus, stone fruit, or anything in a region where phytoplasma diseases are active. In that case, a broom growth warrants faster action. Prune it out, cutting well below the base of the abnormal growth into clean wood, and disinfect your tools between cuts. Monitor the tree over the following season. If new brooms appear, especially on distant branches, the infection is likely systemic and the tree should be removed before it becomes a source of infection for neighbors.

Vector management matters if you are in an area where phytoplasma-type witches’ broom is known. Leafhoppers are the primary spreaders, and anything that reduces their population near susceptible plants helps. Row covers on smaller plants, sticky traps for monitoring, and targeted insecticide applications during peak leafhopper season are all options. Eliminating nearby weed hosts that may harbor the phytoplasma is also worthwhile since many common weeds serve as reservoirs.

Plants Commonly Affected

The range of species susceptible to some form of witches’ broom is broader than most people expect. In agricultural crops, cacao, lime, almond, peanut, cassava, alfalfa, and hemp have all been documented with the disease.³⁹ Paulownia trees, valued for their fast-growing timber, face a phytoplasma-type witches’ broom that causes branch and shoot proliferation severe enough to kill young trees.¹⁵

Among ornamentals, birch trees are probably the most familiar hosts in temperate climates. The brooms are caused by a fungus rather than a phytoplasma and are mostly a cosmetic issue. Willows, elms, and various conifers including spruce and pine can also develop broom growths from fungal, viral, or mite-related causes. In conifers, witches’ brooms are occasionally prized rather than feared: some dwarf conifer cultivars sold in garden centers were originally propagated from witches’ broom mutations that produced compact, attractive growth forms. The abnormal growth that is a disease symptom in one context became a desirable trait in another.

When Witches’ Broom Is Not a Disease at All

It is worth knowing that not all witches’ brooms are caused by pathogens. Some result from random genetic mutations in a single bud, producing a branch with different growth characteristics from the rest of the tree. These are called sport mutations, and they are completely harmless to the tree. Others are caused by eriophyid mites, which are microscopic and feed on developing buds in ways that trigger dense shoot formation. Mite-caused brooms are common on willows and some fruit trees and can be managed with dormant oil sprays if they become excessive.

Environmental stress can also mimic the appearance. Herbicide drift, particularly from growth-regulator herbicides used on lawns, can cause distorted, bushy regrowth on nearby trees that resembles witches’ broom. The giveaway in this case is that the symptoms tend to affect the side of the tree facing the herbicide source and often improve as the plant metabolizes the chemical over time. If every tree on one side of your property suddenly develops odd bushy growths, think about what was sprayed upwind before assuming disease.