If you notice weird, unsightly growths on any of your desert landscape plants, it might be witches’ broom. Here’s a look at the underlying causes of witches’ broom, how to prevent it, and what you should do if you see it on your plants.
“Witches’ broom” is not one disease or organism. The term is an umbrella phrase used to describe a cluster of symptoms that can have a wide variety of underlying causes.
Witches’ broom manifests differently depending on the plant. In trees, there can be large clusters that look like mistletoe while smaller plants exhibit abnormal growth characterized by tight clusters of stunted shoots or leaves.
Causes of Witches’ Broom
There are several known underlying causes of witches’ broom. All are naturally occurring, except for herbicide damage. All cause a plant’s own cells to grow abnormally.
- bacterial infection
- fungal infection
- viral infection
- phytoplasma (parasitic bacteria)
- dwarf mistletoe
- herbicide damage
- genetic mutation
- zinc deficiency
Insects such as aphids and mites are believed to be vectors that
introduce witches’ broom to the host plant, but are not the direct cause.
The exact cause of witches’ broom is unknown in most plants, but there are some exceptions. Here’s a look at a few known causes in popular desert landscape plants.
Witches’ Broom in Roses
Witches’ broom in roses is known to be caused by a virus (Emaravirus spp.) spread by eriophyid mites. This disease is called rose rosette disease. The first sign is the appearance of red pigmentation on leaves.
Witches’ Broom in Palo Verdes
It was recently discovered that witches’ broom in palo verdes is also caused by an Emaravirus virus spread by eriophyid mites. This disease, called palo verde broom virus (PVBV), is primarily found in blue palo verdes (Parkinsonia florida). But symptoms have also been observed in Foothills palo verde (Parkinsonia microphylla) and Mexican palo verde (Parkinsonia aculeata).
Witches’ Broom in Lantana
Witches’ broom in lantana can be caused by lantana gall mite (Aceria lantanae). These microscopic mites cause lantanas to produce broccoli-looking vegetative galls instead of flowers.
Other Known Witches’ Broom Causes
Here are some other known witches’ broom causes. Some of these are grown in the desert Southwest.
| Witches’ Broom Cause | Plants Affected |
| dwarf mistletoe | conifers |
| fungal diseases | cedar, cherry, cypress, juniper, pine, & oak trees |
| nematodes | wheat |
| phytoplasma diseases | ash, elm, citrus, papaya, peach, apple, & nut trees |
Desert Plants Susceptible to Witches’ Broom
I only learned about witches’ broom relatively recently, and it might be new to you, too. But it’s not new — it’s a global problem that’s been observed for hundreds of years.
It largely affects trees and woody shrubs, but it can occur in just about any kind of plant. Many economically important plants can be affected by witches’ broom including blueberries, strawberries, fruit & nut trees, alfalfa, peanuts, potatoes, tomatoes, cacao, and wheat.
Plants that grow in the Southwest that are known to get witches’ broom include:
Trees
- Arizona ash
- citrus (especially limes)
- cypress
- desert willow
- juniper
- mulberry
- oak
- olive
- palo verde (especially blue palo verde)
- pecan
- pine
Shrubs
- bottlebrush
- bougainvillea
- crape myrtle
- hackberry
- hibiscus
- lantana
- manzanita
- oleander
- pyracantha
- rose
- yellow bells
- Texas ranger
Witches’ broom can also affect cactus.
Witches’ Broom Treatment
Currently, there is no cure, but scientists are working on it. It’s not a straightforward problem for many reasons. First, it’s not one disease but many diseases with multiple causes. Additionally, the problem goes deep.
Purdue University Department of Agriculture explains it like this:
Little phytoplasma in something as big as a tree might not seem like a pressing issue, but these phytoplasma do more than colonize. They’re actually able to force a little bit of their genetics into the plants’ genome and control its growth and metabolism.
If you have a plant with witches’ broom, the primary treatment is to prune off affected growth. It’s recommended that you prune away all visibly affected areas plus an additional 6 inches (if possible) to be sure you’ve got it all.
Then, keep an eye on the plant and repeat as needed. Some plants recover after one pruning while some take several prunings over the course of months or even years. Some never recover.
Removing affected areas will not only lessen the burden on the affected plant, it should also reduce the number of insects spreading the disease, thus reducing the risk to nearby plants
Severely infected plants should be removed. In the case of roses, the entire plant, including the roots, should be removed, bagged, and discarded.
For plants where mites are known vectors (i.e., rose, palo verdes, lantana), a secondary treatment to consider is spraying an affected plant with a miticide. This may be feasible for roses or lantanas, but not for large palo verdes.
Mite populations tend to surge in hot, dry weather, but this is the worst time to spray a miticide. Plants are already stressed by heat and drought, and spraying any chemical can add further stress. Even natural miticides such as neem oil should not be used when the temperature exceeds 85°F, since it can burn leaves.
How to Prevent Witches’ Broom
There are two things you can do to dramatically decrease the chance your plants will get witches’ broom.
Stop Using Herbicides
The first thing you can do is to stop using herbicides. Herbicide damage is the one cause of witches’ broom that is completely avoidable. Unfortunately, many gardeners who don’t use herbicides have had their plants damaged by herbicide drift from neighbors who do.
For more on herbicide damage, I recommend this article by Clemson Cooperative Extension. It discusses the symptoms of damage caused by specific herbicides, how to minimize the risk of damage, and what to do if herbicide overspray drifts onto your plants.
Prune with Disinfected Tools
The second thing you can do to prevent witches’ broom is to make sure your plants are pruned with disinfected tools.
Landscapers unknowingly prune an infected plant and proceed to spread it from plant to plant and even from customer to customer. I’ve heard of some heartbreaking incidents where virtually every plant in a yard was infected this way.
You can insist that your landscapers disinfect their pruners before working in your yard or that they use your disinfected tools. They should disinfect their tools when done with one plant before working on the next. If you don’t trust that your landscapers will follow through on this, consider supervising them while they work. Or you may find it easier to prune your own plants.
How to Disinfect Garden Tools
Here are a few proven ways to effectively disinfect garden tools:
- 10% chlorine bleach solution
- household disinfectants or disinfectant wipes
- isopropyl alcohol
- 10% trisodium phosphate solution
I keep a dispenser of disinfectant wipes in my mud room to disinfect my tools. Bleach is not my top pick. It’s highly corrosive and not kind to your tools.
What the Pros Use
A Facebook discussion among Arizona arborists revealed that they use alcohol, bleach, Lysol spray, Simple Green, or Simple Green Pro Disinfectant. My arborist uses a can of spray disinfectant and sprays her tools when moving from one tree to the next.
If only all landscapers were this conscientious, we’d hear a lot less about witches’ broom on desert landscape plants.
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Author Bio
Deane Alban is the creator of Southwest Gardener. She is a science writer with a bachelor’s degree in botany from the University of South Florida. Gardening is her lifelong passion. She’s been gardening in Tucson for over 16 years.
