If you notice weird, unsightly growths on any of your desert landscape plants, it might have witches’ broom. Here’s a look at what witches’ broom is and what should you 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
Here’s a list of the known underlying causes. All are naturally occurring, except for herbicide damage. And all cause a plant’s own cells to grow abnormally.
- bacterial infection
- fungal infection
- viral infection
- phytoplasma (parasitic bacteria)
- insects (aphids, mites)
- dwarf mistletoe
- herbicide damage
- genetic mutation
- zinc deficiency
The exact cause of witches’ broom is unknown in many plants, but there are a few exceptions.
Witches’ broom in roses is known to be caused by a virus (Emaravirus sp.) spread by a mite. This disease even has its own name — rose rosette disease. The first sign is the appearance of red pigmentation on plant leaves.
Witches’ broom in conifers is often caused by dwarf mistletoe (Arceuthobium spp.), a parasitic plant.
And witches’ broom in lantana can be caused by lantana gall mite (Aceria lantanae). These microscopic mites which occur only on lantana cause the plant to produce vegetative galls instead of flowers.
Insects are now believed to be vectors that can introduce witches’ broom to the host plant, but are not the direct cause.
Here are more known witches’ broom causes.
| 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 |
| viral diseases | rose, blue palo verde |
Many economically important plants can be affected by witches’ broom including
blueberries, strawberries, fruit & nut trees, alfalfa, peanuts, potatoes, tomatoes, cacao, and wheat.
Desert Plants Susceptible to Witches’ Broom
I only learned about witches’ broom relatively recently, and might be new to you, too. But it’s not new.
This worldwide problem has been observed for hundreds of years. It largely affects trees and woody shrubs, but it can occur in just about any kind of plant.
Plants grown 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
Scientists are working on a cure, but currently, there is none.
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.
According to Purdue University Department of Agriculture:
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.
The only thing to do if you see it on your plants is to prune it off. It’s recommended that you prune away all the affected growth plus an additional 6 inches to be sure you’ve got it all. If you look at a cut stem and see obvious growths, cut it back further. Do not add the cuttings to your compost pile! Wrap them in a plastic bag and discard.
Severely infected plants should be removed. In the case of roses, the entire plant including the roots should be removed and discarded.
How to Prevent Witches’ Broom
There are two things you can do to dramatically decrease the chance your plants will get witches’ broom.
The first thing you can do is to avoid using herbicides. This is a cause of witches’ broom that is completely avoidable. And if you absolutely must spray herbicides, pick a calm day so it doesn’t drift onto your plants.
The second thing you can do to prevent witches’ broom is to use clean tools to prune your plants. It’s highly suspected that it gets spread from plant to plant and from garden to garden by landscapers’ 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 insist they use your previously-disinfected tools. They should also disinfect their tools when done with one plant before working on to the next. If you don’t trust that your landscapers will follow through on this, consider supervising them while they work.
I find it easier to just prune my own plants. Except for trees. Fortunately, I have a good arborist who does this as a matter of course.
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 clean my tools. Bleach is not my top pick. It is highly corrosive and not kind to your tools. My arborist uses a can of spray disinfectant and sprays her tools before each cut.
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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 15 years.
