Rot on a cactus is a soft, dark patch that grows. These steps are for a cactus that has roots, in a pot or in the ground.
The short version
- Rot is soft, dark and often wet, and the patch gets larger. Brown tissue that is firm and dry is something else.
- Cut it out the day you find it.
- Clean the blade first, and again before each new cut.
- Cut until the exposed tissue is firm and one even color, then take one more thin slice.
- A small spot on the side or a rotted tip is cut away with the plant left in place. Rot at the base means cutting the healthy top off and rooting it again as a cutting (a piece of stem with no roots).
- Keep every cut dry and open to the air until it has sealed over.
- A plant is past saving when no firm, clean tissue is left.
Clean the blade first
Use a sharp knife. Wipe off any sap and soil, because disinfectant does not work through them.
Wipe the blade with 70% isopropyl alcohol so it stays wet for at least 30 seconds. Let it air dry.
Clean the blade again before every new cut, so it does not carry rot into healthy tissue. Wear cactus gloves, or hold the stem with a folded towel.
Find where the rot is
Press the stem between the spines with a gloved fingertip or the blunt end of a pencil, working up from the soil. Rot feels soft.
- On the side. A soft, dark spot, often with a wet-looking edge, that is larger each time you look.
- At the tip. The top of the stem is dark and soft.
- At the base. The stem is dark and soft at the soil line.
- In the roots. The stem is wrinkled or shrunken although the soil is still damp, and the base is still firm. To check a pot, push a wooden skewer to the bottom. It comes out dark and damp if the soil is wet.
Cutting it out
A small spot on the side. Cut or scoop out everything dark or soft until the hollow is lined with firm, pale tissue, then shave off one more thin layer. Leave the hollow open and uncovered. If the rot goes more than halfway through the stem, or runs more than halfway around it, cut the top off as described under rot at the base.
Rot at the tip. Make one cut through the stem below the rot, at a slant of about 45 degrees. The slant protects the rooted stump. If any of the cut face on the stump is dark or soft, clean the blade and cut lower. The stump stays where it is, in its pot or in the ground.
Rot at the base, plant in a pot. Take the plant out of the pot and lay it on its side on a folded towel. Cut square across the stem above the highest soft tissue. Look at the cut face of the top piece. If any of it is dark or soft, clean the blade and cut again higher. When the face is firm and one even color, take one more thin slice. The top is now a cutting.
Rot at the base, plant in the ground. Dig up a small plant and handle it like a potted one. Cut a tall plant where it stands: have a second person steady the top with a folded towel, cut square across above the highest soft tissue, and lay the top down on something soft. Check the cut face, cut higher if any of it is dark or soft, then take one more thin slice. Dig out the rotted base and its roots.
Rot in the roots, plant in a pot. Take the plant out and brush the soil from the roots. Trim off every root that is dark, soft or hollow, cutting back to firm root. If the base of the stem turns out to be soft or dark, handle it as rot at the base.
Rot in the roots, plant in the ground. Stop watering and keep runoff away from the plant. The plant sometimes recovers once the soil dries out. If the base turns soft, cut the top off as for rot at the base.
Bag everything rotted and put it in the trash.
After the cut
Every cut has to callus, meaning dry and seal over into a firm layer.
A hollow or a stump on a plant left in place. Keep water off the cut until it is dry and hard. Move a pot under cover. For a plant in the ground, hold off watering and keep sprinklers off it.
A cut top. Leave it in a dry, airy place with the cut face in the open air, not resting on anything. A thick stem can take from a couple of weeks to several months to callus. It is ready when the whole face is dry and hard. If a soft, dark area appears, cut again. Then set it in fresh, dry cactus soil, deep enough to stand. Give an unrooted cutting no water until it has rooted. Rooting takes 4 to 8 weeks. Roots form when nights are about 60 F or warmer, so in colder weather keep the piece dry and somewhere warmer than that. The steps are in how to root a cactus cutting.
A potted plant with trimmed roots. Leave it out of the soil until the cut roots are dry to the touch. Pot it in fresh, dry cactus soil at the depth it grew at before, and wait a week before watering.
Throw away the soil from a pot that held a rotted plant. Wash the pot with soap and water and let it air dry. Plant a top cut from a plant in the ground in a new spot or in a pot, because the organisms that cause rot can stay in the soil.
A cutting that rots before it has rooted is a different case, covered in why cactus cuttings rot.
When it cannot be saved
A plant is past saving when no firm, clean tissue is left. Once rot has spread through the stem at the base, the lower plant is generally lost, but any firm, clean piece above it can be rooted as a cutting.
If you are unsure what you are seeing, email us with photos at support@thoughtfulpricks.com.
Powders and treatments
Some growers dust a fresh cut with sulfur powder. This step helps but is not essential. What's most important is keeping the cut dry, in moving air.
Cinnamon and hydrogen peroxide are not established treatments for cactus rot, and neither takes the place of cutting.
A fungicide does not repair rotted tissue. It may help keep some fungi from infecting healthy tissue.
How rot gets in
Rot is caused by bacteria and fungi. The affected part turns dark brown or black and soft, and it may leak brown fluid and smell foul. Keeping the plant dry may slow rot, but it often keeps spreading, and it can run farther inside the stem than the patch on the skin shows.
Rot usually enters through a wound: a cut, a scrape, a puncture, a patch killed by frost or sunburn, or a root broken in repotting. Unbroken skin can be infected too when it stays wet. Wet soil is most likely to cause rot in cool weather, when the plant is not growing.
Corking is firm, dry, brown skin on the surface of the stem, and it is not rot. Corking, sunburn and rot covers how to tell the 3 apart.
Preventing the next one
- Let the soil dry between waterings. See watering cactus.
- Stop watering when nights drop below about 60 F, and start again when they are back above it.
- Use soil that drains fast. See cactus soil.
- Plant at the depth the stem grew at before, and never leave water pooled against the base.
Sources
Problems and Pests of Agave, Aloe, Cactus and Yucca. Jack Kelly and Mary W. Olsen, University of Arizona Cooperative Extension, revised 2011. https://extension.arizona.edu/sites/extension.arizona.edu/files/pubs/az1399.pdf
Saguaro Horticulture: Saguaro Problems, Pests And Disease. Michael Chamberland, University of Arizona Cooperative Extension, 2021. https://extension.arizona.edu/sites/extension.arizona.edu/files/pubs/az1962-2021.pdf
Bacterial Necrosis of Saguaro Cacti. Arizona Department of Agriculture. https://agriculture.az.gov/sites/default/files/Saguaro%20Bacterial%20Necrosis%20-%20AZ%20Dept%20of%20Ag.pdf
Cactus (Cactaceae). Plant Pest Handbook, The Connecticut Agricultural Experiment Station. https://portal.ct.gov/CAES/Plant-Pest-Handbook/pphC/Cactus-Cactaceae
Cacti and Succulents. Texas Plant Disease Handbook, Texas A&M AgriLife Extension Service. https://plantdiseasehandbook.tamu.edu/landscaping/flowers/cacti-and-succulents/
Bacterial Soft Rot of Vegetables, Fruits, and Ornamentals. Report on Plant Disease No. 943, Department of Crop Sciences, University of Illinois at Urbana-Champaign, 1990. https://ipm.illinois.edu/diseases/rpds/943.pdf
Cactus, Agave, Yucca and Ocotillo. Jack Kelly and Rob Grumbles, University of Arizona Cooperative Extension, 2009. https://extension.arizona.edu/sites/extension.arizona.edu/files/pubs/az1225.pdf
How to Propagate Agaves and Cacti from Cuttings and Seed. Jack Kelly, University of Arizona Cooperative Extension, 2009. https://extension.arizona.edu/sites/default/files/2025-03/How-to-Propagate-Agaves-and-Cacti-From-Cuttings-and-Seed.pdf
How to Transplant a Cactus. Michael Chamberland and Jack Kelly, University of Arizona Cooperative Extension, 2020. https://extension.arizona.edu/publication/how-transplant-cactus
Common Problems and Issues of Succulents. Aaron Steil, Iowa State University Extension and Outreach, 2023. https://yardandgarden.extension.iastate.edu/how-to/common-problems-and-issues-succulents
Clean and disinfect gardening tools and containers. University of Minnesota Extension. https://extension.umn.edu/planting-and-growing-guides/clean-and-disinfect-gardening-tools
Disinfecting Tools, Equipment, Pots, Flats and Benches. Penn State College of Agricultural Sciences, 2016. https://ccfruitandnuts.ucanr.edu/sites/default/files/2018-12/295410.pdf