Many marks on a cactus are harmless. A few need action as soon as you find them. Two checks sort one from the other: how the spot feels, and whether it is changing.
The short version
- Press the spot. Firm and dry generally points to something harmless. Soft, dark and wet is rot.
- Measure a firm mark and look again over the next few days. A mark that has not grown or softened can wait. One that is growing cannot.
- Rot is cut out as soon as you find it, down to clean, firm flesh. Before you cut, read the method in cactus rot.
- Brown, hard and bark-like at the base of the stem is often corking, which comes with age.
- Yellow or brown on the sunniest side is sunburn. Give light shade. The scar stays and the plant usually lives.
- A wrinkled plant in dry soil is thirsty, so water it. A wrinkled plant in soil that has stayed wet has lost roots, so stop watering and take it out of the pot.
- Growth at the top that is narrower than the stem below it means too little light. Move the plant to brighter light in steps.
- White cottony tufts, small shells that pry off, and fine webbing with tiny brown dots are pests. Move the plant away from your others, then remove them.
- If the plant leans, rocks or collapses, press the stem at the soil line. Soft there is rot at the base.
Press it, then watch it
Use a gloved fingertip or the eraser end of a pencil. Press the mark gently, then press a healthy green part of the same stem and compare. A mark as hard as the healthy part, or harder, is firm. A mark that sinks or gives way is soft.
For a firm mark, measure it with a ruler and write the size and the date down. Check it every day or two for a week. If it has grown, softened or started to leak liquid, treat it as a soft spot.
Find your symptom
Brown patches
Press the patch first.
- Hard, dry and bark-like, on the lowest and oldest part of the stem: often corking. On an older plant the green skin low on the stem slowly turns into a dry, corky surface, much as bark forms on a tree. It is not caused by disease or pests, and it generally needs no treatment. Corking on young growth higher up can be a sign of poor growing conditions, such as too little light or water.
- Irregular corky brown spots or patches scattered over the stem: corky scab. In most cases the spots stay in the skin. Adjusting light and humidity can prevent new ones.
- Brown and dry on the sunniest side only: a healed sunburn.
- Black on the tips or other exposed parts after a freeze: frost damage. After a few weeks the black area dries and turns crisp. If any part turns soft and wet, treat it as a soft spot. Protection is covered in cactus winter care.
How to tell these apart in more detail: corking, sunburn or rot.
Soft spots
The sign is a small spot that looks water-soaked, then grows and turns black, slimy or wet. It may crack and ooze brown liquid. Pressing confirms it: the spot gives way.
This is rot. Stop watering and keep water from splashing on the spot. Rot caught while the spot is small can be cut out, down to fresh flesh with no discoloration, and the wound left open to the air to dry. Once rot is in the base of the main stem, the rooted plant is unlikely to recover. The cutting method, blade cleaning and what can still be saved are in cactus rot.
Wrinkling or shrinking
Push a finger or a wooden stick 2 to 3 inches into the soil.
- Dry: the plant is thirsty. Water it thoroughly.
- Wet, and you have not watered in the last few days: the roots have probably died from staying wet, so the plant cannot take water up. Do not water. Slide the plant out of the pot and look at the roots and the base of the stem.
What to do in each case: overwatered or underwatered cactus.
A cutting that has not grown roots yet is a separate case. Give an unrooted cutting no water until it has rooted. Rooting takes 4 to 8 weeks. See how to root a cactus cutting.
Stretching
The sign is new growth at the top of the stem that is narrower than the stem below it. The check is where it has been growing: indoors or in shade, with too little light. Move it to brighter light a step at a time over a period of weeks, because a plant moved straight into full sun can burn. The steps are in cactus etiolation.
Pale or yellow patches
Yellowing on the sunniest side of the plant is sunburn. It often follows a move into stronger sun. The skin turns yellow, then straw-colored, then brown. Give the plant light shade now, and bring it back to stronger sun gradually over a period of weeks. The plant usually survives and the scar is permanent. See corking, sunburn or rot.
White fluff, bumps and speckling
These 3 signs are pests. Move the plant away from your other plants first, and check the plants that stood next to it.
- White cottony tufts among the spines or where stems join are mealybugs, soft insects 1/5 to 1/3 inch long that suck sap. Dab each tuft with a cotton swab dipped in rubbing alcohol.
- Small dome-shaped shells fixed to the stem are scale insects. The check is a fingernail: scale can be scraped or pried off, and a scar cannot. Scrape them off or use the alcohol swab.
- Tiny brown dots on young growth, often with fine webbing among the spines, are spider mites, about 1/50 inch long. To confirm, tap the stem over a sheet of white paper. Mites show up as tiny specks that move slowly. Wash them off with a strong stream of water.
Repeat treatments, root mealybugs and sprays are covered in cactus pests.
Leaning
Press the stem at the soil line. If it is soft or dark there, or the plant rocks in its pot, treat it as rot at the base: see cactus rot. If the base is firm and the plant bends toward its light, it may need more light. Turn the pot and see "Stretching".
Sudden collapse
A stem that folds or topples has usually rotted at the base. Press along the fallen stem to find where soft tissue ends and firm tissue begins. A firm upper section can often be cut free and rooted again as a cutting. The method is in cactus rot.
If you still cannot tell
If you are unsure what you are seeing, email us with photos at support@thoughtfulpricks.com.
Care that prevents many of these problems is in growing cactus for your conditions.
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 Problems, Pests And Disease. Michael Chamberland, University of Arizona Cooperative Extension, 2021. https://extension.arizona.edu/sites/default/files/2024-08/az1962-2021.pdf
Cactus (Cactaceae). Plant Pest Handbook, The Connecticut Agricultural Experiment Station. https://portal.ct.gov/CAES/Plant-Pest-Handbook/pphC/Cactus-Cactaceae
Cactus Care Tips from Garden Horticulture Staff. Desert Botanical Garden, 2023. https://dbg.org/cactus-care-tips-from-garden-horticulture-staff/
Insect pests of cacti and succulents grown as house plants. Missouri Botanical Garden. https://www.missouribotanicalgarden.org/gardens-gardening/your-garden/help-for-the-home-gardener/advice-tips-resources/insects-pests-and-problems/insects/mealybugs/insect-pests-of-cacti-and-succulents
How can I determine if my houseplant has spider mites? Iowa State University Extension and Outreach, 2022. https://yardandgarden.extension.iastate.edu/faq/how-can-i-determine-if-my-houseplant-has-spider-mites
Cactus type and disease? Ask Extension, answered by the University of Maryland Home and Garden Information Center, 2020. https://ask.extension.org/kb/faq.php?id=633312