A brown patch on a cactus is usually one of 6 things: corking, corky scab, sunburn, a frost mark, an old scar, or rot. The first 5 usually end as dry marks the plant lives with. Rot is the one that spreads through healthy tissue.
Color alone does not separate them. How the patch feels, and whether it changes, does.
The short version
- Press the patch, then press a healthy green part of the same stem. A patch that is soft, wet or gives way under your finger is rot. Stop watering the same day and go to the Rot section below.
- If the patch is as firm as the green part or harder, and dry, draw a pen line around its edge and look at it once a day for 7 days. A patch that stays firm and inside the line is unlikely to be rot.
- Brown, hard, bark-like skin at the base of the stem is corking. It is normal aging.
- The same bark-like skin on recent growth, with green skin below it, can mean the growing conditions are off.
- Rusty, corky spots or patches on the stem are corky scab. They usually sit in the skin only.
- A patch that turned yellow after the plant got more sun, then straw-colored, then dried to a scar, is sunburn. Shade it while it is still yellow.
- Black on exposed parts after a freeze, turning dry and crisp over a few weeks, is frost damage.
The press test and the re-check
Press it. Push on the patch with a fingertip, or with the blunt end of a pencil if spines are in the way. Then press a healthy green part of the same stem and compare. A harmless mark is dry and feels as firm as the green part or harder. Rot feels soft or mushy and gives way, and the surface may be wet or slimy.
Re-check a firm patch. Draw a line around the edge with a ballpoint or felt pen and look once a day for 7 days. Dry marks change little in that time. A patch that stays firm and inside the line is unlikely to be rot. A patch that turns soft, on any day, is treated as rot from that day.
A soft patch is not watched for a week. It is acted on.
A patch that passes both checks is unlikely to be rot. The sections below tell you which of the other 5 it is, and whether it asks anything of you.
Corking
Corking is the cactus version of bark. As a stem ages, the old skin loses its green, turns brown and hardens. It usually appears on the oldest tissue, near the base of the plant. Often it starts at the bottom of the stem and works slowly upward.
What you see: brown, dry skin with a bark-like texture, lowest on the stem.
What you feel: hard and dry.
On a columnar cactus (one that grows as an upright column), such as San Pedro, a corked base needs nothing from you.
When corking is not just age
Corking belongs on old tissue. When the same brown, bark-like skin appears on recent growth, high on the stem with green skin below it, it can be a sign that the plant's health is poor because its growing conditions are off. Too little light may play a part.
The corked skin itself is firm and is not an emergency. Use it as a reason to check the basics:
- Light. A stem that is thinner or paler at the top than lower down is short of light. See cactus growing conditions.
- Water and roots. A stem that is wrinkled or soft along its length has a watering or root problem. See overwatering and underwatering to tell which.
The corked skin stays brown. New growth that comes in green and stays green shows the conditions are right.
Corky scab
Corky scab shows as irregular rusty or brown corky spots and patches on the stem. Corking starts as one area at the base; scabs are separate spots.
What you see: rust-colored or brown corky spots. Where the corky tissue cracks, the skin can break open and curl at the edges.
What you feel: dry and rough.
The spots usually sit in the skin and do not reach the flesh beneath, and scab is usually not serious. It is tied to growing conditions, mainly overwatering and poor air movement. More light and lower humidity help. Let the soil dry fully between waterings, give the plant a place where air moves around it, and increase light gradually so the skin does not burn.
Sunburn
Sunburn is common on plants moved from shade or a greenhouse into open sun, and columnar cacti burn easily.
What you see: the skin turns yellow. If the exposure goes on, it turns straw-colored and dies, leaving a permanent scar. It follows a change: a move into more sun, shade removed, or a replanting that turned a different side of the plant toward the sun.
What you feel: firm, then dry.
The plant usually lives. A patch that is still yellow can still get worse, so give shade now: move the pot out of direct sun, or cover the plant with shade cloth, a fabric rated by how much sun it blocks. 30% shade cloth is used to protect plants that are new to full sun. Then return the plant to sun gradually over a period of weeks. See acclimating a cactus to sun.
Frost marks
Frost damage appears after temperatures stay below freezing for several hours. The exposed parts of the plant turn black. After a few weeks the blackened area dries out and becomes crisp.
When the freeze was short and not severe, the damage may be only cosmetic, and the plant outgrows it over several years. After a hard or long freeze, whole sections of a columnar cactus can yellow and die.
Black is also the color of rot, so press a frost mark as you would any other patch, and press it again each week until it has dried. A frost mark that turns wet and soft, or keeps growing after the cold has passed, may have started to rot. See cactus winter care.
Old scars
A wound that heals cleanly leaves a dry, brown, corky scar. Hail leaves wounds at the points of impact, which scar as anything from small spots to larger marks, and in a windy storm they may be on one side of the plant only. Spines from a neighboring stem, a fall or a scrape do the same.
What you see: a mark with a clear edge and a shape that matches an injury, such as a dent, a line or a puncture.
What you feel: hard and dry.
A scar that is dry and unchanged needs nothing. A fresh wound is different, because wounds are where rot gets in. Keep it dry and press it every few days until it has hardened.
Rot
Rot is decay caused by bacteria or fungi. It is the only cause here that keeps spreading on its own.
What you see: it often begins as a small round spot that looks water-soaked, as if the flesh under the skin were wet. The spot enlarges and turns black. The surface can become slimy, crack, and ooze brown liquid. You may also notice a foul smell.
What you feel: soft, watery, giving way under light pressure.
Where it shows: anywhere, often at a wound or at the base. Rot that circles the base cuts the stem off from its roots, and the plant wilts and collapses.
A soft, dark, growing patch is unlikely to stop without help, so act the same day. The first step is to stop watering and keep water from splashing on the stem. The next is to cut the rot out, and how to do that is in cactus rot.
If you are unsure what you are seeing, email us with photos at support@thoughtfulpricks.com.
Sources
Cactus type and disease? Ask Extension, Home & Garden Information Center (Maryland), 2020. https://ask.extension.org/kb/faq.php?id=633312
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/
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
How to Transplant a Cactus. Michael Chamberland and Jack Kelly, University of Arizona Cooperative Extension, revised 2020. https://extension.arizona.edu/publication/how-transplant-cactus