Pocket Botanist
Check these first
- Look closely for white cottony webs or insects.
- Check if the spots are soft or sunken.
- Ensure the plant is not in a high humidity area.
Likely causes
Fungal infection
Most likelyHow to tell: The spots are dark, sunken, or spreading across the surface of the stem. These often appear after the plant has been kept in humid conditions.
What to do: Improve air circulation around the plant immediately. Use a clean knife to cut away the infected area if the spot is small, then let the wound callous over.
Mealybugs
PossibleHow to tell: You see small white cottony masses tucked into the crevices of the cactus. These pests suck sap and leave behind discolored marks.
What to do: Dip a cotton swab in rubbing alcohol and dab it directly onto the white cottony spots. Repeat this weekly until the insects are gone.
Find out what's wrong with your cactus
The list above covers the usual causes. Pocket Botanist looks at a photo of your actual plant and tells you the likely cause, how serious it is, and what to try first. Identification and a health check are free.
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When to worry
Small, corky brown patches at the base of an old cactus are normal aging. Soft, spreading, or fuzzy spots are serious and require immediate action.
Stopping it coming back
Keep the cactus in a well ventilated area and avoid getting water on the stem when you irrigate.
Keep your cactus healthy from here
Save your plant in Pocket Botanist and you get care reminders timed for this plant, plus a care history that shows what worked. Identification and a health check are free.
Questions people ask
- Are the spots contagious?
- If the spots are caused by pests like mealybugs, they can spread to nearby plants. Fungal spots are also easily spread by water splashes.
- Should I use a fungicide?
- Usually, improving air flow and fixing watering habits is enough. Only use fungicide if the infection is widespread and persistent.
More problems with Cactus
This article was generated with AI from the plant’s care profile and is general information, not horticultural advice. Read about how we build these guides.