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There are 3 of this same cactus sharing the pot. Only one of them is getting these spots. What could it be? Fungus? Sun burn?

They are in my balcony, they don't get rain, and they get like 3-4 hours of direct sunlight in the morning. Not 100% but I think that the one with the spots was facing back (ie: facing the window) when it started.

What can I do to treat it?

Extra info:

  • All cacti are 2 years old
  • The "infected" one is around 25cm (10in) tall and 6cm (2.5in) in diameter.
  • I've found the spots about 2 or 3 weeks ago, and I think they are spreading to a larger area of the cactus.

Pictures below:

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  • Good question and good illustrations; if could include how long the features have been developing and the diameter & height of your cactus would also be helpful.
    – M H
    Commented Aug 3, 2020 at 12:14

2 Answers 2

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I think your cactus has rot, and it will spread to the unaffected ones if you don't take action. I would separate the affected one from the two healthy ones, to prevent spreading. The one that is affected can be saved by cutting off the upper part (which is still healthy green), and try to let it grow roots (propagation by cuttings). While you are cutting, you might also cut the affected brown part to see if it is also brown on the inside, to be sure that it was rot. Don't forget to clean the knife after cutting in the rotted parts.

The causes of cactus rot are too much moist, either in the soil or in the air.

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  • Thanks for you answer, @benn. I should have mentioned that in the question, but the cactus seems pretty firm even in the brown spots area. I mean, it doesn't feel "soggy" or soft under those spots. If it is indeed rot, shouldn't it be soft in that area? What do you think?
    – cbdeveloper
    Commented Aug 3, 2020 at 9:49
  • I have no first hand experience with cactus rot yet (I guess I am lucky, I have been collecting many cacti over the past 2 years now). But if I would see this, I would at least separate the 2 healthy ones and put the affected one in quarantine. Maybe wait with cutting that one up, until you are sure it is rot.
    – benn
    Commented Aug 3, 2020 at 9:59
  • Will separate them. Thanks. But I'm suspecting it is mold. I think I'll try to treat it for mold before cutting it open. If it was mold, what could I do to get rid of the mold by using home ingredients? Does vinegar solution (1%) work? Also heard about milk solution (10%)? Have you ever treated mold in cacti?
    – cbdeveloper
    Commented Aug 3, 2020 at 10:04
  • Yes might be mold (fungus) or bacteria. The only treatment I know about is cutting it away, but maybe others here have experience with your suggested solutions?
    – benn
    Commented Aug 3, 2020 at 10:22
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This cactus has downy mildew. It can occur with a lot of shade and high humidity and colder nights. Get some neem oil. Mix with a few drops of dish detergent and spay after the sun goes away. repeat ever 5 days for one month. The white spots are the fruiting body of the fungus. The brown is another type of fungus. Neem oil prevents spread and the fungus dies. The area will eventual brown over/scar or cork. Not a problem. If you see black spots, do the same thing. If you look at the Areole, (where the spines come out) these are white too. It is on them as well. None of this has anything to do with rot. Remember, only water your cactus when all the soil goes dry. This is because it doesn't get enough direct light, and the rising moisture causes these problems.

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