A few days ago I have noticed dark purple spots on top of my 2 SP cacti. There are a few more such spots on the sides.

What is this disease and how do I treat it?

Thanks.

Has it been over watered?

that is black rot

can be from overwatering during the cold or slow growth times, or other damage to tissue

as its on the tip, and side, it may already be systemic. Is it rooted? new arrivals??

it might heal, and just leave a scar, or the whole thing might turn mushy soon(death)

if it was a larger plant i'd say, cut off all the rot, (keep cutting until you hit white tissue with no rot) then dust cuts with fungicide(sulphur)

as it is, i'd leave it be and hope for the best
some tips, keep it dry, dont water it much-if at all until healed, leave in a mostly shady dry outdoor location

edit: i looked at the pics again, they are likely new arrivals, now being grown indoors, they may not recover with inside conditions

Approx. 6 mo. ago I got these 3 plants from the same source. All 3 hibernated for 5 mo. About a month ago I started to water them once a week instead of once in 3 weeks. I've started to spray them once in 3 days a month ago. One plant is still OK and I've moved it to a different room just in case.



What is it? Can they still survive?



Yes.



2 damaged plants are 4 and 5 inch high. The 3rd one is 3.5 inch high and it is OK. The biggest and the thickest is in the worst condtition.

(keep cutting until you hit white tissue with no rot) then dust cuts with fungicide(sulphur)



I have no experience with cacti but it does not look like I've overwatered them.



No, they are not. They've survived the winter.

Thank you for your help.

so they arrived as rooted cuttings? and have been in a cool dark space for 5 months until recently?

watering once a week for indoor cacti that aren't actively growing(or just barely growing) is too much, make sure the soil medium stays fairly dry and light for a week or two between waterings if you must water at all.

as an example, i have several similarly small tip cuttings i've kept near the window w/o any water at all for months now

why are you spraying them?/what are you spraying-the soil or the cacti? nothing that increases Relative Humidity is good for rot, or needed for these cacti

black rot is just dead tissue, if it has migrated thru the plants system(you'll see brown stains in the plants tissue near the rot site if you cut, that is the poison spreading thru the plant)if the brown stain/"cacti gangrene" continues thru out the core of the plant, it likely wont survive if not in active growth, but i hope they make it for ya

Firstly, I'm no expert, but I'll throw some stuff out there for discussion from my very limited knowledge.

If it were me and I had no internet I'd assume rot and immediately cut out the black bits and dust with sulphur to be safe as in the long term that plant could grow and dwarf the hacked stump it grew from. That tip's growing point isn't going to grow any more anyway if that's rot, it'll be dead. Then I'd chop the cactus in half below the lower black bit (slicing, not sawing, at an angle) to give me two plants to care for instead of one in case one dies. If the cut didn't show infection then I'd leave the bottom half be and just not water if the soil mix was correct. The top half I'd leave for at least a month to dry and heal before planting in a new pot. (Although how big is it, not sure how small a piece you can chop off to root?)

If the bottom half was also infected I'd take it out the soil and let it dry for a few days before repotting in a dry and appropriate mix.

What's your soil mix? I've just put a special kind of cat litter in mine (Tesco low dust cat litter, pink and white bag, soaked and dried first to remove the perfume, 50/50 with loam-free potting soil) that's made of fired diatomous clay that sucks up and holds water to keep the soil from being too wet and is much cheaper than pumice.

I have a Pedro that I water weekly indoors in a temperate zone and he's always been fine. Guess it depends how much water you're watering it with though!

As I say that's just for discussion as I'm a novice, please do correct me as I'm here to learn.

SWIM's plant suffered from the exact same problem (The photos match very well), SWIM slashed off the rots using surgical blade, but new rot formed again (because SWIM lacked bio-cide stuff to powder the fresh flesh with (SWIM could try his luck and add some nicotine or harmalas, but sigh..)) eventually SWIM sadly decided to harvest the plant before it rot completely.

BTW SWIM's plant looked very similar in regards of strain.

SWIM loved that plant so much, and the plant comes to his dreams sometimes. he is very sad about loss of the green friend.

I too have the same problem with a Peruvianus of mine.

I bought it as a cutting a few months ago, and when I receiced it, it looked perfect (very pretty even). A few days after potting it, purple marks started appearing. We're now about 3 months later, and it hasn't grown a bit.

So I guess I'll have to cut off the tip. Shame really, it was a beautiful cactus. I can only hope that pups will sprout from it, even though I doubt it has grown many roots after only 3 months.

here too - same looking peruvianus , same problems with 3-4 new plants ....

This is really coincidental. I got mine from a Dutch vendor.

Same problem appeared on three or four of my peruvianis too! Dutch vendor, in different location and time of the year (outside vs indside, winter vs summer)!
I thinck I really gonna cook the last two I have they are beaten by the mushroom !!