Black spots on TWO totem pole cactuses

[cerclerouge]

So about 10 days ago I received two large totem pole cactuses that I purchased off the internet. I don't think these black marks were there when I received them. I put them in well draining soil, and HAVEN'T watered them, until just 3 hours ago. Now as I move the pots back to their normal area after watering, I'm noticing black marks on BOTH cactus. Especially annoying because they were aesthetically pretty nice before this. Anyone know what this is? I'm located in NYC and both are close to my south facing window with indirect sun. I wish I knew exactly WHEN these marks appeared. I imagine it can't have to do with watering, right? I mean it was only 3 hours ago. The top of the cactus DID get wet while I was watering though, not sure if that makes a difference. Any ideas?

[Mrs.Green]

My first thought when I saw the pics was some form of pressure / mechanical damage from shipping. But then I noticed what looks like dark spots in the middle of the plant (on the right) in the first pic. Is this dark marks or just shadows ?

[cerclerouge]

Those are just shadows… but I think I’ve diagnosed it… humidity. I’m in a normal sized NYC apartment and I have a humidifier for my tropical plants. And very stupidly, this totem pole was basically 6 inches away from the darn humidifier. So I’m reorganizing some things in order to be able to keep tropical plants with high humidity requirements AND cacti in the same living room. I have three hygrometers going in different parts of the room and there can actually be a decent drop off of humidity depending where in the room you are. Would a small desktop dehumidifier in the cactus area help? What about a fan pushing humidity back towards the window? Do we think this rot will spread? I appreciate any help from the experts, as I am clearly not one myself!

[greenknight]

I think Mrs. Green has it right - it's probably bruising from shipping. There are fungi that cause black patches on cacti but the way these appeared all at once shortly after they were shipped, their appearance, and their location, leads me to think mechanical damage is likely the culprit.

That doesn't mean there couldn't be an infection, too - disease organisms can use wounds to gain entry. Keep a close eye on those spots for any spreading or production of spore-bearing bodies. If those occur, cut off the infected part and dust the cut with sulfur. Avoid getting the top of the cacti wet - though I'm sure that's not what caused this, it could help produce more outbreaks. See: https://homeguides.sfgate.com/black-spo ... 37433.html