Disease (?) On Cactus

[moll]

Hello! I have a 7 year old cactus that had just developed this problem this year. It is progressively, but very very slowly, gotten worse. I noticed it about 2 months ago when it was a bit smaller. It does not rub off nor is it squishy in those areas and it occurs on one side of the cactus.
I live in Iowa, so it’s kept inside during fall, winter and early spring. I do not water during the fall and winters.

Any solutions to take care of this?

[jerrytheplater]

Sunburn? Did you change the amount of sun your plant gets suddenly?

[mikethecactusguy]

Looks like a fungal infection. More than likely the damage internally is a lot more advanced than what you see on the exterior. If it was my plant, I'd see how deep it is by digging out the black tissue. If not too deep, treat the cut outs with powdered sulphur. It will obviously leave scars but you will save the plant. If it is isolated to the newer top section, you may consider cutting that off and seeing if the plant will grow offshoots.
Mike M

[moll]

Looks like a fungal infection. More than likely the damage internally is a lot more advanced than what you see on the exterior. If it was my plant, I'd see how deep it is by digging out the black tissue. If not too deep, treat the cut outs with powdered sulphur. It will obviously leave scars but you will save the plant. If it is isolated to the newer top section, you may consider cutting that off and seeing if the plant will grow offshoots.
Mike M

Good idea- I cut open a small area. It’s green and healthy- no internal damage. It looks like it only affects the surface layer. I’m able to just peel the affected “skin” off.

To 1st reply: it happened to be the side that received no sunlight actually- I’m pretty lazy at twirling it around. Maybe that could also have been a factor?

[jerrytheplater]

A reflection off of something? Otherwise, I don't know what is going on.