I've had this cactus for a while now, last spring I moved it to the balcony and kept it there till now.
I thought everything was ok but then I noticed that the side not facing the window was turning yellow and black.
The temperature has dropped under 0C and I think that may be the cause. I moved it inside now. Is there anything else I should or can do to save it?
It could be frost, it's certainly ugly. Regardless of the cause you need to cut the rotten parts off right away. You can use a serrated kitchen knife to cut and hold the plant steady with tongs. Grab the rotten part, so you don't damage any healthy parts.
When you are cutting the top off look at the center of the stem, if there is any rotten-looking discoloration keep cutting lower. You have to cut back to perfect, healthy flesh.
Good luck!
