I am getting black rot on some rhips that I am getting at HD. Some one I think around here was saying the best was to get rid of it was to remove the good leaves and then treat specially to protect against the rot. I forget the details.
Black rot on Rhipsalis
Analog, Wasn't me, but if you have a black rot, I would immediately remove the plant from any damp area it might be in and let dry a bit. The rhips seem to tolerate drying out really well, as long as they are not physically traumatized during the dry spell. Otherwise, they could shatter. My rhips are inside a very dry house all winter. I water about 1 time a week and they are dry the next day. End result is that the plants do not grow over the winter. Point being, I would treat any mold or fungus by immediately reducing the humidity and watering level of a rhipsalis. A follow up of a mold or fungicide would then follow.
I'd like to hear the specific treatment for this, but I have not run into it myself.
Good luck with it. I too got a nice rhip from HD this winter.