Q.My Cactus Is Turning Black And Crusty What Do I Do?
I just noticed today that my cactus is turning brown, crusty and brittle at the top. I usually check on all my plants once a week and just noticed this today. I’ve read a lot and it seems this may be fungus and not frost bite as it is indoors. I usually only water once a month but probably watered a little too much this month so Im not sure if that’s the cause. I plan on repotting with new soil just in case but im not sure how to treat what’s already there. Do I cut all of this off? Hoping there is something I can do to save it.

Certified GKH Gardening Expert
It does sound like too much water has caused rot. Cutting off all infected matter and treating with a fungicide will be your next steps, as you repot.
https://www.gardeningknowhow.com/garden-how-to/info/using-fungicides-in-garden.htm