Fairy castle cactus - I have one that is about 10 years old, big, and it keeps getting black spots on it that I can't figure out how to get rid of. Any idea what is going on? It seems to have room for growth in the pot, and I not sure based on its size if I could find a bigger pot, much less get it in one.
Certified GKH Gardening Expert
If your cactus appears to have room for growth in its pot, it's best to keep it there for now. Here is more information on when to repot your cactus:
Check whether the black spots feel soft and mushy or sunken. This indicates there might be a rot problem, which can be caused by overwatering. Here is more information:
Or, it could be a fungal disease, like those seen in the fungi section of this page: