Barrel cactus problem
I have noticed the skin at the bottom of my barrel cactus is starting to yellow and wrinkled. The cactus receives morning sun only, as it sits by a window facing east. I water it about once a month in winter months and bring it outside in summer months. I fear it is starting to rot and eventually die. What am I doing wrong?
Your Cactus may be suffering from root rot.
You can try lifting the cactus and repotting in a sterile cactus soil mix.
Here is a link with more information.