Sterilize your tools! If you have a problem with rot on your cactus, and have to do some cutting on them…be sterile. Be sure to keep either rubbing alcohol or 10% bleach water handy for cleaning. Every time you make a cut on the plant sterilze the knife or saw with it. Rot can spread to the new clean cut if you don’t. After you have cut out all the rot, rinse the wound with bleach water and let it dry. Never fill the wound(s) with anything! This will trap moisture and promote the rot again.