It appears that your potting medium holds way too much moisture. Take a clean, sharp knife and cut the rotten area away. Be sure to keep only the unaffected part. Leave the cutting laying out in a semi-shaded area (under your porch awning, etc) until it has time to callus over. This will probably take a couple of days, but not more than four or five days. When the wound has sealed over, replant in a sharp draining potting medium or in a dry area of your yard. Be sure to get a good amount of the pad down into the soil. It should root and do well.