Do you know what weather conditions are native to your cactus? Some cactus are extremely sensitive to too much water and some cactus are ok to be watered every day. Low elevation hot desert cactus grow in 115 degree weather with no rain for 11 month a year and cactus that grow at high elevation in the mountains can not take the heat but do good with, snow, ice, cold, wind, rain, blizzard conditions where the desert cactus can not. Most cactus like very well drained soil. Most of the cactus I have experience with get black rot from being too wet. Cut the black rot out with a knife it is like cancer if you don't do something it may kill the whole cactus. Cut the bad rot out make sure you get it all the cactus should heal itself. The worse thing to happen is the cactus rot gets worse so cut the cactus off from the roots. Cut as much of the cactus off as you need to, to remove all the rot then lay it in shade for a week to scab over. Plant it in soil it will grow new roots. Cut all the rot off of the roots too you may be able to get new growth from the roots and end up with 2 cactus. If you manage to save the cactus by cutting out the rot let it scab over then push soil up on the bad spot to hide it. Cactus have the ability to grow roots anywhere soil touches it. San Pedro cactus like lots of shade they do best if full shade.
Cactus can tolerate some of the worse conditions on earth but you give cactus good growing conditions many will grow 12" taller every month, they can grow faster than some weeds. I mix about 25% potting soil with 75% sand and put my cactus where they get full sun early morning until 11 am then full shade the rest of the day they stay green, look nice, make flowers in spring, grow 12" every month.
If you plant a 12" tall cactus on its side cover it with soil so the side is barely visible it will grow 6 or 7 new cactus out of the side. Now you have a row of cactus. When the new cactus are 6" tall break them off the mother plant then plant them all in pots. The mother plant will grow 6 more new cactus.