Question
I bought a barrel cactus last year about October, and when i brought it home, i transplanted it because it was larger than the pot and it was root bound then, it started rotting slowly from the base of the stem. (I think the roots are ok they have been growing) what can i do to heal it?

Answer
Dear Antony,

Sorry to say, but if that cactus is rotting at the base and you can see the rot then the roots are also in bad shape. You need to cut all that off.

I would take a sterilized sharp knife and slice the rot and the roots off.  The rot probably extends higher than you can see and you need to get to totally healthy tissue.  If that requires more than one cut, sterilize the knife each time. Throw out the rotted part and the soil it was in (don't compost it) sterilize the pot.  Let the cactus callous over as long as it takes to feel sort of like cork. I brush it with fungicide initially. A barrel, depending on the size can take awhile.

I would then get a pot not much wider than the plant, fill it with a mixture of half perlite and half spagnum, moisten that just enough so that it crumbles when you squeeze it and not water comes out, and then either place the cactus on top of the medium or whatever depth it needs to stay upright.  It will take a while to root.  You will know it has rooted by seeing new growth at the top.

As to why it rotted, the possibilities are that it was already diseased when you bought it and it can take months before you see anything; the potting medium you put it in held too much water or the pot was too big and that caused it to rot.

Good luck,
Maureen