Daniel wrote:rot i think does not spread but if there is a fungal infection in it that will spread. I personally would keep it away from other plants until you know what to do.
Sorry, but rot
does spread, and if left unchecked it'll kill the plant. The top part of your Mamm mystax tends to suggest that the growth point is still healthy, so your best hope is to remove all the rot and root the top. Here's some instructions.
1. Cut out the rot using a sharp knife sterilized with alcohol. Actually, this will be a series of cuts starting with the initial cut as it looks like this:
- Scarycactus3_cut.jpg (155.11 KiB) Viewed 2230 times
Continue making small cross-section cuts until you see only green tissue. I don't think it'd matter if the skin is light green, but you need to completely remove any brown stuff. If you leave even a little bit behind, the rot will continue spreading. The top part is now the cutting that will hopefully save your plant.
2. Dust the bottom of the cutting with fungicidal powder -- sulfur powder is easily found at nurseries and garden centers, so that should be fine. Let the cutting callous in a dry spot away from direct sunlight, and warm enough to keep it from cold that could damage the plant. It'll take the callous at least a month to heal. When you see shriveling along the edge of the cut, don't panic -- it's a normal part of the callousing process.
3. Once the callous has healed, place the cutting on top of fresh, dry cactus mix, and keep it dry. You won't see any growth in winter, but after spring starts, you can check occasionally for signs of new roots. They'll look like stubby little white protuberances coming out of the bottom. I don't know how long it should take, but I think mystax roots rather easily. However, some cacti can take a year or more to root, so you'll need to be patient.
4. As soon as you see new roots, leave the cutting in cactus mix and continue keeping it dry. Give it about a month, and your plant should be ready for its first watering. Be careful not to over-water -- the building root system won't be able to handle a lot of moisture at once, so light watering is best until you see new growth on the plant's apex really take off.
There are two final things to consider. One, if you cut away at the plant and you see rot going way up into the center, I'm afraid it may be done for. Two, assuming that you have enough left for a salvageable cutting, you'll really need to do something about your mix. The heavy and overly moist soil you were using is precisely what left you with a rotting root ball. Don't know what cactus mixes you have access to, but if it's a commercial mix, you should add a lot of something that'll give you a freely draining mix. Pumice is ideal, although you may not be able to find it. If you can't, either Turface or NAPA Floor-Dri (8822) would be good options. The mix should tilt heavily in favor of drainage, so I'd recommend at least a 50/50 ratio of mineral product to soil. My personal recommendation would be more like 70/30 mineral/soil in your mix.
Hope this helps, and let me know if it works out for you. Best of luck!
. I have no idea what exactly is the matter, or what to do with it. I was taking it out of its soil to repot it because it was rotting (or something) 
, and was horrified to see the following 
.
2: There are a bunch of brown, corky-looking tubercules, slightly mushy to the touch 
WHAT DO I DOOOO??? 
