I have this cactus that I had bought quite a while ago and I am still unsure of what species it is. I had changed its soil quite a few months ago and it seemed that quite a bit of the bottom of the cactus was too far under the soil. For a while it had also seemed pretty dehydrated. But now recently I went to change the soil again as all my cacti needed a soil change and I'm pretty sure I had a fungal issue going on as two of my cacti rotted and died. When I took this one out, I noticed that now the roots almost seem like they're starting to rot. They're discolored and somewhat mushy. I haven't put it in the soil yet since I wanted to wait for the soil to completely dry since it seems like this one could be rotting but I'm still not completely sure. I've never been very successful at saving cactus from rot by cutting them off from the rotted part but that's probably because many of those were too far gone. I really don't want to lose any more of my cacti so I need some help here.

@Solanum_Dulcamara4
I don't see anything that makes me think it has rot on the body of the cactus itself. I can't tell about the roots from the photo. Since it's already unpotted, if it were mine I'd just let it dry in bright, indirect light for a few days, repot it in a 4"-6" container with a fast draining soil mix. Then wait another week before giving it any water. You don't give a location, but here we are just now heading into the warmer days after winter. I have observed that after those winter months, some cacti and succulents tend to lose some of the larger, fleshier root system. That may be due to the care they (don't) get in the winter. I'm proactive about not encouraging growth when plants are inside because I can't provide sufficient light. It may be different with you, but I believe it can't hurt to let the plant air dry a bit and hold off on watering too much, too soon after repotting. If the body of that cactus feels firm and is just sort of woody-looking, it's probably just a natural condition and not rot.

@needrain
Well it originally was firm and discolored but now it's a bit squishy. My one cactus that rotted had gotten discolored at the bottom and it was mostly firm but then a piece of the skin peeled off and there was black mold underneath. That was one of the ones that died. I'm scared of the same happening here. I do have four other cacti that do have the firm discoloring at the bottom but they're definitely not rotting as they haven't gotten mushy or anything. But this one has gotten slightly mushy which is of course concerning.

@Solanum_Dulcamara4
All the more reason to let it air dry for a few days. Easier to keep an eye on and might be helpful overall. If it's rot like Erwinia, a bacterial infection, it will have a strong, unpleasant odor. There is enough moisture stored in the plant that if that is the problem the rot will continue to progress and be visible even lying out, but it will slow down the process. If that is the case, then your plant may be too small to try and salvage it, but if it's not then the air drying should be beneficial. That's my thinking. Sometimes things conspire against maintaining a plants health. It's just part of growing a living organism. It's disappointing, but can be a learning experience.

Good luck and to NGA!

Yes, welcome, and I think Donald has offered some valuable advice. On top of that, I would add a few observations.

Rot is generally a one way street with my plants, especially when it affects the stem. I don't know if there is a reliable treatment (after some point I know there is not) so much as useful measures of protection.

Sometimes it is possible to identify the cause of a plant going mushy, other times it's kind of guesswork. But generally the issue relates to insufficient light, excess water, insufficient air flow. To the extent you can address any of these possible root causes, it's a good idea to revisit the care you are giving in order to fine tune it as appropriate.

Also, as a general rule, it's not a good idea to bare root your small/baby cacti any more than necessary. They are vulnerable at that size and may respond poorly to damage or loss of roots. So extract useful information if you need to, otherwise leave the roots well enough alone. You may end up causing harm to the plant.

Any time you handle the roots (bare rooting, repotting, invasive handling) it's a good idea to avoid water immediately afterwards. Wait a week or whatever first. There is a real risk of rot if damaged parts don't have a chance to heal before they encounter dirty water.