Sorry to say, you acted too late. You need to cut above the collapsed rotted area through green tissue, in hopes of saving the top for rooting. You need to do it now, and let the cut end dry for at least a week before doing anything more.

Not only do these need a fast draining potting medium, they require little watering but need much more light than it was receiving for good growth. Your plant is stretched due to insufficient light, a condition known as etiolation. Too much watering in a low light condition will lead to death of the plant. Read up on care for Jades. There is plenty of information on this forum, as it is the most discussed plant here.

Christopher

That's very reassuring!

Samantha

Don't rely on cactus and succulent mix, especially if you have tendency to over-water. Christopher already mentioned well-draining mix. If you have only this succulent, it may not be feasible to look for other ingredients besides perlite. You get faster drainage with addition of some grit. But if that's the only succulent, make sure you add very generous amount of perlite.

When cutting rotted part off, you need to cut until there is no sigh of rot anymore (blackish marks on inside of trunk, either spots or thread-like). Even if you need to cut most of the plant, it is better than leaving any rot - this will spread and kill the plant. And even small and healthy piece will grow into a nice plant (providing that it won't get over-watered and it will get more light). Btw, it will grow in lower temps.

Jades are very easy to grow, from leaves or from cuttings. But you should consider also improving on light - low light will keep plant growing too stretched and it will never look it's best (it was not top-heavy before, but etiolated as was already mentioned).

Rina

That's good to know! When I cut it, the part that I'm keeping was completely green with no signs of rot. When I pot it, I'll use majority perlite per your suggestion.

As for the light, I'm currently living in a dorm with a single north facing window (none too pleased about it). I've added more light to the room by leaving lights on while I'm away, but that's as much as I think I can do at the moment. Do you have any suggestions for how to improve the lighting?

Samantha

I guess that's all you can do. If you have a floor or desk lamp, get one of 100W CFL bulbs (it uses only 23W) or even higher (they come in 125W, 150W even 300W). Try to find one with high number of kelvins - 4000 or more. You can use that for your plant. They aren't very expensive and last much longer than regular incandescent bulbs, and don't produce so much heat so plant could be closer to it. Don't put it very close at first thou, since plant isn't getting that much light right now - acclimatize it by moving plant closer every few days/week. It would be a good idea to turn the pot about 1/4 turn every couple of weeks if one side is getting more light (side close to window) than other, so it develops more even over all. You could also use a fluorescent tubes if you have more than 1 plant.

Rina