That is definitely rot, sorry to say. You will have to cut the stem above the rot, to where you can't see any discoloration inside the stem, allow the top part of the plant to callous and then re-plant it.

It does look like it is sending out lots of roots further up. Make sure you have all the rot discolored stem out of their and sprinkle some cinnamon on the fresh cut. That is a good fungicide.. Clean of your exacto knife with alcohol for the final cut. Callus and put in soil. Mist for a bit so the roots are reaching out water. Should not take long because I think you do have roots there. The plant has not been getting nutrients through the stem for awhile judging by how many roots you have. Good luck.

Yeah, wantonamara, it's been growing those roots since I got it. I have no idea how fast rot spreads, but it was awful when I pulled it out. I tossed all the potting material (and the pot.)

Discoloration traveled so far up the stems that I had to separate most of the "heads". This is the first I've actually had to behead any of my plants (although I have a few I've been meaning to behead for aesthetic reasons.)

Thank you so much for the help, guys. I hope my plant(s) are able to pull through alright.