too cold

I don't think the cold is the likely cause-- the house is heated, and typically between 70-75 degrees. It has overwintered next to that window before with no issues.

How close to the window? And how cold is the outside temperature?

I had several of my seedlings on the window sill. Well protected from the cold. But our Temps outside got pretty cold and burned the one side of them and one has become mushy

It's fairly close to the windowsill; those branches were about 1-1.5 feet from it. I'm in zone 9b and it honestly doesn't get very cold here. Most nights are 40-45, and it hasn't gone below 37 during the time the rot appeared. Other cold-sensitive plants close to that windowsill seem to be doing fine.

They should be fine on the window sill. We dropped to the very low 20's for 3 days. I didn' notice until afterwards and that's the only reason I can think of that they all lost some/Most of their leaves on the side of the window. So the window isn' the issue. How about your indoor temps?

Relatively steady indoor temps, low-to-mid 70's.

Sounds like maybe Martin has a point

Hmmm. Thanks, Martin; I'd never observed that kind of self-pruning before.

We're having a couple of weeks of warmer weather (mid-60's); I'll try putting it outside to get more light, and watering it a bit more than I have been. We'll see whether it rights itself, because the rot has since spread below the blue arrow, meaning the whole right branch is now in danger. :(

Is the caudex firm? Then don't water it. And a rule of thumb: never water adeniums with temps below 70F.

I would without hesitation remove those rotting branches. Your tree could benefit from a "haircut" anyway.

Im with martin we had a colder than normal winter i have seen this happening to some of my plants you have to cut them back till you see no discoloration in the white of the branch that is a must. also cut on a 45 deg angle then seal off with super glue . Maybe a heat mat to warm the roots they are cheap .just an another thing to think about is i never have this problem in summer it is only when they go semi or fully dormant. Just remember u have to cut the rot out i have had the rot track back to the cautex and i have lost the plant good luck:×)

This has been an issue for many over the dormant/winter months. It is not necessarily attributed to cold conditions. With consideration to over watering during dormancy.

As Martin mentions, self-pruning can play a part in this problem.

The most important thing is to Cut any branch showing this rot back far enough that absolutely No beige/off-white tissue is showing. Always keeping watch the this branch dries hard and if it does not then re-cut.

Some correlation has been known to a branch that does not release dying leaves while losing them. I cut any off w a sterilized blade, if see this.

Rick

Hmm. To all of you suggesting cutting, your advice sort of goes back to my initial issue. I have been cutting this plant, repeatedly, back to the green tissue, no rot left, with a sterilized pruner. (This adenium was actually rather full and branchy at the start of winter.) I've cared for stem rot before and know the technique.

In this case, the rot came back at each cutting, and and new unconnected branches started rotting as well, which is something I have not experienced before in a stem rot case.

At present, if I cut off the rot, I will not be "giving it a haircut"-- I will be cutting off a literal half of the crown, at the main fork where it first branched seven years ago. All of the branches to the right of the fork will go. If that's what it takes to save the plant, I'll do it. But given that pruning over the last few weeks has seemed only to result in rot reappearing closer to the main stem, I'm worried that making a major cut will just result in the same.

If this cut doesn't work, the rot spreads to the main stem. My next step would have to be to cut all of the branches off, back to the base, leaving the plant with no leaves and in a pretty poor position to heal. :) So yeah, I'm a little bit trigger-shy this time. But I'll go ahead and make the cut later today.

Pour alcohol on your shears and while it's still wet, set it on fire.

Then after the cut, apply cinnamon to help desiccate the wound and make it inhospitable to microbes. Make sure you cut with a very sharp knife and all the way to fully healthy tissues. Bismillah!

Or I mean: do you have an update?

No watering in winter as they are dormant-sleeping and resting.

Also I noticed in your comments that you did not mention airing the plant out for a couple of weeks after the cut for the root rot. You cannot plant it right afterwards. After it is cut and cinnamoned make sure you hang it to dry for a few weeks. then check it by touch to make sure it is completely dry. Look at it visually to make sure you do not see anymore root rot-because if you do not get the whole thing if just a touch remained it will grow back into all of your plant.