Commercial cactus and succulent mixes contain way too much peat. Putting rocks and sand on the bottom just plugged the drainage. Never put stones at the bottom of the pot. It does not increase drainage, and raises the effective water table in the pot.

You can save these by transecting the stem ABOVE the rot, or soft areas. Keep cutting towards the tip in 1/4 inch increments until the cross-section is free from dark-colored fungal infection. Callous, and replant on top of dry soil. 3/4 of the mix should be inorganics like pumice, perlite, or gravel. No sand! This will produce a free draining container.

Your plant can be saved, but you need to take bold action. Also, consider repotting all your other plants in a better mix to avoid disaster this Winter. You should be cutting back on watering to almost nothing this time of year because they should be entering dormancy.

Good Luck!

Brad

Unfortunately, it is probably doomed. Put it somewhere warm, out of direct sun, and don't do anything else unless you see new leaves. If you do see new leaves, resume watering and slowly acclimate it back to full light. Sadly, it is either rotting completely, or just the roots rotted, but in either case, it is usually fatal for such a small plant.

The soil mix you are using is terribly inadequate for the more esoteric Pachypodiums. Yeah, P. lamerei and P. geayii could tolerate it, but more unusual species like P. horombense (yours?), P. densiflorum, and most especially P. brevicuale need *superb* drainage.

You would be much better off taking the store-bought mix and adding at least two parts perlite or pumice to it. For even touchier Pachypodiums like P. brecicuale, grow it in pure perlite or pumice. It would also do your plants a great benefit to track down some sort of potting soil/potting compost that doesn't contain any peat moss...

Get rid of the sand altogether. Unless your plants have a specific requirement for a fine-grained substrate, sand will only cause problems.

Lastly, putting coarse rocks at the bottom of a pot ultimately moves the soggiest part of the soil column up towards the plant and its root. Putting a layer of "drainage" rocks in the bottom of a pot is a bad practice with succulents, don't do it.

Ah jeez Brad, you beat me to the post! Good to hear we are basically saying the same thing. Taking cuttings is tricky business though, which is why I personally would take the gamble of seeing if it just lost its roots and is desiccating. If it is rot, doing nothing will assure death. I also much prefer any approach that involves less work... :-)