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My ~3 mo old Sedum rubrotinctum grown from a leaf cutting was doing fine two days ago, but yesterday it looked wilted and today doesn't look any better. I watered it and put it in a slightly warmer spot on Thursday when it was healthy, could that have done something? I moved it back where it had been today, but it doesn't seem to have helped. Anyone have any ideas?

Looks like it either succumbed from its own weight or it received some kind of physical trauma and it bent. The fact that it looks etiolated might be the case in the former. Dont know your growing conditions, but external interference could be the cause of the latter.

I think it was etiolation; there isn't really enough light where it is. Other than more light (which isn't really an option), is there anything I can do to prevent other young plants from falling over?

Most of the "fixes" apply for older plants. For small ones like seedlings, leaf and stem cuttings there is no fix to my knowledge. On top of that, youre probably aware that the species youre growing is a sometimes a sprawling plant too.

Could it be a disease? The sedum is now dead, the plant next to it is wilted, two other plants have died this way too. What happens is the base shrivels up, the plant tips over, and dies. The roots are fine even as the plant dies

It happened once to this crassula of mine(sprawling). Reclone the plant.

Are you in summer or winter at the moment?
I think you may be getting a fungal disease, similar to damping off, which attacks the weak stems.
If you are in the northern hemisphere (guessing so, if you are short of light), keep it cool, and dry, so it doesn't grow weak, lanky growth which is susceptible to disease. Individual leaves or small plants will survive happily over winter at cool temps, and only resume growing in the spring as it warms up and gets brighter.

If it is S. rubrotinctum (it looks too blue/grey to me, and the leaf shape isn't quite right), leaf cuttings stay very compact for me at first, growing a tiny cluster of leaves before they grow any noticeable stem.

It's winter here. Several more plants have died. It seems to happen after they've been dry for awhile (the first few died a day after being watered, but they'd been dry). If it's a fungus, can I treat it with something? I was able to move the plants to a cooler, brighter place

Some plants like succulents, are more needy than cacti in terms of watering in winter- so keeping them dry for a long time is just as bad as keeping them humid. And i can tell you this much- clones are needy.

I found a leaf of the original dead plant in good condition today (5 days after the rest of the plant had died) which makes me almost certain it's a disease. I think the fungus(?) hadn't spread to that leaf yet when it broke off, so it's still looking ok while the rest of the plant's dead

Sometimes even leaves that are separated carry a disease in them - bacteria and fungi wait for an opening until the immune system is weakened and they close in to finish the job. And immune systems in plants are weakened by strain - and in this case its propagation.

If this is a fungus, what can I do to keep it from killing the rest of my plants?

Discard the plant. Avoid watering your plants for a while, use whatever fungicide is adequate for the soil.