Starting medium.

Indeed curious that this happened in TWO plants. The constricted stem at the base makes it looks like these plants were suffering for a long time. This didn't happen suddenly. You might look up Rhizoctania root rot. It's a fungus. A fungicide soil drench is how you get rid of it, but allowing the soil surface to dry may help prevent. If these were in containers, I'd sterilize or dispose of the soil.

Good point! There is one marasa nd three Feher Ozon in the same container, so I'll let the surface dry, but not tossing it yet.

Just a small point. Although this is likely a fungal issue, it isn't, strictly speaking, damping off. That term refers to the collapse of tiny seedlings, not mature plants.

Thanks all! Apparently at this age it's called stem rot. Any thoughts about whether these plus, in a sub irrigation planter, might contribute? Ice never had this before, and I've been growing in these platers for many years.

You just had a bad fungal spore land in your bed, and it did mischief. People never get the flu for many years, and then they suddenly get it. Again, do not reuse that soil without sterilizing it. To the extent that a SIP keeps the surface moist, that's probably not in your interest.

"do not reuse that soil without sterilizing it."

I've found the oven method, but this is a lot of soil. I have about twenty 10 gallon planters full of pro-mix, usually half fresh, sometimes all fresh. Hopefully I only have to worry about THIS one. I seem to have read about ways to do it outside under tarps during the summer. Does anyone know if that works ?

You need 140F+ for 30+ minutes. Under a tarp in full sun, you can get those temps, but only for the top layer. If you have a thick bed, you won't sterilize all of it.

It would be helpful if the planters themselves weren't hurt by those temps. If so, you might be able to fit two in an oven, and you'd be done in an afternoon.