Is it for sure rot or a cut worm? I have had this happen before with tomatoes and peppers. It looks like rot but upon further investigating it was cutworms and grubs doing he initial damage and then yes once the stems were chewed on they got some rot and perished from lack of water

It is called damp-off or dampening off, a fungus problem that develops in overly wet soil

Dave

"Was wet and watered a lot." There you go. Once seedlings are up, soil surface should be allowed to dry out. The roots are down under. They don't need the surface to be wet.

Yep. Yep. Damping off for sure, some call it "wire stem". To me it seems like beets and chard are especially susceptible to this, at least they are in my garden. If you are doing the watering, you need to back off as the two above mentioned. It can be hard to keep it dry if you are direct seeding in the ground and getting a lot of rain.

If it were me, I would go ahead and pull the plant. It may survive, but it will never fully recover and be productive. Next season I will be starting beets, and chard indoors to give them a head start.

In my neighborhood the soil is very sandy. I guess the area was a potato farm before homes were built. Hence I can grow some nice carrots. The downside is trying to keep seedlings and seeds adequately watered until roots are established without overwatering