Tomato seedlings dying

[Ganhada]

Your seedlings are damping off. You will have to start over. They were probably too moist, try not to water the next ones as much.

[Chris]

And if you have them in a little covered tray, take the cover off to let some humidity out. They're essentially rotting while alive in that high humid environment, like gangrene for plants.

[Ganhada]

I wonder if those plastic covers are always advisable. I live on the west coast and I have almost done away with them, I think there might be enough moisture in the air. And then if you live in a drier climate, you could use them judiciously