[Racehorse]

I did cover them with a vented plastic dome. BUT, as soom as I see they’re about to break surface I remove and stop using the dome.

Are you thinking even that much use might have promoted surface fungus growth?

Only other variables are the trays are on a heating mat under grow lights. Heating mat is turned off when ambient temperature reaches 50 degrees and turned on when I go to bed for the night or ambient temperature drops to 50 degrees.

[who knows what evil?]

As long as you removed it as soon as the seedlings came up; that shouldn't have been an issue as long as the soil wasn't damp. I cover mine with saran wrap, but remove it when they break the surface...no problems.

Fifty is kind of borderline, but many growers use the 'cold treatment' on young plants, resulting in the stockier seedlings you see at many of the 'box stores'. I am going to try it with some of mine this year, but will probably keep them around 60 at night.