Disease in seedlings?

[oakley]

Clearly that tray is toast. Start another tray upstairs in a different environment. Make room. It is just one tray. Sterilize your soil
in your oven. If your soil is purchased claiming to be 'soiless/sterile' it may not be. Old stock or bad stock.
Many here have had bad situations with tainted soil.

I had drama in 2017. Similar. I cleaned and sterilized my seed starting room...then found an upstairs shelf to re-start my seeds in my pantry.
I had a tainted 'clean' starting mix that was not such. Disaster until I started over with oven baked soil.

I don't think most diagnosis are easily solved. Sterilize your trays, sterilize your soil.
Plant more than you have room for and cull.

Yes, I do want to solve the reason for disaster. But I want success for the upcoming season.

I do run a small fan 24/7 but colder temps may have contributed to the downstairs 'drying out' ?.

I do have 3 dozen micros growing all winter and fruiting now....not sure what I did correctly....always learning