Have you very closely inspected the plants for bugs? How much are you watering them? What kind of growing medium are they in and where did you get it? Where did you get your seed? What NPK ratio of fertilizer are you using and what frequency are you fertilizing them? Do you live in an area with high air pollution?

Here is a link that might be useful: Start a growers diary here

That does not look like a disease to me. Did you plant in the greenhouse or move them out there after they got started? The reason I ask is it looks a little like what happens when seedlings are not hardened off.
It Could also be possible over watering as you mentioned and it may be getting too cold at night.

Jon