did you harden them off properly.. before taking them from inside to outside??

At this stage of the game, it is more the soil temps that are of concern rather than ambient air temps. If the soil is adequately warm, a few cool nights should not bother them.

Powdery mildew on cucurbits is typically a mid- to late season issue as the disease is most encouraged by warm, dry days; warm, dry soils; still air and lots of nighttime humidity. It is rarely a spring concern.

No, they cannot. As GG mentioned, it's more about the soil temp. And they like it WARM - not lukewarm and certainly not cool. Ambient temps haven't been consistently warm enough to thoroughly heat the soil yet, at least not here, and I doubt they've been in your location, either. If you're lucky, they'll sulk and recover fine, but more likely they're going to perform cr*ppily, if they don't rot out in cool, wet soil first. Buy fresh transplants, wait until the soil has thoroughly warmed, and chalk it up to a lesson learned.

most veg is best in full sun ..... and they would need to be hardened off not only to temps ... but also to sunshine ...

did you save any of the seed ... if so... plant those in the ground in a week or two ... and then compare your early started.. to those ... in the long run.. you might be surprised how little difference it makes.. come september ... and the crop you end up with ...

ken

40’s will not kill them, mine see these temps at night for months while in the greenhouse early on and then outside later on. But i agree with what everyone else has said, melons and watermelons (in fact all warm season crops) need consistent warmth overall. A few nights in the 40’s or a few days in the 50’s should not hurt if they are otherwise hardened off and they eventually get the heat they need.

FINALLY, someone who knows what they're talking about.. I have a crap load of melons growing and it's hit the 40s many times. Slow down a little but , not much. They will grow til frost