I'm trying my hand at winter gardening under a hoop house. Lately I've had some issues with some of the plants: Some of them look fine, but some have suddenly wilted and have become lifeless. First up, my lettuce. Some of them look very happy, but a few have suddenly gone limp.






Same thing for beets. Some look happy, some have wilted.




My broccoli shows different signs of being unhappy, with yellowed leaves and brown edges.



Meanwhile, my kale and spinach all seem to be doing great. If I were to make a guess, I'd say it might be because I'm growing these veggies under a Remay hoop house. The weather is unusually warm this week, and before I could get out to open up the hoop houses in the morning, at least twice the temperature has gotten up to the mid-90s in the hoop houses. Could it be these brief high temps that are making my veggies unhappy?

Yes, those brief high temps could wilt those cool-weather crops, especially if they are somewhat dry. I would think they could tolerate your current outdoor daytime temps. Even on days that seem cool by your standards, if the sun is shining it generates quite a bit of heat inside the hoop house. You might experiment with leaving the hoop house partly open more often during the day. Also check your watering.

In that first photo, try pressing the soil firmer around the roots of that one lettuce that wilted, and water it. Since it seems to be the exception in that section, it could be rooted more loosely than the others. Those others look lovely

The broccoli has Black rot: those yellow V-shaped lesions drying on the inside are textbook first symptoms.
If you are growing kale nearby (another Crucifera), dispose of that plant, rake up any material and for the next couple of years do not plant any other Crucifera there.

The other plants suffer of a common problem when growing under poly-tunnels: air temperatures are too high while soil temperatures remain low. In several plants this induces a very fast vegetative growth (read: leaves) while the roots develop much slower. When a certain tipping point is reached the far less developed roots cannot support the plant's leaves anymore and there's a sudden collapse. These plants cannot be salvaged. This problem can be remediated by removing the tunnel covering during the hottest hours of the day but it may prove unpractical in case of old fashioned poly-tunnels. In that case switch to less susceptible leaf vegetables like mizuna and romaine lettuce.