too much water from watering and the tips of the leaves die. The cells burst from water. Too much water from rain and the fungus spores in the soil get splashed on the lower foliage and the fungus spreads.
If the leaves are normal looking, but wilted, you probably have this bad nematode or some sort of parasite that blocks the plants from taking up water. It basically eats up all the little root hairs until the plant is just unable to give itself water. If it looks ok in the am, but shitty after being in the hot sun, then it is probably those parasites. My book isn't nearby, but I'll look it up for you.
For now, just give it long, deep waterings. I'll check if there is a chemical or cultural solution. Did you buy the plants or start them? Sterilized potting soil to start?
http://vegetablemdonline.ppath.corne...omleafkey.html
http://blogs.cornell.edu/hort/2010/0...-blight-blues/
I might be confusing two plant diseases, follow the top link for diagnostic