Some of mine are doing the same thing. I assumed those leaves stayed too damp and got diseased b/c it's been so wet and they were all close to the ground. Since it's dried out some, no new leaves have shown this problem, however, in the heat of the day, about 75% of my cuke plants' leaves are wilting. they perk right back up after they get some shade or it gets cloudy. There are a few plants that don't show any ill effects from the heat...and by heat, I mean 90 degrees since this is one of the coolest springs I can remember here. I may have planted them too densly (2 plants every 18 inches), but I know they are getting enough water since they are mulched and I water deeply when I water (which hasn't been much since it's been extra rainy here too). I noticed some aphids on a couple of the plants today, but they aren't on all the wilting plants. Just wondered if there is anything I could do to help them out. There are tons of flowers and I've harvested a couple of good-tasting cukes, but I am worried they might not make it long as it gets hotter.