Might be bacterial wilt, spread by cucumber beetles.

Before jumping on the disease bandwagon consider all the other much more common possibilities first. Could be just insufficient water especially during the heat of the day. So first you try giving it a good watering to see if everything perks back up.

Or a damaged stem somewhere under the growth where it can't be seen that causes only random leaves to wilt so if you can find the damaged area you bury it under soil or you jest remove the damaged portion and the rest is fine.

And when those options are checked then you look for the little gray squash bug nymphs sucking on the underside of the wilting leaves or nearby and remove them. Lastly, especially on whole wilted plants you look for SVB.

Once all those options are exhausted then you start thinking about the possibility of a disease.

Dave

+1 what Dave said.

Look for a broken stem on the single leaf drooping and the SVB when the whole plant is fine in AM but droops by PM.

I agree with digdirt. Disease isn't always the problem. Cultural issues are much more common. For example, the wilted plant is right under a conifer. Not a good spot for a water hungry, leafy vegetable. Start with applying a deluge of water and work from there.

http://u.osu.edu/vegetablediseasefacts/cucurbit-diseases/bacterial-wilt/