It sounds like bacterial wilt, which is transmitted to the plant by insects, primarily cucumber beetles, but any insect (squash bug) can vector the disease. To check, cut off an infected runner close to the plant with a sharp knife, press the cut surfaces back together and hold for a moment, then slowly draw them apart. If whitish strands form between the cut surfaces, it's bacterial wilt, and can't be cured.

Wayne - I went out and checked the bad vine, but there was no discoloration of the stem tissue, as I think Fusarium is supposed to cause. The good thing would be that the Charentais are resistant to F wilt. Of course, this variety of Galia is supposed to be, too. Bad thing would be, nothing to do about it now. Charentais still looking good. Rest of Galias holding on. [I only grow the Galias to have melons before the Charentais are ripe.]

Thank you. guys! In my reading last night I did figure out that it was squash bugs. I understand that the threat from them is that they can chew the leaves or fruit and damage them. In this process. They can also be a vector for introducing bacterial wilt? I definitely have a few leaves that have been eaten and these guys are probably the culprits. A few of the vines that are in full sun definitely do wilt during the day, but perk back up late in the day. I read that this is a sort of self-protective behavior and like you had said, not always caused by under-watering. I cut off the end of the wilty vine and buried most of my vines. I read about the test for bacterial wilt, so I may attempt that - because if this plant DOES have BW, I need to remove it entirely, correct? I'm evaluating my options for purging the squash bugs. I hate the idea of collateral damage.

Next year order a batch of preying mantis eggs ahead of time.