"All I can suppose is that the nasturtiums I planted in front of it are out of control, and maybe they're taking too much of it's water? I ususally water in the afternoon"

Are the nasturtiums planted only in front of this sick plant? I also think that water in the morning is much better. Do you also mulch heavily around the head of the plants? Touch around and see if it is particularly dry around the head of this particular plant. Don't just pull up this sick plant now before you find out why, do some experimentation and learn something!

It's possibly the Squash Vine Borer. I've found SVB eggs on my cukes before. And your symptoms seem to match -- wilting and coming back. Look for a hole in the fine, esp. near the base, with frass coming out.

That should have said, look for a hole in the VINE. . . .

Hmmm...well, there are two plants next to each other and only one is wilty. The nasturtiums are in front of both plants. I don't have any mulch down in the garden (a huge sin, I know), but the garden patch holds water very well. I'll try my best to get out and water in the mornings, but I have a newborn and a toddler and it's usually 11am before I get my first cup of coffee! I guess I should just be happy I got any plants in the ground at all this year.

I hadn't thought of SVB on cucumber plants. I actually chased one moth around the garden with a huge metal spoon one day, cursing and screaming (I never got her). Since it's been more than ten days since that incident (the neighbors are convinced I'm insane), I assumed somehow I'd escaped as my zucchini plants so far are fine. Though today I trimmed some branches off them that had wilted leaves and found the stems completely hollow.

I feel the beginning of the end...

Ah, well, at least we got a dozen or so zucchini and some cucumbers. I won't give up on the plants yet, I guess.

Thanks for your responses.

It's always this part of the summer when I feel I don't control the garden, it controls me.