Could be the heat - do they refresh overnight?

And that last photo looks like an older leaf - they don't last forever. The new growth looks pretty healthy to me...

Thanks Carol,

You know it is not clear from the photos, but some of the affected leaves are new leaves, not just the old ones. It seems to me to be some sort of a disease that is only affecting some of the plant, not all of it. I have something similar, and more extreme going on with my zucchini. Whole sections of the zucchini have yellowed, wilted and died. And, it does not refresh over night. It is a puzzle. I have grown squash for over 50 years, and never seen this.... Well - there is something new every day in this wondrous and mysterious world!!!! Thanks for your reply.,.

Another possibility might be if you see cucumber beetles (striped or spotted) on the plant, they can spread a bacterial wilt disease on both cuke and squash plants.

Thought about borers too - those guys are sneaky, and you need to do some sleuthing along the stems to find where they've tunneled in.

I seem to recall seeing that after removing the pests, you can bury the affected stems so they'll form new roots...?

I tend to have leaves start to look like that on all my zuc's, squash, cuc's etc. Old age and likely some fungus or dieses. I spray 3 fungicide's rotating them and have no issues with squash bug or borers. I have never been able to stop leaf issues like that. We are very humid here so it is different.

Thanks to all. I will look for signs of the squash borer, or the cucumber beetles. So far, I have not seen them. It is quite a mystery. Part of the plant turns yellow and dies, and the rest of the plant just keeps growing and making fruit!!!! Oh well, as I said, life is full of mystery, and I think that I will get a fair crop in the end, so not all is lost. Not yet at least...

Sqash bug hatchlings also spread a wilt that causes leaves to look like that. Butternut can take much more of it than zucchini.