Looks like downy mildew.

Pick that cuke or it will shut down further production.

My thought about the 2nd image is heat/sun damage -- ran short of water.

Looks like the same thing I have which I think is mildew, I spray with Immunox for garden, It seems too help but I plant 6 hills an 3 plants to a hill so I have plenty of cukes if a plant or 2 dies, I just now have all my canning of cukes done an I have plenty to give away.

Agree - Downey Mildew.

Dave

Immunox is good at preventing powdery mildew but not downy mildew, which despite the name is an entirely different disease.

A mix of chlorothanolil and mancozeb is probably as effective as anything against it. But this is another of the diseases that it's easier to prevent than cure.