Looks like powdery mildew has taken control of your cucurbits ;-(

At this stage with yield being affected and some appearing to be near death it may be too late to do much.

There are fungicides you can use to try to control the fungus, but they aren't 100% effective and if plant damage is too severe it will still die.

Anyway, a rounded tablespoon of baking soda with 1 gallon of water sprayed on the plants to the point of run off is a non toxic and tested fungicide.

If you prefer the chemical route Daconil is available for edible plants as well (but check the label, not all are suitable for edibles).

Regardless of what choice you make please take the time to google on 'powdery mildew' and read up on ways to prevent it. Prevention isn't 100% either, but every little thing we can do helps.

Good luck.