Answer: I can't tell from your description whether the leaves are turning white (losing chlorophyll) or they are getting a white coating on the surface. A white coating indicates a disease called powdery mildew and once it attacks your plants, it's difficult to control. Watering at night shouldn't make a difference to your plants, but shady conditions and poor air circulation can encourage diseases. You might want to take a few of the affected leaves to your local cooperative extension office or well staffed nursery for a diagnosis. Helpful folks there can suggest controls for the problem. Good luck with your veggies!
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