While it is OK to spray in the morning, a better time to spray would be just before sunset. Then the spray will dry slowly and have all night to help acidify the leaf surfaces. Your entire cucurbit crop can be given a preventative spray every week to ten days.

The following are some homemade powdery mildew sprays:

Three tablespoons of baking soda, three drops of detergent, and three tablespoons of vegetable oil per gallon of water. Mix well.

Two tablespoons of Apple Cider Vinegar per one quart of water.

Twelve tablespoons of a 3% Hydrogen Peroxide solution per gallon of water.

As with any homemade solution, test the solution on several leaves and wait a couple of days to see if the solution damages your plants before spraying the entire crop.

Thank you for your question and have a great gardening experience.

Copper fungicides kill the pathogens that cause powdery mildew while the acidic fungicides ( homemade) inhibit their growth. The copper based fungicides can be used on the entire crop to prevent the further development of fungus based diseases.