If you should get mildew on your squash leaves, trim all affected leaves from the plant and do one of the following.
Spray the non affected leaves (after trimming bad leaves) with 1 tablespoon of wettable sulfur in 1 gallon of water, every two weeks. And spray in the evening so that the sulfur won’t burn the leaves. Most leaves need to cool off before you spray anything on them. Spraying on hot leaves might saute them. Be sure that you buy wettable sulfur NOT soil sulfur.
Another cure is to merely buy a gallon of skim (non fat) milk and spray it on the squash leaves. Follow the directions above for time of day and trimming all bad leaves off before spraying. You can also mix up your own non fat milk by using one part dry skim milk with 9 parts of water.
Just wanted to pass this along to those that are plagued with mildew and we here on the Southern California coast are and we deal with it constantly.