....too late, after to many plants had spots and lost most of the plants.  This year we were planning on spraying our plants with a homemade remedy for powdery mildew (see recipe below), but procrastinated and let them go.  We did get several weeks of good fruiting and put several gallons of squash and zucchini in the freezer.  We would probably have had a few more good weeks of fruitng if the plants had stayed healthy. 
In my experience the best way to deal with powdery mildew is to apply a fungicide as soon as the first signs appear.  Once the powdery white spots are  spread throught your garden, its probably to late to get good results from a fungicide.  Although leaves with spots usually die, new stalks and leaves can continue to grow from the stem, but rarely get large enough to fruit.  Most infected plants in our garden stop fruiting shortly after the white spots are visible, although it may take several more days for the leaves to wilt and turn brown.  Neem Oil is a good natural fungicide which we have used in the past.  It works well if you apply it as soon as the first white spots appear.

Lately we have been using a home made mixture of 1 part milk to 9 parts water, apllied with a spray bottle to the top and bottom sides of the leaves.  It seems to work just as well as the commercial products, as long it's applied as soon as the white spots are discovered.