Powdery mildew is a fungal disease which affects many types of plants.  Cucurbits like zucchini, squash, pumpkins, melons and gourds are all succeptable to powdery mildew.  It usually appears on leaves as small round white powdery spots which quickly spread and cover the entire leaf.  In our garden here at the yardstead we've lost many zucchini, squash and pumpkin plants to powdery mildew.  This year powdery mildew killed all of our zucchini and most of our paty pan sqash and yellow squash.   
It is usually easy to identify powdery mildew by its appearance on the top of the leaves.  It usually looks like white powder on the leaves.  Infected leaves usually turn yellow and wilt after just a few days.  Powdery mildew usually appears in our garden on one or two plants then quickly spreads to all the other zucchini and squash plants.   The infected plants usually don't die completely and frequently send out new leaves and flowers.  We have been able to recover from powdery mildew in the garden a few times, when we caught it early and sprayed all the plants with neem oil.
On a few other occasions I applied the funigicide ...........


....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.