Powdery mildew is a disease caused by fungus that affects a wide variety of plants.   Plants of the family known as Cucurbits, which includes cucumbers, squash, zucchini, gourds, melons and pumpkins are very succeptable to powdery mildew.  We lost all of pumpkins here at the yardstead last year to powdery mildew.  Fortunately powdery mildew is easy to identify and treat.  It can be identified by white powdery spots, usually starting on the leaves of the plant.  It usually starts as small white spots which become larger and more numerous pretty quickly.  If left untreated the white spots can consume most of the leaves which turn yellow and wither, causing the plant to die.  The key to treatment for powdery mildew is to identify and treat plants as soon as any sign of powdery mildew is detected.  The treatment for powdery mildew usually consists of a fungicide applied by spraying.  There are several commercial fungicides available, some of which are made from natural plant oils.  Last year we succesfully treated our zuchinni plants.....
with Neem Oil. 
Neem oil is a vegetable oil harvested from Neem trees which are native to the indian sub-continent.  Neem oil has many uses from homeopathic medicine to insect repellent and is a potent fungicide.  Here is another green product that works great for powdery mildew and many other fungal infections:

The picture above was taken today of a zucchini plant in our garden.  Almost all of our squash and zucchini has the small white spots seen in the picture above.  I first noticed the white spots about 5 days ago.  I should have treated the plants right away, but we are out of neem oil and my schedule has prevented me from working on it until today.  I have seen plants die in as little as two weeks after the white spots first appear.  Powdery mildew spreads very quickly to nearby plants due to the huge nubers of spore produced by the fungus.  Hopefully its not too late for my squash and zucchini plants, which are producing very nice vegetables.  I would hate to lose these plants, but we have more squash seedlings ready for planting, which were intended to extend our harvest season.   These will go in the garden as soon as the fungus is eradicated or at least controlled. 
Kathleen has found a recipe for a homemade remedy for powdery mildew which I plan to apply with a pump-up sprayer today.  I will be writing another article in about a week to report the progress of efforts to stop the fungus.  If the homemade remedy works well, I will also include the recipe in the article.  If it does not appear to be working after a few days, I will be heading to the store for some more neem oil to try and save what is left of our squash and zucchini.  If you would like more information in the mean time you can post question, comments or suggestions in the vegetable gardening forum.  Don't forget to check back in about a week for full details and results of our experiment with the homemade powdery mildew remedy.