Powdery mildew is a white, powdery looking fungus the grows on leaves. Pumpkin plants seem to be particularly susceptible to it and once you get it, you can't really get rid of it unless you use some very powerful fungicides that I prefer not to use at all. An ounce of prevention is worth a pound of cure in this case.
Usually around the first few weeks of August I'll start seeing some signs of powdery mildew. Getting a little isn't a big deal, but if it gets out of control it can take a plant down. Usually that isn't a problem for healthy plants, but powdery mildew will damage leaves and as a result affect pumpkin growth. So what I typically do around this time of year is use Actinovate. It is an organic fungicide that seems to work well if applied a couple of weeks before the powdery mildew starts showing up. It won't knock out powdery mildew once you have it, but from experience I can say that it reduces the amount of powdery mildew and can help slow it down when you do get it.
The day before yesterday I started brewing some compost tea and put a touch of Actinovate in the brew at the beginning. I then added some more the last 15 minutes of the brew. That compost tea was then splashed on all of the leaves of the plants. I found that works pretty well. So far this year I've seen no signs of powdery mildew (knock on wood).
When powdery mildew does start, another organic way to help contain it is spray some silica on the leaves. It will actually be used by the plant to form a silica barrier around the infected area to help contain the infection.