Jeanette: In my area, too much rain, or overwatering will cause it. Moreso, in the coolish weather of season's end. Wet plus cool equals Powdery Mildew on Squash, Pumpkins (and other cucurbits). How much rain have you been getting there?

Milk is a good preventive remedy but if the mildew has already taken hold then you may have to use copper or sulfur sprays to get rid of the mildew.
Safer's has a garden fungicide that is available at most garden centers or even Lowes and Home Depot.

Spray late at night when beneficial insect activity is low as the spray can kill some beneficials.

Personally, I use organic Neem oil to stop any mildew problems, its more expensive than the copper or sulfur and usually cost prohibitive for home gardeners but its an option.

Powdery mildew spores are carried by wind to new hosts. Although humidity requirements for germination vary, all powdery mildew species can germinate and infect in the absence of free water. In fact, spores of some powdery mildew fungi are killed and germination is inhibited by water on plant surfaces for extended periods. Moderate temperatures (60° to 80°F) and shady conditions generally are the most favorable for powdery mildew development. Spores and fungal growth are sensitive to extreme heat (above 90°F) and direct sunlight.

MANAGEMENT
The best method of control is prevention. Planting resistant vegetable varieties when available, or avoiding the most susceptible varieties, planting in the full sun, and following good cultural practices will adequately control powdery mildew in many cases.
However, very susceptible vegetables such as cucurbits (cucumber, melons, squash, and pumpkins) may require fungicide treatment. Several least-toxic fungicides are available but must be applied no later than the first sign of disease.

Resistant Varieties
In some cases, varieties resistant to powdery mildew may be available. If available, plant resistant varieties of cantaloupe, cole crops, cucumber, melons, peas, pumpkins, and squash. If you plant more susceptible varieties, you may need to take control measures.

Cultural Practices
Plant in sunny areas as much as possible, provide good air circulation, and avoid applying excess fertilizer. A good alternative is to use a slow-release fertilizer. Overhead sprinkling may help reduce powdery mildew because spores are washed off the plant. However, overhead sprinklers are not usually recommended as a control method in vegetables because their use may contribute to other pest problems.

A spray of either 1 teaspoon of baking soda in 1 quart of water, with a few drops of a light oil added, or a 50/50 mix of fat free milk and water applied every 5 days t9 infected plants does wonders. Check the link below for a good discussion on PM and what causes it, but disregard the solutions offered.

Here is a link that might be useful: Powdery Mildew

Jeanette, if you live in the Twin Cities and decide to try neem oil, you can get it at Patel's E.Indian grocery on 19th(?) & Central N.E. for a lot less $ than a greenhouse. It is shelved in the health & beauty section.

Marylandmojo, we had near constant rain from mid-April until last week.

Althea: Unreal. Two years ago in my area, we had way too much rain--standing water in low-lying areas. Blackened and killed nearly every Tomato plant in the area (though many gardeners with raised beds dodged the bullet), and Powdery Mildew on every cucurbit.

We had powdery mildew last year. Is there anything we can do here in Michigan prior to planting that acts as a preventative.

I know this post is pretty old but...
My zucchini plants started with white spots (powdery mildew) I sprayed neem oil on it and it seems to have gotten worse? Any suggestions as to why? A majority of the leaves are yellow and dying..it hasn't bore any fruit either.

Powdery Mildew is a general dusty white substance that forms on the leaves of plants and can easily be wiped off, it is usually not white spots. The link below may be of some help.
Yellowing of plant leaves, chlorosis, can be the result of a very bad case of PM but it also has numerous other causes. Plant leaves turn yellow when they loose the chlorophyll that makes them green and that usually stars with the soil the plant is growing in.
http://urbanext.illinois.edu/focus/chlorosis.cfm

Here is a link that might be useful: About Powdery Mildew