Answer: It sounds as though powdery mildew was the culprit. Powdery mildew is common early and late in the season, when days are warm and nights are cool. You can't control the weather, but you can help your plants avoid this malady by planting in the sunniest spot possible (at least 8 hours of direct sunshine), and making sure there's lots of good air circulation all around. There are some protectant fungicides you can use to keep powdery mildew from attacking your plants, but these will not eradicate the disease once it is present. Safer's Garden Fungicide is one such product. Be sure to follow label directions carefully.
Hope you grow a prize winning giant pumpkin!
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