Answer: What you describe sounds like powdery mildew and yes, it's quite common this time of year. It attacks plants most readily when daytime temperatures are warm and nights are cool. It means the season is winding down. It will eventually kill the plants but it won't harm the squash. You can harvest your squash when the rinds harden - when you cannot pierce the skin with your fingernail. If they do not harden before the vines die or the weather turns cold you can still harvest them, but they won't store well and you'll want to use them within a few weeks of harvest.
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