Answer: Once powdery mildew takes hold, you can't cure it but that doesn't mean your plants will develop it again next year. This is especially true if they are getting full sunshine and are not overcrowded. It's getting late in the season and the tops will die down with first frost so if there's any green left in the leaves, I wouldn't cut them back until after frost kills them. Any green in the leaves will still be actively processing energy from the sun and transporting it back to the roots. If the leaves are dead, cutting them down to ground level is fine - they've done their job for this year.
Be sure to remove all the plant debris this winter so your plants will get off to a healthy start next spring.
Best wishes with your garden!
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