Roses are more vulnerable to PM when their foliage isn't getting enough water from their roots, which often happens on young or transplanted roses. Keep the plant well watered though not drowning and as it settles in it may be more resistant, if it is not a cultivar prone to PM. Some are. You can spray off the foliage with water every morning, provided it is sunny/warm enough to dry in a few hours, to wash off spores. You could spray with a fungicide if you are really concerned. PM spores float through the air for miles and miles, there isn't anything you can do about that except keep your plants as strong and healthy as possible, so their natural resistance can best protect them.

Cadence, one caution about horticultural oils. If the temperatures rise up to 80 or so, you'll begin to exchange mildewed foliage for burned foliage. I do like horticultural oils for control of mild transient mildew, but I am cautious. I've even had burned foliage in temperatures barely over 75, so I've become very cautious about using hort oils. I have had good luck using SERENADE, a bacterial compound, and you might try that. Jeri Coastal Ventura Co., SoCal

You can go ahead a cut them back. Just keep them watered and they'll come back nice. I use Bayer Fungicide for trees and shrubs. You only need to spray it every 2 weeks. Works great for me.

Thank you Henry, I had powdery mildew ony Winchester Cathedral. It is in the best light with drip system. I will be more vigilant with the Neem oil