Attached is a link to a study from NCSU. They have 'David' as highly resistant, but in my experience it gets infected just as often and just as bad as other phlox. If you are in an area where humid summers are the norm, mildew will always be a problem you'll need to deal with. While many will say air circulation will be important, and it is, it won't do much if you don't have air movement. Powdery mildew likes moisture in the form of water vapor, but not as free moisture sitting on leaves. Some of the best phlox I've seen in someone's garden was in a part with overhead irrigation that had almost no mildew while the same cultivar less than 10 feet away but with no irrigation was covered in mildew.

Here is a link that might be useful: Mildew Resistant Garden Phlox