Reporting
Yes, powdery mildew is a

Yes, powdery mildew is a well-known nuisance on phlox. Usually, this is not a serious enough problem to kill the plant in its first season, but cut back all the disease and remove the stems this fall (after frost) s the fungus does not survive the winter. In the spring, watch for new development and spray the foliage with a fungicide or dust with sulfur. Also, make sure the plants are well spaced to allow good air movement throughout the foliage. Water plants early in the day so leaves dry quickly.
Do not over fertilize (especially with nitrogen).