is there any way to rid my garden of powdery mildew during winter now that my infected
phlox are over?
edited September 2019
You don't need to do anything like that. The emphasis is on good plant care for the growing season. Plants affected with Powdery Mildew are weakened by unfavourable conditions. Usually too dry conditions by lack of watering or in some shrubs, long periods of dry weather. That can be local weather at the time, growing in an area where there is too much sun when the plant prefers some dapple shade, and finally, some plants are just more prone to it.