Phlox, white and powdery

Meomye

How do i stop this please, as it happens every year. They start off fine then they go all white and powdery then the leaves shrivel and they don't flower.

K67

I would cut your losses and replace the plants, I assume you are referring to Phlox panticulata. My local supermarket sells them, or at least they did last year,  haven't shopped since March!
  You can buy sprays or try a homemade remedy. Dryness and lack of airflow doesn't help.
Or you could replace with more mildew resistant plants.

Palustris

If you really like Phlox paniculata, then try replanting them with a lot of water retaining crystals in the soil. They really do like a moisture retentive growing medium.