the browning leaves at the base show drought from earlier on this year, insufficient water at the time, phlox are thirsty plants. If the white on the leaves is like a powdery deposit, its mildew, spray with a 1 in 10 solution of milk to water, or spray with an appropriate fungicide.

I'm glad i read this, i didn't realise that Phlox were that thirsty, i'll remember that.