The GARDENA gardening expert
Dear Ms. M., mildew has attacked your phlox plant. The summer phlox (Phlox paniculata) is comparatively susceptible to this disease, in particular the older varieties. Have a look under Phlox paniculata for some varieties which would suit you. Also, have a look at the large-leaf phloxes (Phlox amplifolia), which are less susceptible to mildew. The old garden saying applies here: “A summer garden without phlox is not a summer garden at all”!
In addition to planting “better” varieties, it is important that you optimise the location conditions for your phlox plants, for example by achieving sufficient location ventilation. The phlox plants must not stand too close to each other, and the neighbouring plants may not grow too densely or high.