My tall phlox do this regularly, I don't know why. I don't agonize over it, sometimes I pull off the brown leaves, sometimes I don't bother. It doesn't prevent me from keeping the phlox in my gardens, tho -- I love them anyway!

I suppose if I really was sensible, I would have grown other plants near the base of them, to hide this ugliness. But at age 78 now, I am past being too persnickety. I am just happy they still bloom.. They are fond memories of my childhood.

IF you get mildew on yours, which phlox can be prone to getting, I have found the following to be helpful, and it has worked quite well for me: Mix one part of milk (any % of fat - skim will work) with 9 parts of water in a spray bottle and apply to the leaves. It will get rid of the mildew, altho you may need to apply more than once. Worth the effort IMO.