It looks like mildew. A lot of my plants had it this this year. Some are more prone to it than others but in my garden it affects phlox, verbena bonariensis, some some hardy geraniums and honeysuckle. I never use chemicals and just hope it doesn't come back next year. I do take off the affected leaves when I first spot it but it seems to be a bit like blackspot on roses; once it's there, it's there.

6 parts water to 4 parts milk and spray at regular intervals. (perhaps weekly) If the plant is prone to mildew you can begin spraying earlier next year as a preventative.

Thank you both. I wondered about mildew but phlox is not a plant I know much about. Thanks.

Very susceptible to it are Phlox, especially when planted in free draining soil. They do like a bit of dampness.
You could scrabble round in the soil at the base of it and add some water retaining crystals. They would keep the soil moist when the plant needs it.

I've just chopped mine right down as it was covered in mildew and the leaves were all going brown from the bottom up. I've tried the milk spray and another one I found where you mix bicarbonate of soda with water and spray it. I have given it up as a bad job and may have to move the plant. Such a shame as it was a beautiful colour.

We have some gorgeous blue phlox that so far haven't succumbed to mildew and they are 3 years old. I must try Owdboggy's idea about adding moisture retaining crystals.

Thanks everyone. I think it may be best that she tries the crystals as well.