Tall Garden Phlox; no, this is not the perennial that flowers in the Spring; that’s its vertically challenged cousin the Creeping Phlox. This one comes into bloom in the mid to late summer; which makes it a much sought plant if you have a sunny location.
It does have a nemesis though; a powdery mildew. And when the weather gets hot and sticky this fungus comes to life.
Dormant spores on the ground, underneath the plant, get kicked up onto the plants by means of rain, sprinklers, or the occasional water balloon fight. Once firmly ensconced on the leaves they marry and begin to have offspring of their own at a rate that would make a rabbit jealous.
They do seem to co-exist for a while, but in time will cause the leaves to drop off. But truthfully I don’t really know if they are dropping off because of the fungus. Remember that car you had with the nasty dent; it never did see the car wash much after that structural anomaly; we sometimes treat our “less than perfect” plants like that. We look at them and say in our mind “shoot, you’re dying anyway, why waste water on you”.
A Multi-propose Fungicide will help them recover. Better yet in the spring and fall soak the ground around them with the Fungicide and kill the spores before they start traveling.