Also picking up the effected leaves when they drop, *cleaning up in the fall is best, so it doesn't over winter on top of the soil*
and thinning the plant
*ie: bee balm is famous for powdery mildew...
thin every two or three plants
out to give room
in between the plants
for aeration and free moving air flow*
and watch your own watering practices,
to water early in the morning....
so the soil has a chance to dry out during the day...
and to "water their toes, not their nose"
meaning=water the soil, not on top of the plant