Black-eyed Susan (Rudbeckia) often suffers from mildews. The best defense is a good offense: buy mildew-resistent varieties and keep the plants spaced apart to encourge good airflow.

Rudbeckia is considered to be a short-lived perennial, usually blooming best during its second, third and fourth year -yearlings sometimes bloom as well :) Since rudbeckia usually multiplies nicely, it is not difficult to have new plants available to replace very diseased or elderly plants. Because of the ease with which it gets mildew, I usually pull up & discard infected plants as soon as they show signs, although many gardeners simply prune the mildewy stems back and the plants apparently will bloom the following year; while other gardeners often simply purchase new plants on a triennial basis.