Wish I could help you as I love my prairie sun rudbeckia. Maybe if you could put a picture of the leaves it would make for easier identification.

Good luck with them they are truly stunning.

SCG

Sounds like powdery mildew or downy mildew. I have been trying milk and baking soda to control powdery mildew on a few of my plants. Hard to tell if it is working though. GL!

From UCONN IPM site:

Downy Mildew
Downy mildews symptoms appear very rapidly and tend to be much more difficult to control than powdery mildew. Conditions that promote leaf wetness, such as high relative humidity, overhead irrigation and close spacing, favor downy mildews. With a hand lens, closely examine the bloom of sporangia (microscopic stalks bearing spores) on the underside of the oldest leaves. (They will look like many tiny branched trees, each bearing tiny lemons). This can help you distinguish downy mildew from powdery mildew. Aster, Artemisia, Centaurea, Geranium, Lupinus, Potentilla, Rudbeckia and Viola are susceptible to down mildew.

Here is a link that might be useful: UConn IPM site