Certified GKH Gardening Expert
If you're not seeing powdery mildew on the plants, there are two possibilities that come to mind. Your plants like a neutral pH soil, so if you've fertilized them, it may be too rich for them. The other possibility is that they may be wanting more sun.
Check for powdery mildew and leaf spot: https://www.gardeningknowhow.com/ornamental/flowers/black-eyed-susan/rudbeckia-leaf-spot.htm