Answer: It's really difficult to diagnose a plant problem without being able to see it so I can only offer the most common problems with Black Eyed Susan plants: Angular Leaf Spots appear as angular, brown spots on the leaves that may cover the entire leaf. Leaf spots may appear water soaked. Symptoms begin on the lower leaves first and progress up the stem.
Septoria leaf spot is one of the most common foliar pathogens of Rudbeckia.
Symptoms begin as small, dark brown lesions that enlarge from 1/8 to 1/4 inch in diameter. Although the lesions are usually rounded, there may be angles where leaf veins limit the spread of the fungus. Symptoms of this disease may be difficult to distinguish from those of angular leaf spot of Rudbeckia.
Fungicides containing chlorothalonil or copper may protect new growth and reduce the spread of the disease, but preventative applications of fungicides should begin in early to mid June prior to the onset of symptoms.
Or, the problem might simply be weather related. Over or under-watering, dessicating winds, or hot summer weather can all cause similar leaf appearance. I'd pinch off the affected leaves (just in case it is a fungal or bacterial blight).
Best wishes with your plants!
Q&A Library Searching Tips