Answer: Based on your description it is possible they were affected by a disease such as Septoria leaf spot. To some extent this will be dependent on the weather. The best things to do are clean up all the fallen leaves and stems and do a thorough clean up of the bed in the fall to limit carryover to the next season. (Put the debris in the trash, not the compost pile.) Make sure they are planted in a full sun location with good air circulation. Keep them divided and thinned as needed so the plants are not overcrowded. Avoid wetting the foliage if you water them.
There may be a need for fungicide. Please consult with your local Rutgers county extension for their recommendations as to what to use and when/how is best to apply it for the maximum results. They should also be able to verify the diagnosis -- you really need to know exactly what it is before you can treat it.
In the meantime, here is some information you may find helpful, click on the photos for enlarged views. You may need to cut and paste the complete url into your browser to make it work correctly.
Good luck with your black eyed Susans this summer.
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