Answer: What you are describing could be several things at work. First, if you don't dead head or trim away the faded blackeyed Susans they do tend to darken as they fade and both of these perennials will turn somewhat shabby after blooming especially in a dry year. Deadheading will help both to stay more presentable but is more important on the purple coneflowers. On the other hand, the birds love the seeds so many gardeners do not deadhead these plants. Second, Japanese beetles and grasshoppers and other insects will occasionally munch on purple coneflower petals. These plants may also succomb to mildew and a sort of "yellows" similar to aster yellows, but these are fairly unusual. If you suspect a disease, remove and destroy all plant trash as it occurs and do the same with any trimmings from those plants this fall.
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