If the foliage on your coneflowers looks dead but are still blooming, they likely have gotten infected with a foliar fungal disease. It's also possible that they have root rot. In either case it may be related to wet weather conditions. Poor drainage can cause the roots to rot. In this instance, a different location may be the answer. In the case of foliar fungal problems, fungicidal spray may be necessary to protect the leaves from getting the fungus. It's too late this year, but if it's wet next year, consider applying a spray.