Answer: Because it sounds as though you're doing everything right in terms of culture and care, I wonder if your coneflowers have developed a disease called Aster Yellows. This disease is spread by leafhopper insects and there is no cure. The best recourse is to remove the plants from your garden. Before doing anything drastic, though, why not take a sample of the problem to your local Cooperative Extension office for a positive diagnosis? If it is Aster Yellows, there is a list of susceptible plants you'll want to avoid planting in your garden. Good luck with your coneflowers!
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