Your description sounds like a disease called "aster yellows." Aster yellows cannot be treated. It is transmitted by leafhoppers, and it is simply not feasible to significantly reduce the leafhopper population. Therefore, the only approach is to pull and destroy infected plants right away. Since aster yellows also lives on some weeds, you should keep your garden weeded. The good news is that purple coneflowers self-seed prolifically, so the ones you lose to aster yellows are usually easily replaced.

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