It's hard to determine exactly which disease(s) may be causing these symptoms, if any; it's also possible this is simply cold damage if the young plants were grown outside and experienced frosts and freezes after they sprouted. Cold damage sometimes mimics infection symptoms. If these were greenhouse- or hoop-house-grown (in a non-commercial setting), this might be a fungal or bacterial infection, given the typically reduced airflow and high humidity inside such structures. (Commercial greenhouses typically do vent well and nurseries maintain good airflow with multiple fans.)

For now, all you can do is to remove the dead leaves, weeds, and any individual leaves with heavier spotting and dead zones. Keep the plants in full sun (if they are already acclimated to that much light) and where they will get good airflow. Depending on when your plant sale takes place, the Echinacea might produce cleaner leaves by then, at which point you can remove a few more of any marred older leaves.

Leaf spot diseases can be difficult to tell apart from symptoms alone, especially if the pathogen is not yet producing spores. You can look at the undersides of spotted leaves for signs of spores, which usually look like gray mold or black specks. We do not recommend a fungicide treatment until it can be determined that an infection is the cause of the spotting. Even then, leaves with spotting will not heal. Additionally, fungicides only protect healthy growth from infection; they cannot cure existing infection, and symptoms tend to manifest days or weeks after infection has taken place, making it difficult to know if treating clean-looking leaves will be effective. Fungicide use also bears the risk of killing beneficial microbes that colonize leaf surfaces and out-compete pathogens, similar to the risks to beneficial insects of using broad-spectrum insecticides. Any plants that are too weakened or badly infected are best just replaced.

Miri