We aren't able to confirm with certainty the presence of either organism from the photos, and the Coneflower Rosette Mites are nearly microscopic, but you can use the information and comparison images on the pages linked below to help judge which may be causing the symptoms you're seeing. If this is Aster Yellows, the symptoms aren't well-developed enough for us to be more certain that it's the cause. Mite-damaged flower heads can be pruned off (and that's the simplest way to deal with outbreaks anyway), but Aster Yellows is systemic and symptoms will worsen over time (perhaps not this year, given how late in the season it is, but next year) and cannot be pruned off to save the plant. If you decide to trim this plant, you should clean the pruner blades with a sanitizer like a disinfectant wipe or rubbing alcohol. (Avoid bleach, as it can damage the metal and make microscopic pits that are not easy to sterilize.)

Miri