Okay... I just found this line: (7) Destroy volunteer overwintering plants and avoid planting near established diseased crops. from the following site: http://plantpathology.tamu.edu/Texlab/Multicrop/asteryellows.html Does a volunteer overwintering plant mean from seed too or just from a from a plant that survived the winter? I wonder if the warning for not planting new plants near currently infected plants only means during the same season, or if it also includes a new growing season where once infect plants once were. The only spaces in my garden for the new coneflowers are where the diseased were pulled up. :c( Should I not even bother?

I am not find similar photos... though I am not the best on filtering searches. Anyway, I would still be suspect of this growth, it doesn't look anywhere normal to how an aster would originally emerge from the spring, and it doesn't look like the existing growth. There are other issues that can cause witch's broom type growth, not just AY. I would keep an eye on it

Potting soil that has any sort of degradable organic matter in it - peat, coir, bark fines, compost, etc. - can generate fungal organisms. Typically they will be saprophytic or only affecting the decaying organic matter and pretty much harmless to the plants. I certainly wouldn't throw the plant away!! Just scrape the offending material off. And maybe look at your watering regime (consistently moist soil is more likely for these to occur than a faster draining, less organic mix).

Ah, Heterotheca subaxillaris (Camphorweed) confirmed by its mentholy aroma when rubbing its sticky leaves...thanks!