You seem to have done some homework on this. The websites below describe the yellow aster problem. What I don’t see in your pictures are the typical green leaves that invade the flower petals or the yellowing leaves. You could try and capture mites on fly paper. The best way to resolve this is to send a sample to the plant disease clinic. Another option is to take a sample to a Ramsey County Master Gardener diagnostic clinic. See the websites below. 1. https://hortnews.extension.iastate.edu/aster-yellows 2. http://www.missouribotanicalgarden.org/gardens-gardening/your-garden/help-for-the-home-gardener/advice-tips-resources/pests-and-problems/diseases/viruses/aster-yellows.aspx. 3. https://pdc.umn.edu/ 4. https://www.ramseycounty.us/residents/environment/master-gardeners