It could be aster yellows. There doesn't appear to be yellowing of foliage or green flowers that I can see. Those are common symptoms. You could send a sample to the Plant Disease Clinic if you like. There is a fee. You can remove them and replant also.https://pdc.umn.edu/
Someone else has mentioned that eriophyid mites have been a problem this year. Maybe you should wait to pull them and watch for any other signs before you do.