I've grown many Echinacea over the last 25 years and have never had Aster Yellow . but everything I've read says the plant must be destroyed as soon as you see it so it does not spread.
Since we were also talking about Hellebore , they can get a virus that leads to Hellebore Black Death. If you see this plant must be destroyed ASAP so it does not spread. This I did get on one plant just glade I saw it in time.
Both good reasons to not overcrowd your plants.
