Typical symptoms of aster yellows include chlorosis or pale leaf veins, yellowing of leaves, abnormally bushy growth, deformed flowers, and stunting. Flowers may be deformed with unusual growths of deformed leaves inside the flower or replacing the flowers.
Prevention is best achieved by clean garden practices. Removal of infected plants and keeping the garden free of weeds is the best way to control the spread of this disease. Here are some tips that I have used to control this disease in my plants:
1.) Leafhoppers spread the disease from plant to plant; therefore, don't provide them with conditions they enjoy. Leafhoppers prefer tall grass and weeds so it's important to keep your grass cut short.
2.) Try to edge your flower beds so that the grass is further away from your Echinaceas. Remove the grass and edge the flower beds about 4-5 inches beyond the plastic edging.
3.) Apply Bug-B-Gone to your yard as per the instructions on the bag. The bag indicates 3 months of protection. I used the product in a red bag and applied it with a spreader.
4.) Plant Echinaceas in flower beds against the house or other grass-free areas. Any Echinaceas that are out in the open more are more vulnerable to leafhoppers. If the flower bed is bordered by grass on more sides, there are more areas for the leafhoppers to enter from.
5.) Remove any infected plants as soon as possible. Discard them with the trash. Don't leave debris from infected plants near your other plants.
I hope these tips help others deal with this problem, which seems to be widespread this year.
Note: I'm sure some will not agree with using chemicals; however, I feel the problem is so severe this year that chemicals are required. If you have something to add, please feel free to comment.
Update: Echinacea tennesseensis, pallida and paradoxa appear to be more resistant to Aster Yellows than Echinacea purpurea or hybrids involving it. Some have asked me for info on resistant cultivars. These are the only 3 varieties I've never seen with Aster Yellows. I'm not guaranteeing this information, but I thought it would be beneficial to others looking for tougher plants.