Aster Yellows is a common disease that affects many ornamental flowers. Susceptible plants include asters, chrysanthemum, coreopsis, cosmos, echinacea (coneflowers), dianthus, gladiola, marigold and petunias. Vein clearing, or loss of green pigment within the veins, is often the first symptom. Stunting, stiff extra bushy yellow growth, deformed or poorly developed flowers which remain green are all common symptoms. There is no cure for infected plants. Remove and discard them to reduce further spread.
I learned something about those funny looking purple coneflower plants that I have randomly growing in the garden. At the Master Garden plant diagnostic clinic last Friday, there was a table with the same funny looking Echinacea plants that I have growing at home. Fortunately, I have already been getting rid of them when I see them. The plants have what is called aster yellow and should be dug out of the garden and thrown into the garbage so this does not spread to the other cone flowers or other plants. So today, I went around and removed about 3 large plants with the distorted aster like flowers and threw them in the garbage.
Here is some info that I found describing this disease:
Purple cone flowers display some of the most dramatic evidence of this non-fatal, but potentially prolific disease. Secondary flower heads emerging from primary flowers can be a common sight. However, Aster Yellows is a disease that affects over 300 species of plants including herbaceous ornamentals, vegetables and even weeds.
Infected plants can serve as the starting place for the spread to other non-infected plants. The source of the problem is a bacteria-like microscopic organism known as a phytoplasma. A tiny insect known as a leafhopper most commonly spreads it. As the leafhopper feeds on infected plants, it is taking in this phytoplasma through the plant sap. Once inside the insect’s body, the disease organisms multiply rapidly. Eventually, the phytoplasma is then reintroduced as the leafhopper feeds on healthy plants.
Symptoms of Aster Yellows develop most quickly and are more severe when temperatures are warm. In cooler temperatures, plants may be infected without any visible indication. Unique symptoms also vary between plant types. Symptoms common to most infected plants include yellow foliage, stunted growth, flowers that remain on the green side, and an overall distorted look.
Controlling Aster Yellows is challenging. There is no known cure for the disease, and chemical control of the vectoring leafhopper is usually not effective and therefore not recommended. The best means of control for this problem, as well as many other garden diseases is good sanitation. Remove and destroy any infected plants you see immediately. This includes all weeds, since they can be a common host source for this disease.
Lastly, not all plants are susceptible to the disease. Most woody shrubs seem to avoid the problem as well as a few herbaceous plants such as salvia, geraniums and impatiens. Choosing resistant plants and removal of infected plants are the best control methods for controlling Aster Yellows in the home landscape.
This info was found here