If you have noticed that your coneflowers (Echinacea) or black-eyed Susans (Rudbeckia) are looking strange, they may have “aster yellows” which is caused by a phytoplasma, a small bacterium. This is a disease that affects more than 300 species of plants, including asters, coneflowers, zinnias, marigolds, heleniums, and chrysanthemums. It also appears on vegetables such as garlic, lettuce, carrots, tomatoes, and celery.
Aster yellows is primarily transmitted via leafhoppers. Leafhoppers are small insects, only a few centimeters long, with wedge-shaped, brown, yellow, or green bodies. As the name suggests, they quickly hop from plant to plant. When a leafhopper feeds on a plant infected with aster yellows, the pathogen enters the leafhopper’s body and stays within for as long as that leafhopper lives. So as it feeds on plants and moves around from plant to plant, it spreads the phytoplasma thus spreading the disease. Once a plant is infected, if it is not removed, it remains a host plant — a source of phytoplasma for the rest of the plants in the garden.
Symptoms vary depending on the plant but in my garden, the flowers are grossly deformed. The flower heads are twisted and some are producing small tufts of green growth in the center of the flower. Some petals are too short or green–often there is more green than the color the flower should be. Once plants are infected, they should be removed from the garden. They cannot be cured.
The only thing you can do to prevent aster yellows is to remove and destroy diseased plants as soon as possible to prevent the spread, control weeds which may harbor the disease, and purchase ornamentals that are not as susceptible such as verbena, salvia, nicotiana, geraniums, impatiens, and cockscomb.
If your plant does look weird but you are not sure if it is because of aster yellows, take a clipping to a local plant clinic (usually at farmer’s markets or libraries) or ask your extension agent.