The first symptom of aster yellows is yellowing of the foliage, followed by excessive growth and bunching of shoots. Older leaves become twisted and may fall off. Roots are misshapen, and of poor quality with many adventitious roots. The organism is introduced in the field by leafhoppers from overwintering infected weeds. The disease usually occurs sporadically with little economic loss.
Symptoms begin as small, yellowish spots on both sides of the foliage.
Spots enlarge quickly changing from yellow to ash-gray with the tissue drying in affected areas. The fungus also attacks leaf petioles and stems. Under conditions of high humidity and warm weather, a gray velvety growth can be seen on the surface of affected spots. The fungus is seed-borne and spores can also be blown in air currents for long distances. Warm temperatures are required for fungus development.