Aster yellows is a fairly common disease that can cause bizarre symptoms on a wide variety of plants. The disease can be found on hundreds of plants, including many flowers and vegetables, such as coneflower, daisy, marigold, zinnia, snapdragon, chrysanthemum, tomato, carrot and lettuce.
Typical symptoms of aster yellows include veins that turn pale, yellowing of new leaves, abnormally bushy growth, deformed flowers, and stunting. Infected plants often have a stiff, upright appearance, with branches joining the stem at narrow angles. The appearance of this disease varies with the host plant. On coneflowers, the most typical symptom is replacement of flowers by tufts of small, green, deformed leaves. Infected carrots roots may be excessively hairy and bitter, and in lettuce, inner leaves may be curled and twisted, whereas outer leaves do not develop fully and have pink or tan spots. On onion, leaves are small, twisted and yellow. Symptoms are more pronounced in hot weather, and in cool weather a plant can be infected without showing any symptoms.
Aster yellows is caused by a tiny organism called a phytoplasma, similar to a bacterium. The phytoplasma is carried from plant to plant by aster leafhoppers, which feed on the sap of the plants.
No treatment is available to save a plant infected with aster yellows. Aster yellows is best managed by removing infected plants from the garden to minimize spread. Management of the insect vector is not usually feasible in a home garden.