Cause Candidatus (Ca.) Phytoplasma asteris is spread by leafhoppers. The organism overwinters on wild weed hosts. Aster yellows also affects carrot, lettuce, endive, tomato, parsley, onion, salsify, many weeds, and several ornamental flowering plants. The disease is rare in Oregon.
Symptoms Plants are chlorotic, dwarfed, and many secondary shoots develop from the leaf axil. Many shoots also develop from leaf axils on secondary shoots, so the plant has a bushy or witches' broom look. Some younger leaves show vein clearing, and most are dwarfed, yellow, and misshapen. All leaves are usually stiff, rigid, and somewhat thickened with enlarged midribs and lateral veins. Fruits are small, malformed, and lighter than normal in color.
Cultural control No control attempt has been completely successful. Controlling weed hosts and leafhoppers is of some value.