Symptoms and Signs
Symptoms of aster yellows on lettuce include blanching and chlorosis of the young heart leaves. The center leaves fail to develop normally and remain as short, thickened stubs in the middle of the head. Outer leaves become yellow and twisted. Pink to tan latex deposits collect on the underside of midribs of affected leaves. If young plants are infected, outer leaves yellow and the plants may be severely stunted. If older plants are infected, latex spots in the interior of the head may be the only symptoms.
Comments on the Disease
The aster yellows phytoplasma overwinters in the body of adult leafhoppers and in perennial or biennial host plants, including native plants, ornamentals, and weeds such as Russian thistle (Salsola tragus), sowthistle (Sonchus spp.), prickly lettuce (Lactuca serriola), dandelion (Taraxacum officinale), and many others. The principal vector is the aster leafhopper, Macrosteles quadrilineatus, although the phytoplasma can be transmitted by many other species of leafhoppers. Leafhoppers can remain inoculative for at least 100 days. In some areas, the occurrence of aster yellows may follow the pattern of leafhopper migration.
To control aster yellows:
- Remove weed reservoirs near lettuce fields.
- Control vectors with insecticides.
- Avoid planting lettuce in regions and locations that have a history of aster yellows, as this indicates reservoir hosts in the area.
However, control measures are not usually warranted. There are no resistant or immune lettuce cultivars.