Aster Yellows

Aster yellows symptoms and severity can be highly variable depending on the strain of the pathogen, age of plant when infected, and other factors. Celery plants are usually severely stunted and yellowed. Inner petioles are characteristically short, yellow to white in color, and moderately to severely curved and twisted. On older plants, petioles become brittle in texture and the epidermis and underlying tissues can crack and peel. In later stages of the disease, the inner heart of the plant turns brown and can decay. Fusarium yellows can cause similar stunting and yellowing. However, Fusarium yellows causes distinct vascular browning in the roots and crowns and does not cause petiole deformities.

Aster yellows has viruslike symptoms but is actually caused by the aster yellows phytoplasma, a single-celled organism that, like bacteria, lacks a nucleus and is therefore classified as a prokaryote.

Aster yellows is transmitted to crops by leafhopper insect vectors. Overwintering leafhoppers can harbor the phytoplasma, or leafhoppers can obtain the pathogen while feeding on infected plants. Important weed hosts include dandelion, plantains, pineapple-weed, Russian thistle, sowthistle, wild lettuce, wild chicory, horseweed, and wild asters. When leafhoppers migrate from pasture or noncrop land to vegetable fields, or when drying vegetation drives leafhoppers from foothills and other areas, the insects encounter celery and other crops and transmit the phytoplasma during feeding. Therefore, significant aster yellows outbreaks almost always occur in fields near pastures, rivers, ditchbanks, foothills, and weedy noncrop land. Because of fluctuations in the populations and flight patterns of leafhoppers, and fluctuations in the populations of infected reservoir plants, aster yellows incidence varies greatly from year to year. Overall impact on celery is generally low.

Aster yellows is difficult to control, in part, because of the extensive host range of the phytoplasma. Over 300 species of food, forage, ornamental, and weed plants are susceptible. While weed management should be practiced, this will have little effect on aster yellows. For particularly valuable plantings, don't plant celery on ranches that are near known phytoplasma and leafhopper reservoirs.

Cultural controls are acceptable for use on organically grown produce.

There are no chemical controls for the aster yellows phytoplasma. Insecticides will have little effect on leafhopper transmission of the pathogen and are therefore not recommended.