Aster yellows

Aster yellows
Symptom range: healthy carrot is on right, seriously damaged carrot on left
Common names	Aster yellows phytoplasma, AYP
Causal agents	Phytoplasma
Hosts	Several, see text
Vectors	Aster leafhopper (Macrosteles quadrilineatus)

Aster yellows is a chronic, systemic plant disease caused by a bacterium-like organism called a phytoplasma. The aster yellows phytoplasma (AYP) affects 300 species in 38 families of broad-leaf herbaceous plants, primarily in the aster family, as well as important cereal crops such as wheat and barley. Symptoms are variable and can include phyllody, virescence, chlorosis, stunting, and sterility of flowers. The aster leafhopper vector, Macrosteles quadrilineatus, moves the aster yellows phytoplasma from plant to plant. Its economic burden is primarily felt in the carrot (Daucus carota ssp. sativus) crop industry, as well as the nursery industry. No cure is known for plants infected with aster yellows. Infected plants should be removed immediately to limit the continued spread of the phytoplasma to other susceptible plants. However, in agricultural settings such as carrot fields, some application of chemical insecticides has proven to minimize the rate of infection by killing the vector.

Aster yellows affects a long list of plant species including native plants, ornamentals, weeds, and vegetables crops. The largest family affected is the Asteraceae, and ornamental plants commonly infected are asters, marigolds, coreopsis, and purple coneflower. Regarding vegetable crops, onion, lettuce, celery, and carrot are affected with the latter suffering the greatest losses.

The range of characteristic symptoms varies by the phytoplasma strain, timing of infection, plant species, temperature, age, and size of the plant. The symptoms can be mistaken for herbicide damage. They include vein clearing until the entire leaf becomes chlorotic, stunting, deformation, virescence (greening of flowers), phyllody (development of leaf-like flower petals), reddening of foliage, reduced root system, and sterility. Aster yellows does not typically kill perennial host plants.

Characteristic symptoms specific to the carrot include initial vein clearing and chlorosis, followed by production of many adventitious shoots, with the tops looking like a witches’-broom. The internodes of such shoots are short as are the leaf petioles. Young leaves are smaller and dry up while the petioles of older leaves twist and break off. Any remaining older leaves turn bronze or red late in the season. Floral parts are deformed and roots are smaller, abnormally shaped, and have woolly secondary roots. The carrot roots are predisposed to soft rots in the field and storage and taste unpleasant to the consumer.