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Aster yellows
Aster yellows is a chronic, systemic plant disease caused by a bacterium-like organism called a phytoplasma.〔Bai et al. Living with genome instability: the adaptation of phytoplasmas to diverse environments of their insect and plant hosts. ''J. Bacteriol.'' 2006 May;188(10):3682-96.〕 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.〔Davis, M. R. & Raid, R. N., eds. Compendium of Umbelliferous Crop Diseases. St. Paul: The American Phytopathological Society. 2002. pp. 58-59.〕 Its economic burden is primarily felt in the carrot (''Daucus carota ssp. sativus'') crop industry as well as the nursery industry. There is no cure for plants infected with Aster yellows.〔Hudelson, Brian. Aster Yellows. University of Wisconsin Garden Facts. 2006.〕 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.〔
== Hosts and symptoms ==
Aster yellows affects a long list of plant species including native plants, ornamentals, weeds, and vegetables crops. The largest family affected is 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 from the greatest losses.
There are a range of characteristic symptoms which vary with the phytoplasma strain, timing of infection, plant species, temperature, age, and/or size of the plant.〔O’Mara, J., Bauernfeind, R., Stevens, A., Gast, K., & Steven, S. Aster Yellows. Cooperative Extension Service, Kansas State University. 1993. http://www.ksre.ksu.edu/library/hort2/mf1086.pdf〕 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.〔Agrios, George N. Plant Pathology. Burlington: Elsevier Academic Press. 2005. pp.691-694.〕
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