Aster yellows is a plant disease which affects a wide range of plants including flowers, vegetables, and weeds. Susceptible flowers include aster, chrysanthemum, cockscomb, coreopsis, cosmos, daisy, dianthus, echinacea (coneflower), gladiolus, marigold, petunia, and phlox. Susceptible vegetables include carrots, onions, potatoes, and tomatoes. Weeds such as dandelions, plantain, and thistle are also susceptible and can serve as a source of inoculum in home gardens.
The cause of aster yellows is a microscopic organism called a phytoplasma (formerly called a mycoplasma-like organism or MLO). Phytoplasmas are most similar in size and composition to bacteria. Aster yellows is vectored (transmitted) by insects called leafhoppers, and through grafting. The phytoplasma survives winter in perennial and biennial plants. In the spring, leafhoppers feed on infected plants. During feeding, plant sap containing the phytoplasma is sucked into the leafhopper's body where multiplication of the microorganism takes place. Following an incubation period, the phytoplasma is transferred to healthy plants during leafhopper feeding.
Specific symptoms vary from plant to plant. Symptoms of aster yellows are more severe and appear more quickly during warm weather. At lower temperatures, plants may be infected without symptom expression. Symptoms of the disease usually include stunting or dwarfing, yellowing of the foliage, and production of many spindly stems and/or flower stalks. Flowers often fail to develop color, remain green and distorted, and seeds or fruit never develop.
Carrots and marigolds can be severely affected by aster yellows. Leaves of infected carrots grow in tight bunches. The inner leaves are yellow and stunted, while outer leaves turn rusty red to reddish purple. The roots are bitter, stunted, and deformed with tiny, hair-like roots developing all over the main root. Infected marigolds have small, yellow leaves. Infected plants may also produce a proliferation of spindly, secondary shoots, giving the plant a bushy appearance. Flowers that are produced are often irregular, deformed, and green. The whole plant is generally stunted and spindly.
Control of this disease is best accomplished by preventing the entrance of phytoplasmas into the garden. Although certain varieties are more resistant than others, once infected there is no cure. Promptly destroy and discard diseased plants to prevent further spread. Remove weeds (many act as reservoirs for the microorganism) and monitor plants for leafhoppers. Insecticides are generally not recommended for the control of leafhoppers in the home garden.
The Ortho Problem Solver. 1982. M.D. Smith, ed., Ortho Info. Serv., San Francisco, CA. 1022 pp. (see pages 384, 396, 721, 861 and 994).
Agrios, G.N. 1978. Plant Pathology. 2nd Ed. Academic Press, NY. 703 pp. (see pages 521-525).
Chad Behrendt, Crystal Floyd