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Crystal Ball Garden
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Aster Yellows
  by Gail Ruhl, Plant Disease Diagnostician and Peggy Sellers, P&PDL Director

Aster yellows is a disease caused by a phytoplasma (formerly known as a mycoplasma) that attacks a wide range of vegetables, ornamentals and weeds. Common plants affected include carrot, chrysanthemum, cockscomb, coreopsis, marigold, statice, and strawflower.

Aster Yellows on Mum

Phytoplasmas are smaller than bacteria but larger than viruses. Phytoplasmas can only survive and reproduce in live plant tissue and cannot be isolated and cultured using conventional laboratory methods. Positive identification of phytoplasmas requires the use of an electron microscope. Diagnosis of aster yellows in most labs is done by comparing the suspect plants to the typical symptoms in disease identification photos.

Symptoms of the disease can include yellowing (chlorosis), stunting, and abnormal flower development. This disease causes parts of flowers (petals, anthers and pistils) to revert to vegetative or leafy structures that remain green. Aster yellows overwinters on perennial host plants and is spread by leafhoppers (primarily by the aster leafhopper).

The leafhoppers serve as vectors of the phytoplasmas. Leafhoppers must first feed on a plant that is infected. Approximately 10 to 14 days must pass between the time a leafhopper feeds on an infected plant and when it is capable of transmitting the phytoplasma to new plants. During this period (latent period), the aster yellow's phytoplasma actually migrates into the body of the insect and reproduces within its cells. The leafhoppers are rendered infective (capable of infecting new plants) only after sufficient amounts of the phytoplasma have migrated to their salivary glands and remain infective until they die.

Control of aster yellows is best accomplished by preventing the entrance of the phytoplasma into the garden. Plant only healthy seeds, cuttings, and plants. Many weeds, including dandelion, plantain, and ragweed are also infected with this disease and can serve as a source of the phytoplasma in your garden, thus weed control is important in disease prevention. Diseased plants should be promptly destroyed and discarded to prevent further spread. Insecticides to control leafhoppers in home gardens are generally not recommended.

--from Plants and Pests, Purdue University 
This page is updated monthly.
Revised:  September 04, 2017