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Aster Yellows

July 22, 1998

This disease can occur at any time during the growing season but stands out now, in the middle of summer. Infected plants characteristically have green flowers with leafy petals. Plants are stunted, stiff, erect, and bushy. In Illinois this disease occurs most commonly on chrysanthemum, aster, daisy, marigold, and petunia, but it can occur on many other species as well. The plants look like they have been infected by a virus; for many years that was thought to be the case.

Aster yellows is a disease caused by a phytoplasma. Phytoplasmas (formerly known as mycoplasmas) are pathogens similar to the more familiar fungi, bacteria, and viruses. This group is intermediate between a bacterium and a viral pathogen. Important facts to remember about phytoplasmas are that they are transmitted by sucking insects (in this case, leafhoppers) and they cannot be cultured in a laboratory.

Because the pathogen can be transmitted by leafhoppers, control measures include destroying all affected plants when they are first seen, eliminating broadleaf weeds, and, of course, buying symptom-free plants. Spraying regularly to keep leafhoppers from feeding may be beneficial in a commercial setting. Refer to the article on flower hoppers in issue No. 11 of this newsletter for more on control of the insect vector.

We do not see many aster yellows samples in the lab. Part of the reason is that these plants are rogued before they reach the retail outlets. Symptoms are easy to identify by comparing suspect plants to disease identification photos. The pathogen cannot be cultured in the lab, so sending a sample to the Plant Clinic is not necessary. For a detailed description of these diseases, consult Report on Plant Diseases No. 903, Aster Yellows.

Author: Nancy Pataky


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