Aster yellows is a disease caused by a phytoplasma, formerly known as a mycoplasma. Phytoplasmas are the pathogens most similar to bacteria. They grow in the phloem tissue of plants and cause distortions in plant growth. The aster yellows phytoplasma is transmitted by sucking insects—in this case, leafhoppers—and cannot be cultured in a laboratory.
You see this disease most commonly on chrysanthemum, aster, daisy, marigold, phlox, and petunia, but it occurs on many other species as well. Affected plants are easy to distinguish because they are yellowish, stunted, stiff, erect, and bushy due to their witches’-brooms. The flowers may be deformed with partially or totally green leafy petals. The plants look as if they have been affected by a virus.
Because the pathogen can be transmitted by leafhoppers, control measures include destroying all affected plants when they are first seen, eliminating broadleaf weeds that can host the pathogen, and, of course, buying symptom-free plants. Spraying regularly to keep leafhoppers from feeding may be beneficial in a commercial setting.
We do not see many aster yellows samples in the lab, partly because these plants are rogued before they reach the retail outlets. Symptoms are easy to identify by comparing plants to disease identification photos. Because the pathogen cannot be cultured in the lab, sending a sample to the Plant Clinic is not necessary. For a detailed description of these diseases, consult RPD No. 903.