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.