Aster yellows is caused by a phytoplasma, a small bacteria that lives only within the vascular system of a plant or within the aster leafhopper that carries it from plant to plant. Once a plant is infected, the aster yellows phytoplasma moves systemically through the plant, infecting every part from the roots through the flowers. Symptoms of the disease include yellowing of leaves and stems, unusual flower formation and clusters of weak stems known as witches brooms. The aster yellows phytoplasma can infect over 350 plants including many common vegetables, flowers and weeds. Once a plant is infect, it can never be cured.
M.Grabowski, UMN Extension
Photo 2: Petunias with yellow leaves and small discolored flowers from aster yellows
In 2012, unusually high numbers of aster leafhoppers migrated into Minnesota from southern states where they overwinter. Many of these leafhoppers were carrying the AY phytoplasma. As a result exceptionally high rates of aster yellows was seen throughout Minnesota.
So what is the status of aster yellows in Minnesota in 2013. Happily there have been significantly fewer reports of disease throughout Minnesota in 2013. This is likely due to a combination of factors. Aster yellows can survive Minnesota's winter in perennial plant parts like the crown of an infected purple cone flower. Gardeners that did not remove infected perennials last year may see symptoms again this year. The aster yellows phytoplasma that infected any annual plant like a tomato or cosmos, however, would have died with the plant at the first hard frost. Very few aster leaf hoppers overwinter in Minnesota as eggs and these do not carry the aster yellows phytoplasma. In addition, surveys indicate that lower than average numbers of aster leafhoppers migrated into Minnesota in 2013. Gardeners should continue to watch for symptoms of aster yellows and remove any infected plants.