The plant disease called aster yellows seems to be hitting St Louis pretty hard this year. It thrives in cool wet weather which we have had plenty of this spring. Aster yellows will effect many common garden plants, in my garden it is on the purple coneflowers and black eyed Susan's. Aster yellows is caused by a leafhopper that transmits a phytoplasma to the plant that causes it to deform. The plant will be deformed by having mini flowers sticking out of the flower head, looking chlorotic, and twisting growth. The only way to keep the disease from spreading to other plants is to remove the infected plants, and don't put them in the compost. It is hard for me to rip out a plant but I remind myself that it is no longer able to perform it's ecological function and may spread the disease. I made myself a nice bouquet of aster yellows infected flowers for my dinning room table while I was at it.
In addition to aster yellows there is a similar looking deformity that is caused by a mite living in the flower head. These plants will have a normal looking foliage and flower heads except for a deformity in the center of the head. If the plant is infected with the mite the flower head can be removed and not the whole plant.
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