Michelle Grabowski, UMN Extension Educator
Aster yellows is caused by a tiny bacteria - like organism known as a phytoplasma. Phytoplasmas live with the phloem (nutrient conducting vascular system) of plants. The aster yellows group of phytoplasmas can infect over 60 families of plants including many common garden perennials, vegetables and weeds. Although many gardeners are noticing disease symptoms on coneflowers, infection of other plants such as cosmos, golden rod and carrot have also been reported.
Leaves of plants infected with aster yellows are often discolored yellow to red. The phytoplasma affects normal growth and development of the plant, resulting in clumps of weak shoots known as witches broom developing throughout the plant or on flower stalks. Flowers are often the most visibly affected part of the plant. Flowers often remain green and are distorted.
The aster yellows phytoplasmas are moved from plant to plant by aster leaf hoppers (Macrosteles spp.). These insects have sucking mouth parts and they inadvertently suck up phytoplasmas with the plant sap they are feeding on. From then, the phytoplasmas actually live and reproduce within the aster leaf hopper.
M. Grabowski, UMN Extension
Photo 2: Witches broom on the a goldenrod flower infected with aster yellows
Unfortunately once a plant is infected with Aster Yellows, there is no way to cure it. Perennial plants may survive many years with the aster yellows phytoplasma and can serve as a source of the disease within the garden. The best way to deal with aster yellows is to remove infected plants. Plants infected with aster yellows can be thrown on the compost pile or buried because the phytoplasma will not survive once the plant dies. Unfortunately removal of infected plants is no guarantee that the disease will not return to the same garden. Infected leaf hoppers and infected perennial weeds can both be a source of new infections in the garden.