By Susan Albert
The first time I saw the symptoms of aster yellows disease on my purple coneflowers, I thought aliens had invaded. The flowers had mutated into something out of a horror movie -- twisted leaves; stunted stems and petals; and tiny, green flowers helicoptering off the main cone in bizarre projections.
I rushed inside to look up aster yellows on the Internet and learned the disease affects many of the most popular garden plants: coneflower, aster, marigold, zinnia, petunia, snapdragon, periwinkle, and chrysanthemum. (I’ve seen the disease twice on purple coneflowers, and I believe my Mexican sunflowers (Tithonia) showed symptoms one summer.) Susceptible edibles include lettuce, tomato, celery, parsley, dill, and carrot. Other symptoms include yellowing or purpling of the leaves, and a “witch’s broom” effect. Ornamentals thought to be less susceptible to the disease include salvia, geraniums, verbena, and impatiens.
Migrating aster leafhoppers are the culprits. Using their sucking mouth parts to ingest plant juices, the leafhoppers pick up the bacteria from infected plants. After an incubation period, the insects transmit the disease when feeding on healthy plants.
Broadleaf weeds, such as dandelion and plantain, can harbor the disease, so controlling those weeds can minimize the spread of the virus. Covering fast-growing edibles, such as lettuce, with a screen may prevent leafhoppers from gaining access to the leaves.
Unfortunately there is no cure. It is best to remove the affected plants when you notice the symptoms. Discard them in the trash rather than the compost bin.