Recent reports of distorted flowers on purple coneflower (Echinacea purpurea) in Allegany County, New York, remind us that aster yellows remains a threat to iconic fall plants. Despite the name, however, the disease affects more than 300 species of plants, including vegetables and field crops as well as ornamentals.
Aster yellows is a viral-like affliction caused by a phytoplasma vectored by sap-sucking insects – especially the aster leafhopper. Here’s how it works: When a leafhopper feeds on an infected plant, the bug itself becomes permanently infected with the phytoplasma. Within weeks of contact, the phytoplasma cells multiply and affect the insect’s salivary glands. The leafhopper then feeds on a healthy plant, injecting the phytoplasma cells into the plant phloem, thus completing the cycle of infection.
Susceptible plants will begin to show signs of infection anywhere from 10 to 40 days following the onset of infection. Chlorosis is a significant symptom; growth also tends to slow, and leaves may appear smaller and narrower than normal. Foliage can also curl.
Among the most obvious symptoms, however, is the appearance of a bizarrely deformed flower. Strange, prolific tufts of deformed leaves emerge inside the flower or in place of a bloom. It’s all too creepy.
What to do?
Sanitation is key. Once a plant is infected with aster yellows, it’s a lost cause. The disease is not lethal, but considering the havoc it wreaks upon otherwise lovely blooms, it might as well be. Aesthetic value is all but eliminated, and it’s best to start again with a clean crop. Prompt removal – at the first emergence of symptoms – may help to stem the spread of the disease to nearby plants.
Planting less susceptible plants can help to prevent aster yellows, but because so many popular varieties are vulnerable, this is no easy task. So far, verbena, salvia, nicotiana, geranium, cockscomb and impatiens have proved to be among the least susceptible. If you want that iconic coneflower look, be vigilant and diagnose early.
Remove weeds that may harbor the disease. You don’t want them in the area, anyway.
Insect control is critical, and here’s a fascinating tip: Placing strips of foil between plants – encouraging bright reflections of sunlight – can confuse the heck out of leafhoppers. Of course, in a garden setting this may not be the look your clients are going for. Again, keeping plants clean is important.
And here’s another fact that’ll add to the confusion: Researchers have discovered that an unnamed eriophyid mite may cause symptoms that appear quite similar to those of aster yellows. No surprisingly, sanitation can help to control this problem.
Photos courtesy of Missouri Botanic Garden