The summer of 2012 brought to Wisconsin an increase in “aster yellows” a plant disease that occurs throughout North America. The disease affects over 300 species of flowers, vegetables, and weeds. Aster yellows is caused by a bacteria-like organism called “aster yellows phytoplasma.”
This pathogen is spread by adult leafhoppers, green, wedge-shaped insects about 4 mm long. Aster leafhoppers, which attack a broad range of plants, acquire the phytoplasma by using their piercing mouthparts to suck plant juices from an infected plant. After a minimum of 11 days, the insect is capable of transmitting the pathogen by feeding on uninfected plants. Two or three generations of leafhoppers are expected during a season, but 2012 was special. According to Brian Hudelson, director of the Plant Disease Diagnostics Clinic at UW-Madison, there was a huge influx of leafhoppers very early last year. The leafhoppers presented a 10% infectivity rate compared to a typical 1 or 2%. As a result, many plants exhibited symptoms of the disease.
Often, symptoms, which may not appear until 20 or 30 days after infection, are mistaken for damage due to unintended herbicide exposure. Twisting, stunted growth, wilting and a bushy, yellowish appearance are some symptoms but they vary depending on the plant. New foliage on carrots becomes yellow and twisted. Older leaves turn bronze and break off easily. Carrot roots become covered with root hairs and develop a bitter taste. Coneflowers may display a tight rosette sometimes referred to as “witches’broom”. Secondary flower heads may emerge in a cluster from the primary coneflower head. Leaves can form where flowers should be present and often petals are green or nonexistent. In marigolds, the flowers may appear leafy and a muddy greenish color. Carrots and lettuce suffer the greatest losses in vegetables and many popular annual and perennial flowers are severely affected. A partial list of susceptible plants can be found at: http://ipm.illinois.edu/diseases/rpds/903.pdf
Aster yellows infection is permanent and there is no known cure. The best way to deal with the disease is to remove infected plants. They can be composted or buried because the phytoplasma will not survive once the plant dies. Removal of infected plants is no guarantee that the disease will not return to the same garden. Migrating infected leafhoppers and infected perennial weeds such as Queen Ann’s Lace, ragweed, and dandelions serve as a reservoir of new infections.
Commercial growers are encouraged to contact the nearest extension office for treatment thresholds and infectivity rates. Placing aluminum foil strips between rows as mulch is thought to protect plants by disorienting the leafhoppers. Floating row covers can also be used to keep them away. Spraying for leafhoppers in home landscapes is not recommended. Most trees, shrubs, and some less susceptible plants (geraniums and impatiens) can be planted where aster yellows is a problem. Keep a sharp eye out for symptoms in your garden.
Master Gardener Intern