Vector

• An organism called a phytoplasma causes aster yellows disease. Several different strains infect plants depending upon their growing location. Aster leafhoppers transmit this pathogen, which is similar in form to a bacterium, between plants. Aster leafhoppers are too small to clearly see without magnification. They are approximately 0.15 inches long with wedge-shaped bodies and nearly transparent wings with a pale green or light-brown tinge. These insects spend the winter near the Gulf of Mexico and migrate north during the summer, carrying the infection with them.
  
  

Transmission

• Aster leafhoppers become infected with the phytoplasma by feeding on the sap of infected plants. The organism multiplies inside the insect's intestinal tract and saliva, and feeding insects inject the pathogen into healthy plants. The phytoplasma can also attack the eggs in the leafhopper's ovary, so leafhoppers can become infected even before birth. The pathogen needs time to multiply inside the insect's body after the initial infection occurs, so leafhoppers cannot spread the disease between plants for approximately three weeks.
  
  

Symptoms

• The symptoms of aster yellows usually appear between 10 and 40 days after the plant has become infected. Some of the symptoms include stunted growth, curled foliage or deformed flowers. Lettuce leaves may have pinkish splotches, while carrots have unusually hairy or bitter roots and purplish-red outer leaves. Many infected plants develop rigid, stiff forms because of deformities at the juncture between the stems and branches. The leaves lose their color, but the veins stay green. Sometimes the flowers stay green as well, and fruit and seeds do not follow the blossoms.
  
  

Control

• Aster leafhopper populations are difficult to control in home landscapes, and pesticides offer limited benefits, according to David Whiting from the Colorado State University Cooperative Extension. Avoid yellow aster outbreaks by planting resistant species such as impatiens, salvia, geranium and verbena. Since dandelions and other weeds harbor insects and can become infected with yellow aster, promptly remove all weeds from around vulnerable plants. You can protect susceptible vegetables from leafhopper attacks by covering them with mesh fabrics. Straw mulch may also help reduce leafhopper populations.