How Can a Planting of Coneflower Be Treated for Aster Yellows?

Check for leafhoppers in the coneflower bed. Adult insects are 4mm long, about the size of the white seeds in a watermelon. The leafhoppers have tinted, almost transparent wings, and their heads may have black spots. The insects hop or fly away quickly when plants are disturbed, and may look like gnats fleeing the garden.

Confirm the presence of aster yellows. Examine the foliage and flowers. Aster yellows, unlike "normal" chlorosis, causes the entire leaf, including the veins, to pale, and turns flowers bronze, yellow or greenish-yellow, even if the flowers are normally another color.

Dig up all infected coneflower plants with a shovel or spade. Place the unearthed plants in a bucket or wheelbarrow. Consider packing the plants into a plastic bag to keep any remaining insects or bacterium contained until the plant material is safely out of range of healthy coneflower plants.

Dig up weeds, grasses and other unwanted plants growing near the coneflower bed. Add this material to the plastic bag, if used, and place it in the bucket or wheelbarrow. Transport the material out of the garden and destroy it.

Spread oat straw mulch around the remaining coneflower plants. According to the Colorado State University Cooperative Extension website, oat straw mulch may reduce the leafhopper population.

Spray an insecticide formulated specifically for use against leafhoppers while the insects are present in large numbers. Insecticides are often ineffective and may not save the plants. Follow the label directions for application rates and timing.