A healthy coneflower (Echinacea purpurea) shows purple ray flowers on a cone-shaped head. Infected with the yellows organism, these flowers turn into distorted, green leaflike structures. Infected plants are usually stunted, often with yellow or reddish leaves.
Yellows disease is caused by a microorganism called a phytoplasma. Phytoplasmas are smaller than a bacteria but larger than a virus. They can be difficult to detect and identify because they only survive and reproduce in living plant tissue. They cannot be isolated and cultured in a laboratory. An electron microscope can be used to detect structures of phytoplasmas in the cells of host plants.
Phytoplasmas live in the phloem (food conducting tissues) of their host plant. The disease is spread from plant to plant primarily by phloem feeding leafhoppers. The leafhopper acquires the phytoplasma while feeding by inserting its stylet (a long, slender hollow feeding structure) into the phloem of infected plants and withdrawing the phytoplasma with the plant sap. After an incubation period, the insect spreads the disease organism to healthy plants as it feeds.
The yellows phytoplasma has a wide host range, including echinacea, monarda, caraway, marigold, snapdragon, aster, mum, daisy, carnation, strawflower, carrot, broccoli, tomato, radish, squash, ragweed, thistle, plantain, and dandelion, and others. Symptoms are similar on these plants, and include yellowing, stunting, abnormal production of shoots, sterility of flowers, and malformation of organs.
Phytoplasmas survive from year to year in perennial ornamental, vegetable, and weed plants. The incidence of yellows diseases often depends on leafhopper populations.
Infected plants cannot be cured and should be removed. Removal of perennial weed hosts in the area will help reduce losses. Control of the leafhopper vector may help reduce transmission of the disease. At this time, little effort has been made to develop plants with resistance of phytoplasmas. Some work has been done with the use of antibiotics to alleviate phytoplasma infections in high-value perennial plants.
This article originally appeared in the July 24, 1998 issue, p. 103.