By David Whiting, Colorado State University Cooperative Extension, Horticulture
Aster yellows is a common plant disease that affects a wide range of flowers, vegetables and weeds. Susceptible flowers include asters, cockscomb, chrysanthemum, coreopsis, cosmos, echinacea (purple coneflower), dianthus, glads, marigolds and petunias. Entire plantings of marigolds can be destroyed. Susceptible vegetables include carrots, potatoes, onions and tomatoes. Dandelions and plantain are also susceptible and can serve as sources of the disease in home gardens.
The first symptom of the disease is vein clearing, the loss of chlorophyll or green pigment in the leaf veins. This is followed by yellowing of newly formed leaves, sporadic bushy growth, erect growing habit, and stunting. Stems and flower stalks may be numerous and spindly. Flowers often remain green and become distorted. Seeds and fruit do not develop. Specific symptoms vary with the kind of plant.
The leaves of infected carrots grow in tight bunches. The inner leaves are yellow and stunted, while outer leaves turn rusty red to reddish purple. The roots are bitter, stunted and deformed, with tiny hair-like roots growing all over the main root.
Infected glads may have thin, weak, yellow leaves, and the flower spikes may be twisted and deformed, while the flowers remain green. The whole plant is generally stunted and spindly, and the top is often killed.
Asters have stiff yellow growth with many secondary shoots and are stunted. In warm weather, symptoms are more severe, and appear more quickly. At 55 degrees or less, plants may be infected without the symptoms being obvious.
Aster yellows is caused by a tiny organism known as a phytoplasma and is spread from plant to plant by leafhopper feeding. Touching a plant will cause them to hop or fly away quickly.
Once infected, there is no cure. Diseased plants should be promptly removed and discarded to reduce further spread. Control weeds, such as dandelions and plantain, which may harbor the pathogen. Plants and some varieties vary in resistance. Some research indicates that the use of oat straw mulch may reduce leafhopper numbers. Insecticides are generally not recommended for control of leafhoppers in the home garden.
Photos: Judy Sedbrook
© CSU/Denver County Extension Master Gardener 2010
888 E. Iliff Avenue, Denver, CO 80210
Date last revised: 01/05/2010