Aster yellows of garden flowers and vegetables
Aster yellows is caused by a small bacteria, called a phytoplasma. It can occur in over 350 plants including many common vegetables, annual flowering plants, perennial flowering plants and weeds. Once infected with aster yellows, a plant will never recover.
- Leaves are discolored pale green to yellow or white. In some plants, red to purple discoloration of leaves occurs.
- Leaves may be small and stunted.
- Flowers are small, malformed, and often remain green or fail to develop the proper color.
- Plants infected early in the growing season may remain small and stunted.
- Many thin weak stems grow close together forming a witches' broom.
- Tap roots of carrots are thin, small, covered in many root hairs, and often taste bitter.
Aster yellows is caused by a phytoplasma, a small bacteria that lives only within the vascular system of a plant or within the leafhopper that vectors it from plant to plant. Once a plant is infected, the AY phytoplasma moves systemically through the plant, infecting every part from the roots through the flowers. The pathogen affects the plant's growth, development and ability to store nutrients.
When adult and nymph aster leafhoppers feed on plants infected with AY, they ingest some of the phytoplasma along with the plant sap. In a minimum of two weeks, the phytoplasma moves through the insect's gut into the salivary glands. At this point, when the leafhopper feeds, a small amount of phytoplasma is released into the new plant starting a new infection. The leafhopper will transmit the pathogen every time it feeds for the rest of its life.
Only a few aster leafhoppers overwinter in Minnesota as eggs. The majority of the aster leafhopper population overwinters in southern states, feeding on grain crops and other plants. Weather systems carry the insects north early in the growing season. The aster leafhopper population that arrives in Minnesota each year varies in size and in the percent of the population carrying the AY phytoplasma. In years where leafhopper populations are high, many cases of aster yellows are reported in landscape flowers and vegetable gardens. In years where leafhopper populations are low, only a few cases of aster yellows occur.
The AY phytoplasma will not survive in plant debris of infected plants but can survive in the crown and roots of infected perennial plants.
Once a plant is infected with aster yellows, there is no way to cure it. Infected plants should be completely removed from the garden. Infected plant material can be composted because the aster yellows phytoplasma will not survive once the plant material is dead.
Perennial weeds should be removed from the yard and garden. If infected with aster yellows, the pathogen will survive in these unwanted plants from one season to the next.
Light colored or reflective mulches will disorient aster leafhoppers and can reduce feeding on plants. Covering plants with floating row covers will prevent leafhoppers from reaching the plants and transmitting the disease but will also block pollinators from reaching the flowers. These strategies are more practical in a vegetable garden than in an ornamental bed.
Insecticides are not effective in reducing aster yellows in the home garden.