By Carl Wilson, Horticulturist, Denver Cooperative Extension
Aster yellows is a severe disease of many flowers and vegetables. It is caused by a
bacterial-like organism and is carried by the aster leafhopper. The leafhoppers annually
migrate to Colorado from overwintering areas along the Gulf of Mexico. Once insects feed
and acquire the disease organism, they remain infective for life.
Aster yellows affect some 300 species of plants including aster, marigold, zinnia,
petunia, lettuce, carrots, beets and onions. Though symptoms vary, plants usually show
yellowed, bronzed, and twisted new shoot or flower growth. The most severe symptoms are
seen in late July and August and don't let up until frost.
Control is difficult. As long as leafhoppers abound, the disease can be carried to plants.
And leafhoppers may move into gardens from many areas. The most practical remedy for
repeated disease occurrences over several seasons may be to grow less susceptible flowers.
These include nicotiana, geraniums, salvia, cockscomb, impatiens, portulaca and verbena.
Photo of Aster Yellows on Cosmos: Whitney Cranshaw
Photograph of Aster Leafhopper courtesy of University of California
Statewide Integrated Pest Management Project.
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