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Roots Growing Along the Stems of Cosmos and Zinnias

I've raised cosmos and zinnias and have a problem with the roots growing out all along the stems of these plants. Sometimes, these roots become very large and unsightly by mid to late summer.  What causes this and how can I prevent it?

These adventitious roots may be a symptom of aster yellows. This disease is caused by a phytoplasm transferred from sick to healthy plants by the aster leafhopper. This disease affects about 300 species of plants including cosmos, zinnias, aster, marigold, petunia, lettuce, carrots, beets and onions. In addition to the adventitious roots the plants may have yellow, bronze, 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. The only way to control the disease is remove infected plants and kill the leafhoppers to stop the spread of the disease. You may want to grow less susceptible plants like Nicotiana (flowering tobacco), geraniums, Salvia, cockscomb (Celosia), Impatiens, Portulaca (moss rose) and Verbena.


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