The Q&A Archives: Aster Yellows on Marigolds

Question: Last summer a lot of my marigolds developed aster yellows. A few of my petunias did as well. What causes this disease and what can I do about it next year?

Answer: Aster yellows is caused by a bacteria-like organism called a phytoplasma and is spread by sucking insects such as leafhoppers, the same way mosquitos spread malaria among people. Although symptoms vary with the plant affected, the first symptom of the disease is usually a yellowing of leaf veins. Then entire leaves may yellow, growth may become oddly shrubby or unusually erect and spindly, or plant may be stunted. Flowers may remain green or become distorted.

In addition to marigolds and petunias, susceptible flowers include asters, mums, gladiolas, coreopsis, cosmos, purple coneflower, and dianthus. Susceptible vegetables include carrots, potatoes, onions and tomatoes.

Once a plant is infected, there is no cure. Diseased plants should be promptly pulled and discarded to keep the disease from spreading to nearby plants. Control weeds, especially dandelions and plantain, which may harbor the pathogen. Cover plants with row covers to exclude vector insects (the ones who spread the disease). Keep plants healthy and growing vigorously so they can fight disease, and grow resistant varieties if they are a.vailable. Finally, plant a diversity of crops and mix up your plantings to confuse the pests.

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