Warm Wet Weather Promotes Plant Problems

By C. Rae Hozer, Cumberland County Master Gardener

I enjoy working the Ask-a-Master-Gardener (MG) desk at our county UT Extension office because it involves unraveling horticultural mysteries and solving plant insect and disease problems. I typically consult reference books and do Internet research to help identify pathogens or insect pests causing plant problems in a homeowner’s landscape. When  plant samples are provided, a microscope often discloses clues not visible to the naked eye. If other efforts fail, the county agent can submit a diagnostics request to the University of Tennessee Plant Pathology Laboratory specialists via computer. Studying replies from the UT scientists at the lab after they identify causes for submitted plant problems and suggest treatment options, is great on-the-job training for MGs doing this type of volunteer service.

In years with wet springs and summers like 2009, we see certain plant problems brought to the Extension office that aren’t seen with the same frequency in dry, droughty years. That happens because wet conditions favor certain insects along with the diseases they spread while going from plant to plant to feed and high air temperatures combined with foliage which stays damp from rain (or overhead watering) promotes an upsurge in fungal diseases.

This spring I noticed that some yellow coneflower and purple coneflower plants in my flower gardens had pale leaf veins and yellow blotches on the leaves that could be symptomatic of disease. By the end of June, coneflower blossoms confirmed a plant problem I became acquainted with in 2004 (another year with excessively wet summer weather) when a local gardener brought strange-looking purple coneflowers that were weirdly distorted and entirely green to the Extension office. I was stumped and sent a request for diagnosis along with digital pictures to the UT Plant Pathology Lab. The lab report diagnosed “Aster Yellows” disease. See 2009 photo: The lowest bloom in each vase looks normal for the coneflower species but the other flowers are misshapened. Diseased purple coneflowers are leaf-green (not displaying the normal orange-colored center cone with pink petals surrounding it) while diseased yellow coneflowers have a typical brown cone with yellow rays. 

Aster yellows is an incurable disease that is spread by insects called leafhoppers which suck plant sap. Once a leafhopper feeds on an infected plant, disease cells remain in the insect and stay active during its lifetime. Nine to forty days after an infected leafhopper bites a healthy plant, that plant shows symptoms. Symptoms vary with different plants but some are fairly common. Flowers are green, stunted and distorted. Seeds may not form. Leaf veins don’t have their normal green color. Leaves are unusually yellow or yellow-green. Plants have stunted, bushy growth with many spindly stems. Perennial plants over winter showing symptoms each spring after their infection. Seeds from diseased plants also carry the illness.

Over 300 species of flowering plants are susceptible: ornamentals such as aster, chrysanthemum, coreopsis, cosmos, coneflower, dianthus, gladiola, marigold, petunia, and zinnia plants; weeds like dandelion and ragweed; as well as edible crops including broccoli, cabbage, carrot, cauliflower, celery, flax, onion, potato, parsnip, spinach, strawberry and tomato. 

Check your yard for infected plants. When uncertain whether a plant is infected, bring it to your local extension office. Once Aster Yellows disease is confirmed, uproot and destroy any sick plants. Otherwise, they will be an on-going source of infection.

Plateau Gardening is written by Master Gardeners for those tending home landscapes and gardens in Tennessee’s Upper Cumberland Region. Contact UT Extension Cumberland County, P.O. Box 483, Crossville, TN 38557, (phone 931-484-6743) for quick answers to specific questions, free publications, or to learn about becoming a Master Gardener. Email comments or yard and garden inquiries to Master Gardener Rae, mgardenerrae@frontiernet.net.