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Archive for Sunday, August 12, 2012

Garden Calendar: Abnormal blooms a sign of disease

August 12, 2012

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The coneflower bloom on the lower right is stunted and lacking petals  — probably due to infection with aster yellows disease. The other two blooms appear normal for comparison.

The coneflower bloom on the lower right is stunted and lacking petals — probably due to infection with aster yellows disease. The other two blooms appear normal for comparison.

Coneflowers and black-eyed Susans are tough Kansas natives that withstand many of the challenges of the Kansas landscape, but this year a disease called aster yellows is taking its toll on these perennial flowers.

One of the most recognizable symptoms of aster yellows is an abnormality in flower growth. On coneflowers especially, flowers of infected plants are often smaller than normal, misshapen, green or greenish-yellow, and have petals coming out of the center of the flower rather than just the outside edges.

In other plants — more than 300 species are susceptible to aster yellows — symptoms include yellowing, stunting and witches’ brooming of growth.

Plants infected with aster yellows should be removed from the landscape and destroyed to prevent the spread. There is no treatment for the disease, and there are no resistant or immune plant varieties. Luckily, aster yellows is not aggressive and typically only affects a small number of plants in a given area.

In large plantings or commercial production, an integrated pest management approach includes control of the aster leafhopper, which transmits the disease. Nearby weeds, which can host and harbor the disease, should also be controlled.

If you have a plant that you suspect is infected with aster yellows, you can submit a portion of the plant, the whole plant, or pictures of the plant to the Douglas County Extension Master Gardener Hotline at the K-State Research and Extension—Douglas County office, 2110 Harper St. Trained garden center staff will also be able to recognize the disease.

Perhaps the most interesting thing about the prevalence of aster yellows this season is that the disease is typically more of a problem in cool, wet summers. Researchers are unsure about what is causing its prevalence this season, although infection may have occurred during the prolonged spring temperatures experienced in this area this year.

Aster yellows also has an interesting life cycle. The disease is caused by an organism called a phytoplasma, which scientists used to call a mycoplasmalike organism. A phytoplasma is somewhere between a virus and a bacterium, although it is sometimes called virus-like. It spreads when aster leafhoppers feed on infected plants and ingest the phytoplasma. The phytoplasma multiplies within the leafhopper. The leafhopper then transmits the phytoplasma to new plants as it feeds throughout the insect’s approximate 100-day life cycle.

Because aster leafhoppers lay their eggs in leaf tissue, the insects may acquire the phytoplasma as soon as they begin feeding.

Plants commonly affected by aster yellows include:

• Ornamentals: asters, black-eyed Susans, butterfly weed, canna, chrysanthemum, coreopsis, cosmos, coneflower, daisy, gaillardia, gladiolus, marigolds, nasturtium, petunias, phlox, salvia, sunflower, veronica, zinnias

• Vegetables: broccoli, cabbage, carrot, cauliflower, cucumber, parsley, pumpkin, spinach, squash, tomato

• Crops and weeds: barley, buckwheat, daisy, dandelion, fleabane, milkweed, plantains, purslane, thistle, wild lettuce

— Jennifer Smith is the Horticulture Extension Agent for K-State Research and Extension in Douglas County. Contact her or an Extension Master Gardener at 843-7058 or dgemg@sunflower.com.

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