Extension News

Ask the ISU Garden Experts: Lilac Mildew, Magnolia Scale and Cockscomb

Note to media editors: Got gardening questions? Call the Hortline at (515) 294-3108, Monday-Friday from 10 a.m. to noon and 1 to 4:30 p.m., or e-mail us at hortline@iastate.edu. For more gardening information, visit us at Yard and Garden Online, http://www.yardandgarden.extension.iastate.edu

8/26/2009

The leaves on my lilac are covered with a white substance. Is this a serious problem?

The white substance is likely powdery mildew. Powdery mildew is a fungal disease. It appears as a white, dusty growth on plant foliage. Many plants can get powdery mildew. However, it is especially common on lilac, phlox, zinnia and turfgrass.

Powdery mildew is favored by high humidity, cool nights and warm days. Plants growing in partial to heavy shade are most susceptible to powdery mildew.

Powdery mildew does not cause serious harm to lilacs. The damage is mainly aesthetic. Spraying with a fungicide is usually not recommended. When planting lilacs, select a site that receives partial to full sun. To bloom well, lilacs need at least six hours of direct sun each day. Also, powdery mildew will not be a serious problem in sunny areas.

My magnolia is infested with magnolia scale. What are my control options?

Magnolia scale is the largest scale insect in Iowa. Sap feeding by the scales causes stress to heavily infested plants and can result in stunted growth, yellowish foliage, branch dieback or death of the plant. Magnolia scales produce large quantities of honeydew (sugary excretion) that accumulate on the tree’s leaves and twigs. Sooty mold, a black fungus that grows on honeydew, turns the honeydew-covered leaves and twigs black.

To control magnolia scale, remove and destroy heavily-infested branches. Treat with a contact insecticide (horticultural oil, insecticidal soap, other ornamental synthetic insecticide) or a systemic insecticide (imidacloprid) in late August or early September. Dormant horticultural oil also can be used in late fall or early spring.

How do I preserve cockscomb?

Air-drying is the best way to preserve cockscomb. After harvesting cockscomb, strip the foliage from the stems. Tie the stems into small bunches with rubber bands. Hang the loose bunches upside down in a warm, dark, dry, well-ventilated place, such as an attic or shed. (Harvested flowers are usually hung to dry so that the stems dry straight.) Dry the plant materials thoroughly. Two to four weeks is usually sufficient.

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Contacts :

Richard Jauron, Horticulture, (515) 294-1871, rjauron@iastate.edu

Del Marks, Extension Communications and External Relations, (515) 294-9807, delmarks@iastate.edu