Mildew diseases spread in hot, humid conditions

  • The leaves on this peony show signs of powdery mildew.

    The leaves on this peony show signs of powdery mildew. Photos Courtesy of Chicago Botanic Garden

  • Downy mildew can damage your basil plants, causing the leaves to brown and fall off.

    Downy mildew can damage your basil plants, causing the leaves to brown and fall off.

By Tim Johnson
Chicago Botanic Garden
Posted9/8/2019 7:00 AM

Powdery mildew (a disease that causes a grayish, powdery film on leaves) is present now. Common plants that can get powdery mildew are lilacs, phlox, bee balm, zinnias and nannyberry viburnum.

This disease typically occurs in the late summer when the weather is warm and humid. Fungicides can help control powdery mildew if applied before the infection becomes severe.


In most cases. there is no need to apply any controls for powdery mildew at this time of year or any reason to apply any preventive controls. Other options include selecting more mildew-resistant varieties, or moving the infected plants to an area of the garden that has better air circulation.

• Basil downy mildew, which flourishes in warm, humid environments, is present again this year.

Hot, dry weather will cause the pathogen to go dormant. The first symptoms will be diffuse yellow areas on the top side of leaves, with the plants taking on an overall sick look with brown spots as the disease progresses. You may mistake it for a nutrient deficiency.

Look for small black spots, which are spores on the undersides of the leaves. You may also see threadlike structures on the undersides of leaves that will give them a dirty, gray appearance. This disease will develop quickly on your basil plants, with the affected leaves turning brown and eventually falling from the plant. The plant may lose all of its leaves.

Remove and destroy all of the diseased plants in your garden. Do not put the diseased plants in your compost pile. There is nothing else to do at this point.

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Look closely at basil that is being sold in grocery stores or farmers' markets to make sure it has not been affected with downy mildew before purchasing.

• Crabgrass may be evident in your lawn now, especially if it has been neglected or cut too short. This annual weedy grass seeds readily in the fall. Keep it closely cut to prevent seed formation or pull it out by hand.

There are herbicides that will control crabgrass at this time, but they will not be as effective after the crabgrass has set seed.

Make note of bad areas in your lawn and apply a pre-emergent herbicide (one that prevents weed seeds from germinating) in early spring before lilacs flower.

• Tim Johnson is director of horticulture at Chicago Botanic Garden,

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