No need to spray for most cases of powdery mildew

  • Powdery mildew is a disease that causes a grayish, powdery film on leaves.

    Powdery mildew is a disease that causes a grayish, powdery film on leaves. Courtesy of Chicago Botanic Garden

By Tim Johnson
Chicago Botanic Garden
Posted9/12/2021 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.


Other options include selecting more mildew-resistant varieties, or moving the infected plants to an area of the garden that has better air circulation.

• Many lawns are showing the effects of the recent run of hot, humid and dry weather. Lots of lawns infested with bent grass have sections of it that have completely died out, as this grass is very disease prone. Large dead areas could also indicate grub damage, so check just below the surface of the soil for the grubs.

If treating your lawn for grubs is necessary, be sure you use the right product that will kill grubs within 24 hours after being watered in.

• It's a good time of year to reseed areas of your lawn. Remove the dead grass, loosen up the soil and add some topsoil if the areas is low before spreading grass seed. The grass seed will need to be watered frequently enough (one to two times a day) to keep it moist for good germination in a couple of weeks.

• It is time to order spring-flowering bulbs. Choose a site with well-drained soil. Any area in the garden that remains wet for long periods -- or has standing water for any length of time -- is unsuitable for bulbs. They prefer moisture in spring and fall and drought in summer. Most prefer full sun. When planted beneath a high branching tree, spring bulbs will often flower before the tree leafs out and will have only light shade to contend with.

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Leucojum aestivum, summer snowflake, does have some tolerance to moist conditions.

• Crabgrass may be evident in your lawn now, especially if it has been neglected. 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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