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posted: 8/30/2014 6:15 AM

Crabgrass, powdery mildew common ailments

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  • Zinnias, pictured healthy here, are among the plants that can get powdery mildew this time of the year.

      Zinnias, pictured healthy here, are among the plants that can get powdery mildew this time of the year.

 
By Tim Johnson
Chicago Botanic Garden

You may be noticing crabgrass at this time of year, especially in a neglected lawn.  Crabgrass is an annual weedy grass that readily forms seeds in the fall.

To prevent seed formation, keep crabgrass closely cut or pull out the weeds by hand.

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

It is more effective to control crabgrass in spring by applying a pre-emergent herbicide (one that prevents weed seeds from germinating). Make note of bad areas in your lawn now so you can apply such an herbicide next spring before lilacs flower.

Powdery mildew, a disease that causes a grayish, powdery film on leaves, can be seen in many plants now.

This disease typically occurs in the late summer when the weather is warm and humid.

Common plants that can get powdery mildew include lilacs, phlox, bee balm, zinnias and nannyberry viburnum.

In most cases there is no need to apply any controls for powdery mildew at this time of year.

Fungicides may help control the disease, but only if applied before the infection becomes severe.

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 cool, wet weather, has been widespread this year and is killing plants. The first symptoms will be yellow areas on the top sides of leaves, followed by brown spots and an overall sickly appearance. The disease will develop quickly, with the affected leaves turning brown and eventually falling from the plant. The plant may lose all its leaves.

Look for the fungus spores, which appear as small black spots on the undersides of the leaves. You may also see threadlike structures that give the undersides of leaves a dirty, gray appearance. Before you purchase basil at a grocery store or farmers market, examine it closely to make sure it has not been affected with downy mildew.

Remove and destroy all diseased plants in your garden.

Do not put the diseased plants in your compost pile. There is nothing else to do at this point.

Bulb time: It is time to order spring-flowering bulbs. As you plan for bulb planting, choose a site with well-drained soil. Spring-flowering bulbs prefer moisture in spring and fall and drought in summer.

Any area in the garden that remains wet for long periods or has standing water for any length of time is unsuitable for them. Only Leucojum aestivum, summer snowflake, has some tolerance for moist conditions.

Most bulbs prefer full sun. Some early season bulbs will flower when planted beneath a high-branching tree, because they get enough sunlight before the tree leafs out and casts more shade.

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

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