Control crabgrass before it goes to seed

I am seeing lots of crabgrass in lawns now, and in some places, I have noticed crabgrass moving into diseased areas of lawns.

Crabgrass is an annoying lawn weed but it’s relatively easy to control. It is more prevalent in lawns that have been neglected or cut too short. This annual weedy grass seeds readily in the fall. Pull it out by hand or keep it closely cut to prevent seed formation.

There are herbicides that control crabgrass, but they are not as effective after the crabgrass has gone to 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. The pre-emergent herbicide to control crabgrass will also prevent grass seed from germinating, so plan any lawn renovations accordingly.

Powdery mildew on the rise

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 late summer when the weather is warm and humid.

Fungicides can help control powdery mildew if they are applied before the infection becomes severe. In most cases, there is no need to apply controls for powdery mildew at this time of year (or any reason to apply preventive controls earlier in the 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. I don’t pay much attention to this garden problem.

Eradicate basil downy mildew

Basil downy mildew that flourishes in warm, humid environments is present again this year. Hot, dry weather causes the pathogen to go dormant.

The first symptoms are diffuse yellow areas on the top side of leaves, with the plants exhibiting an overall sick look with brown spots as the disease progresses. You may mistake this disease for a nutrient deficiency.

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

Remove and destroy all 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. Look closely at basil that is being sold in grocery stores or farmers’ markets to make sure it is not affected with downy mildew before purchasing.

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