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Turfgrass Diseases

By Laura Pottorff, Colorado State University Cooperative Extension agent, horticulture/plant pathology

If your lawn seems plagued with disease, you probably don't need to spend much time asking why. Most of the time turfgrass diseases stem from one basic factor: SOIL.

That's why correct management -- starting with good soil preparation -- is the key to controlling most diseases in home lawns. It's a lot better to get it right in the beginning than to apply gallons of chemicals once problems begin.

Most soil in the Denver area is heavy clay. When clay becomes wet, it's very wet and when it is dry, it's about the same as concrete. In either situation, turf roots have difficulty taking hold. Roots tend to stay in the shallow zone, when the turf would benefit if they grew deeply.

Sometimes we set the stage for lawn problems by trying to give the lawn good care. We irrigate too frequently and fertilize too heavily. These actions, combined with poor soil, lead to thatch build-up. That's when the problems -- from winter kill to drought, disease and insect damage -- begin.

All turf diseases in Colorado are caused by fungi and all cause the grass to thin out and to appear patchy. To identify exactly what's happening to your grass, however, you need to get down on your hands and knees and look at individual grass blades.

Here are the common turf diseases, followed by a list of non-chemical controls that will help promote a healthy lawn.

Ascochyta leaf blight occurs during cooler parts of the growing season and is particularly prevalent in areas that lack moisture (near side walks and driveways, on south-facing exposures and slopes). Symptoms begin at the tip of the grass blade. Blades will be straw-colored and, where healthy tissue meets dead, you'll find a definite pinch in the blade.

Leaf spot and melting out disease probably is one of the most common diseases of the home lawn. Leaf spot is the first stage; it occurs in spring or fall. Elliptical-shaped spots are surrounded by a dark purple border. Tissue in the center of the spot may turn straw-colored. If the spot extends across the leaf, the blades wither and die. PHOTO

As the disease progresses and temperatures warm, the fungus works its way to the plant's base and attacks roots and crowns. Basal tissues near the ground may become dark brown and rot. This is the melting-out stage, when grass gradually thins and "melts out" the diseased area.

Dollar spot occurs during warmer summer months. It appears as small circular spots (about the size of a silver dollar) in the lawn. Individual leaf blades (at the margins of the dead area) display a characteristic band across the middle. These straw-colored bands are pinched in the middle giving them an hour-glass shape. The straw-colored banded tissue on the blade might be bordered by darker-colored tissue between healthy and diseased areas. PHOTO

Powdery mildew is prevalent in shady areas and areas that are sheltered from wind and air movement. The fungus appears on the leaf blades as a white talcum-like growth or substance. PHOTO

The following list of non-chemical control methods will promote a healthy lawn and help prevent turf disease as well as other problems.

  • When establishing or renovating a turf area, till the soil as deeply as possible and improve soil conditions by incorporating organic matter.

  • Fertilize to meet the nutritional needs of turfgrass, but avoid over-fertilization that stimulates lush, succulent growth. Such growth is more susceptible to disease-causing fungi. One pound of nitrogen per 1,000 square feet should be sufficient.

  • Raise the cutting height of the lawn mower so grass is cut 2 1/2 to 3 inches. Keep mower blades sharp to reduce the area of open wounds through which fungi can enter.

  • Reduce excess thatch accumulation by aerating annually either in spring or fall. Thatch is a layer of slowly decomposing grass stems, dead roots and debris that accumulates above the soil line and below the grass blades.

A thin layer of thatch -- one-fourth to one-half inch -- may benefit the lawn as it buffers soil temperatures and can help reduce soil compaction. When this layer reaches more than one-half inch, however, it can cause turfgrass stress. Turf roots begin to grow in the thatch rather than in the soil. Plants from these roots are less temperature-and-drought-resistant. A thick thatch layer also can enhance the build-up of diseases and an increase in insects.

  • Irrigate infrequently, but deeply to a 6-to-8-inch depth. This encourages deep root growth.

  • If powdery mildew and shade are a problem, overseed with varieties of shade-tolerant grass.

  • When controlling any type of disease, insect or weed problems, find out what you can do to discourage pests.

  • Use pesticides only as a last resort and even then only in conjunction with proper turf management.

For more information and for diagnostic services, contact your local office of Colorado State University Cooperative Extension

Photo: Judy Sedbrook

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Date last revised: 01/05/2010