Identification
Necrotic ring spot disease, or N-R-S, is a serious lawn disease. The symptoms start with patches or rings of dead grass that appear suddenly. This usually occurs during dry periods following periods of wet weather. Scattered light yellow patches, typically two to six inches wide, appear first, and quickly change to a tan, brown or straw color.

Older spots may have a bleached or white appearance. The disease kills the turf in affected areas. In some spots, the center may remain healthy, producing a ring, or "donut" shape. Because of this, some people call it the "frog-eye" disease. The spots may also be crescent-shaped and elongated. Sometimes much of a lawn is affected. In severe instances, the disease kills so much turf you cannot detect the spotted pattern.

Do not confuse NRS with fairy ring disease, a common but much less serious disease. Fairy rings are almost always much larger in diameter. Several other problems such as white grub infestation, Pythium blight and drought produce symptoms that resemble NRS.

Lawns from two to eight years old are especially prone to NRS. There is evidence that some bluegrass varieties are much more susceptible than other varieties. Red fescues are susceptible, too, but perennial ryegrass is not.

Control
Control is difficult. We suggest the following cultural modifications as the best way to slow down the disease in Minnesota.

• Fertilize moderately to promote growth during April to mid-May.
• Fertilize again in late August or early September.
• Don't starve the lawn, but avoid excessive nitrogen fertilization as well.
• You can use a fungicide if you detect the problem early. Chemical treatment is expensive and not always effective. Lawns that are badly infected, with poor root development due to compacted soils, or thatch, are not good candidates for effective chemical treatment.

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