Date: April 1989 (Revised April 1995)
Source: NDSU Extension Service Horticulturists
Necrotic Ring Spot disease (NRS) symptoms start with conspicuous 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, but in some spots the center may remain healthy, producing a ring, or distinct "doughnut" 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.
You can identify a typical infection of NRS (Necrotic Ring Spot) with reasonable accuracy. Unusual cases may be difficult to diagnose.
Do not confuse NRS (Necrotic Ring Spot) with fairy ring disease, a common but much less serious disease affecting turf in North Dakota. Fairy rings are almost always much larger in diameter and rarely kill grass in well fertilized lawns. Several other problems may also cause confusion, such as white grub infestation or Pythium blight. Even drought-produced symptoms can resemble NRS.
Lawns from two to eight years old are especially prone to NRS problems. 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 is difficult. It is suggested that cultural modifications are the best way to slow down the disease. Fertilize moderately to promote growth during April to mid-May, late August and early September, or late fall. Do not starve the lawn. Compaction is often times associated with affected areas. Aeration with a spading fork may be beneficial.
You can use a fungicide if you detect the problem early. Chemical treatment is expensive and not always effective. Lawns that are badly infected, layered with poor root development, or thatchy, are not good candidates for effective chemical treatment.
You can find illustrations and additional information at your county extension office. For future reference, you may want to obtain a copy of Extension bulletin PP-950, "Lawn Disease," which is available at your county office of the NDSU Extension Service.
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