Summer Lawn Diseases: Necrotic Ring Spot

Necrotic Ring Spot is a common turf damaging disease of bluegrass lawns across the northern United States.While this root-destroying disease infects grass roots in the spring, symptoms do not typically become evident until the hot summer months. To identify NRS, look for a ‘frog-eye’ shaped circle in the lawn. In the center of the browned-out circle, a green ‘eye’ appears. As the disease progresses, circles grow in size and can coalesce or come together.

Necrotic Ring Spot Disease. Courtesy of UMass.

NRS is primarily a Kentucky Bluegrass problem. Turf-Type Tall Fescue and Ryegrass are not affected. The disease is most active on well-watered or regularly irrigated lawns, where roots are constantly moist.  It is especially active in the greater Denver area where most lawns are irrigated.

Managing NRS is difficult. While “over-the-counter” fungicides are available, only licensed lawn care applicators have access to the most effective materials. Even using the very best products, application rate and timing are critical. A professional service will make an initial treatment in the late Spring, when soil temperatures at a depth of two inches reaches 65 degrees. A second and possibly a third application, approximately 28 days apart are also required. And, while the lawn should recover in Fall, the disease is likely to re-appear next season.

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