April 1996 - Volume III, Issue 4

Necrotic Ring Spot (NRS) and Summer Patch (SP) of Kentucky Bluegrass Turf

David M. Kopec, Extension Turfgrass Specialist

NRS appears in the spring or late summer months during cooler weather (irrigation or rainfall is necessary). Patches range from 5-30 cm. in diameter. The outside of the patches turn yellow, brown and may be bleached. KBG leaves on the inside of the patch turn bronze or purple to wine-red in color. Later on, they turn brown and die. On infected grass with a thick thatch layer, plants within the patch area collapse to form crater like depressions. Sometimes, plants located in the center of the patch survive, resulting in a "frog eye" or ring appearance. As multiple patches cross over into each other, large areas may become thinned or scar-like in appearance.

Inspection of roots, rhizomes and crowns may show dark colored tissues which have been invaded by the fungus (Leptospheria korrae).

These "rings" will repeat for 2-3 years and eventually subside. NRS is most prevalent on newly seeded KBG plantings, and on two to four year old sod installations. NRS occurs at cooler temperatures than Fusarium blight or summer patch, but the loss of rooting causes the visible decline of turf in the summer.

Summer Patch (SP) is a major disease of KBG, and was first named in 1984. The causal fungus is Magnaporthe poae. It is very prevalent in the northeast, central Midwest, and southern coastal California. I do not know for sure if we have it here.

Outbreaks occur mostly from June through September under sustained high temperature conditions. It occurs on mature KBG lawns, usually three years or more in age. On KBG, gray-green wilting plants initially appear in poorly defined patches 3-8 cm. in diameter. While banded lesions occur may also be present on the leaf blades during hear stress periods. Circular or irregular patches, rings or serpentine patterns appear later as withered turf turns tan or brown. The fungus is more difficult to find on infected plants than that of NRS. A laboratory identification is suggested.

For summer patch (SP), aerify the turf in mid spring, and leave the holes open. Raise the mower height (easy on home lawns) to 7.5 cm., and use acidifying type nitrogen carriers.

KBG cultures which are more resistant to (SP) include Adelphi, Aspen, Enmundi, Rugby, Sydsport and Touchdown. The most susceptible varieties include Dormie, Fylking, S-21, Merion and Windsor.

For (NRS), cultivar resistance is not as great as that of (SP). The varieties Midnight, Wabash, Eclipse, Adelphi, Park, American and Mystic show some resistance. Very susceptible cultivars include Baron, Birka, Colombia, Georgetown, Glade, Hage, Nassau, NewPort, Ram1, Sydsport and Trampas.

Chemical control requires repeat applications of triadimefon, fenarimol, iprodione, or cyproconazole. Propiconazole and benomyl provided minimal or no control in reported literature.

Slow release fertilizers (nitrogen carriers) and prevention of drought stress may minimize the disease. Early spring applications of fenarimol, propiconizole or thiophanate methyl may reduce disease incidence when applied on a preventative basis.

Chemical control with fungicides for (SP) include benomyl, fenarimol*, propiconizole*, thiophanate-methyl, triadimefon*. (*Best when applied before maximum development/disease expression).

Chlorothalonil and iprodione have been shown to enhance severity of summer patch in certain trials.

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