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Kentucky Bluegrass Diseases

Sloping landscape with trees and fog 20080615-1200634 image by SIGNSofMIND from

Disease resistant cultivars were developed to resist Helminthosporium mold, once a major problem with Kentucky bluegrass. New cultivars are being developed to deal with diseases that took its place. Selecting the right cultivar and proper cultivation remain the best ways to prevent diseases. Because they show similar symptoms, two diseases that frequently strike Kentucky bluegrass, summer patch and necrotic ring ppot, were once lumped together as Fusarium blight.

Necrotic Ring Spot

Necrotic ring spot (Leptosphaeria korrae) appears in early June as 1-½ to 3-foot-wide, bluish-green patches of wilted grass. The patches turn brown and the turf dies. Patches of healthy grass in the center of diseased turf sometimes give a donut appearance. Decaying roots turn light to dark brown. Treat necrotic ring spot with fungicides containing the active ingredients fenarimol, propiconazole, iprodione, thiophanate-methyl, cyproconazole, myclobutanil or azoxystrobin.

Summer Patch

Summer patch (Magnaporthe poae) produces dead patches of grass a few inches to 2 to 3 feet wide. These appear in mid- to late summer. These are crescent-shaped or circular. Dying grass on the edges of the patches can have a slightly bronze color. Patches of healthy grass sometimes appear in the center. Symptoms are most severe on knolls, slopes and dry parts of the turf. Cool weather may allow the grass to recover. Summer patch can be treated with fungicides containing fenarimol, triadimefon, propiconazole and cyproconazole, among others. Apply fungicides containing thiophanate-methal to cure infections of summer patch.

Fusarium Blight

Fusarium blight (Fusarium roseum) strikes in hot, humid conditions when Kentucky bluegrass is under stress. Symptoms include light-green circular, elongated or crescent-shaped patches 2 to 6 inches wide. These can grow up to 3 feet wide with the same donut look that describes necrotic ring spot. Several cultivars of Kentucky bluegrass resist Fusarium blight and necrotic ring spot. These include adel phi, baron, galaxy, majestic, parade and touchdown. Fungicides containing the active ingredient fenarimol, iprodione, myclobutanil, propiconazole or thiophanate-methyl are used to treat this blight. Fungicides containing triadimefon are most effective.

Powdery Mildew

Powdery mildew, caused by Erysiphe graminis and other fungi, appears as a gray to dusty white powdery coating over the leaf surface. As the spots grow larger they come together to form a mat of mildew that looks like dirt or dust. Powdery mildew is a problem with baron, flyking and merion cultivars of Kentucky bluegrass if they grow in the shade. Warren and nugget are varieties adapted to the shade. Treat powdery mildew with fungicides containing triadimefon, fenarimol or propiconazole, among others.

Striped Smut

Striped smut, caused by a variety of fungi, causes yellow-green streaks that turn gray. The gray streaks then rupture, revealing masses of black spores. The disease shreds the grass leaves, which turn brown and die. Striped smut usually strikes in cooler spring weather and the grass dies in the hottest parts of summer. Merion is a highly susceptible Kentucky bluegrass cultivar. Numerous varieties have been developed that will resist striped smut to some degree. Treat striped smut with fungicides containing propiconazole, triadimefon, myclobutanil or cyproconazole. Apply in late autumn or early spring.

Gray Snow Mold

Gray snow mold (Typhula incarnate) appears in circular patches up to 2 feet wide under snow. When the snow melts, the patches of infections are usually matted and surrounded by a halo of gray to white, fluffy fungal growth. This disease, which rarely kills the grass, is found more often on bentgrass than bluegrass. Fungicides containing the active ingredients azoxystrobin, thiophanate-methyl, chlorothalonil and propiconazole, among others, can be used to treat snow mold.

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