Pest Watch 23

Snow Molds

Pink snow mold (Microdochium nivale) and gray snow mold (Typhula incarnata and T. ishikariensis) are two common winter diseases of turf grass. Gray snow mold is the less injurious of the two because it commonly infects only the grass blades; new growth emerges from the unharmed crowns in the spring. Pink snow mold, however, kills both crowns and roots.

A weblike mycelium is one symptom of snow mold.

BIOLOGY The warm, dry weather of spring and summer keeps snow mold dormant. In fall, the cooler, wetter conditions reactivate the fungi. Growth is most rapid between 32°F and 45°F. Gray snow mold is most severe when the snow cover is prolonged, deep, and compacted, but fallen leaves or an early snowfall prior to the ground freezing also encourage its spread. Pink snow mold infection also requires moisture and cold temperature but can occur with or without snow.

SYMPTOMS Gray snow mold injury typically becomes evident in late winter when the snow melts and exposes roughly circular patches of dead grass that may have coalesced to give the appearance of one large affected area. In the infected area, the advancing margin will have a halo of grayish white mycelial growth. Lawns infected with pink snow mold, by contrast, exhibit patches that are rounder than gray mold infections, and about 8 to 12 inches in diameter. When the grass blades are wet a spiderweb-like mycelium may be visible. White at first, it ages to a pink and salmon color when exposed to sunlight.

CONTROL Lawns attacked by gray snow mold will recover. In the spring, rake away dead and matted leaves from damaged areas to help new growth fill in. Lawn areas killed by pink snow mold, however, will have to be replanted.

To avoid or reduce snow mold infections, mow the lawn in the fall until it stops growing. Fertilize in the fall no later than six weeks before the grass is expected to go dormant. Remove any fallen leaves. Pink snow mold favors a soil pH above 6.5, so delaying applications of lime until spring may help.

During the winter avoid piling snow beside walks and driveways where it will remain for several months. In important turf areas restrict walking, sledding, snowmobiling, or other activities that will compact the snow.

Fungicide applications can be used, especially in extremely serious situations where snow molds have been severe or widespread year after year, but these must be used preventively and are best applied by lawn care professionals. —B.P.

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