matted down turfgrass with snow mold (215760 bytes)

Snow Molds

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By Judy Sedbrook, Colorado Master GardenerSM, Colorado State University Cooperative Extension, Denver County

In early spring, as the snow melts, you may notice patchy, matted areas of turfgrass that don't green up as quickly as the surrounding grass in spring.  Snow mold is a turfgrass fungal disease that appears when areas of your yard remain covered with snow for long periods.

Fungal growth begins in the winter, beneath a cover of snow on unfrozen ground. Growth can take place at temperatures slightly below freezing and may continue after snow melt, as long as the grass remains cool and wet. There are two types of snow molds, gray and pink, that become active under the snow cover. Gray snow mold (also called Typhula blight) is caused by Typhula spp., while pink snow mold (also called Fusarium patch) is caused by  Microdochium
nivalis
.
Gray snow mold activity stops when the temperature exceeds 45 F or the surface dries. Pink snow mold activity may continue during wet weather in the fall and spring, as long as the temperature is between 32 F and 60 F.

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Matted grass with pink snow mold

Symptoms first appear in the lawn as circular, straw colored patches when the snow melts in the spring. These patches continue to enlarge as long as the grass remains cold and wet. Grass within the patch often has a matted appearance and colored fungal growth. The fungal growth may cover the entire patch or develop along the margins, with gray snow mold being white to gray in color and pink snow mold being white to pink in color. Occasionally, fungal fruiting bodies (mushrooms) may be seen emerging from infected turf. Hard structures, called sclerotia, may also develop on the leaves and crowns of plants infected by gray snow mold, not pink. The sclerotia are spherical in shape and roughly the size of a pinhead. Their presence helps to distinguish gray snow mold from pink snow mold.

The damage caused by snow molds is seldom serious. Generally, infected areas are just a little slower to green up. Gently rake affected areas of the lawn to promote drying and prevent further fungal growth. The grass usually will begin to green up within a few weeks after raking. If it doesn't come back, overseed the area with the appropriate turfgrass variety. Use of fungicides isn't necessary, nor are they particularly effective.

Photos: Jon Nelson

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Date last revised: 01/05/2010