Snow mold after melting in spring. Source: https://gardeningafterfive.files.wordpress.com/2010/01/snowmold5.jpg
The fungal disease Snow mold occurs in early spring after snow melting. The two types of snow molds (gray and pink) become active under the snow cover and are caused by the fungus Typhula spp. (gray snow mold) and Microdochium nivalis (pink snow mold).
Gray snow mold survives hot summer temperatures in the soil or in infected plant debris as sclerotia, resistant fungal structures, while pink snow mold survives as mycelium or spores in infected plant debris.
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. Gray snow mold activity stops when the temperature exceeds 7°C 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 0°C and 15°C.
Symptoms are firsty seen 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.
Snow molds do not occur in the home lawn every year, but are most common during years when an early, deep snow cover prevents the ground from freezing. A cold, open winter will not promote snow mold, but may cause winter injury. 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. Fungicides are not usually recommended, but in severe cases a preventative spray may be helpful.
The following steps can be taken to minimize damage in future years:
- Avoid excessive applications of nitrogen fertilizer in the fall.
- Continue to mow the lawn at the recommended height until it is no longer actively growing. The taller the grass, the more likely it will mat down and encourage snow mold development.
- Rake up leaves in the fall.
- Manage the thatch layer to avoid accumulations of more than ½ inch.
- Spread out large snow piles to encourage rapid melting. Use snow fencing to minimize snow accumulation in problem spots.
Lamey, H. Arthur, Cynthia L. Ash, and Ward C. Stienstra. 1988. Lawn Diseases, PP-950(ND) or AG- FO-3386(MN). NDSU Extension Service, North Dakota State University. MN Extension Service, University of Minnesota