Gray snow mold (Typhula blight) and pink snow mold (Microdochium nivale) occurs on grass in the late winter. It first shows up when the final layer of snow melts. The fungi of gray mold normally clears up once the grass completely dries out with no permanent effects, but pink mold can invade the root system of the grass causing considerable damage.
Both gray and pink molds may co-exist on grass following the winter snow but they also may occur singularly. Any type of lawn grass is susceptible to the molds but Kentucky bluegrass and fescue lawns appear to suffer more than other grass varieties, according to the University of Rhode Island. Gray mold appears on the grass as round white spots that vary in size. Tiny black masses also appear on the blades of the grass with gray mold fungus infections. Snow mold first looks similar to gray mold spots but the infected area quickly takes on a pink or salmon color.
Both types of snow mold thrive in temperatures that hover just above freezing in moist conditions. The mold thrives in locations of the lawn that have sustained heavy snowfall, are shaded by trees and suffer leaf accumulation. Pink mold often thrives under the snow from the beginning of fall through winter. It also does not require a heavy, moist cover to thrive within, unlike gray mold.
During the hot summer months the fungi hides in the thatch of the grass and remains inactive. During the fall months the mold often produces tiny pink or white projectiles that produce spores for reproduction. Gardeners can often detect the projectiles if the grass has recently been mowed low. The fungus spores are easily transmitted by cultivation, walking, watering and the wind, according to Cornell University.
Mow lawns closely during the fall months to prevent an overgrowth of green grass, which can easily support both pink and gray snow molds. Fertilize the lawn six weeks before the first expected frost so the lawn can adequately cease growing. Raking up all leaves and lawn debris can also help prevent or reduce snow molds from occurring. Try to avoid deep snow drifts from accumulating on the lawn over the winter months.
Fungicides can help control snow mold but they must be applied in the fall prior to the first snowfall. Once the mold establishes itself and the spring weather arrives, fungicides offer little or no help at treatment or control. Fungicides containing azoxystrobin, propiconazole and thiophanate-methyl can help control and prevent the mold if applied in the fall. If the pink snow mold has been extensive and damaging in the past, alternate two fungicide applications in the fall using two different fungicides.