Gray snow mold, also known as Typhula blight, is common in areas where snow cover is present throughout much of the winter. All cool-season turfgrasses are susceptible, with bentgrasses, annual bluegrass and ryegrasses being most susceptible. Kentucky bluegrass varieties differ in resistance.
During and after snow-melt, gray snow mold appears in roughly circular yellow to whitish-gray patches, from 1 to 3 feet or more in diameter. Wet grass may be matted together and covered with fluffy white to gray mold speckled with numerous amber to dark brown sclerotia. When conditions favor disease development, larger turf areas may be killed. More commonly, only the leaves are killed and new leaves emerge from the overwintered plant crowns. Old gray mold "scars" may be evident until May or early June.
During warm and dry weather, the pathogen lies dormant in the form of sclerotia (survival structures measuring 1 to 5 mm in diameter). With the onset of cold and wet weather in late autumn, the pathogen becomes active. A deep and prolonged snow cover on unfrozen soil, tall matted-down grass, and unbalanced nitrogen fertilization produce favorable conditions for disease development. The optimum temperature for infection is between 30 and 45 F. In winter or early spring when the snow melts and the turf thaws, the gray snow mold pathogen may again become active, and diseased patches enlarge. As the spring weather warms and the turf dries, the pathogen becomes dormant again until middle to late autumn.
To repair damage, rake matted grass, fertilize, and re-seed or resod as necessary. Somewhat resistant varieties of Kentucky bluegrass, bentgrasses, and ryegrasses are available. Keep the turf mowed in the autumn until growth stops. The turf should not go into winter dormancy in a succulent condition. Do not fertilize with nitrogen within about 6 weeks of a killing frost or when the first snow is expected. Remove mulches of fallen leaves and thatch accumulation.
Apply fungicides preventively only to areas which have a history of gray snow mold. One to three fungicide applications during cold, wet weather from mid- to late autumn through early spring is often sufficient.