Snow mold is a fungal disease that affects turfgrass, becoming noticeable as the snow cover melts away in late winter or early spring. This condition is caused by specific cold-tolerant fungi that require a unique combination of environmental and turf conditions to proliferate. The disease manifests as circular patches of matted, discolored grass that can range from a few inches to several feet in diameter. Understanding the biological agents and the specific conditions that encourage their growth is key to managing this winter lawn problem.
The Pathogens: Two Distinct Types
Snow mold is primarily caused by two different groups of fungi, resulting in two distinct types of the disease: Gray Snow Mold and Pink Snow Mold.
Gray Snow Mold, also known as Typhula blight, is caused by Typhula species. This type of fungus is an obligate psychrophile, meaning it requires the presence of prolonged snow cover to activate and spread. Gray Snow Mold typically only damages the grass blades and leaves, rarely killing the plant’s crown or roots, allowing for relatively quick recovery in the spring.
The second type, Pink Snow Mold, is caused by the fungus Microdochium nivale. This pathogen is generally more destructive than its gray counterpart because it can infect and kill the grass crown and roots, leading to more permanent damage. Unlike Gray Snow Mold, this fungus does not strictly require snow cover; it can remain active and damage turf during cool, wet periods in the late fall or early spring.
Environmental Triggers: The Role of Snow Cover
The development of snow mold is heavily dependent on a unique set of weather conditions that allow the fungi to thrive beneath the snowpack. A deep layer of snow acts as an insulator, creating a consistent, near-freezing environment at the turf-snow interface. This insulation prevents the temperature from dropping to levels that would kill the fungus and keeps it within its optimal growth range. The ideal temperature for the fungi is typically between 32°F and 40°F, though Pink Snow Mold can remain active up to 60°F.
The snow cover also creates a dark, high-humidity chamber, which is necessary for fungal proliferation. Water saturation and prolonged wetness are major factors, especially when an early, heavy snowfall occurs before the ground has had a chance to freeze solid. This scenario traps moisture on the turf, preventing drying and providing the continuous dampness the pathogens require to spread their mycelium.
Extended periods of snow cover, especially exceeding 40 to 60 days, significantly increase the risk and severity of Gray Snow Mold infections. While the fungi may be present in the soil year-round, the combination of darkness, high moisture, and insulated temperatures allows them to rapidly colonize the turf. The active growth of the fungus occurs throughout the winter beneath the snow blanket, becoming visible only as the snow melts.
Contributing Turf Conditions
While the presence of the pathogens and the appropriate weather are necessary, certain localized turf conditions can dramatically increase a lawn’s susceptibility to snow mold.
Thatch and Moisture Retention
One major factor is the presence of excessive thatch, the layer of dead and decaying organic material between the grass blades and the soil surface. A thick thatch layer holds moisture like a sponge, providing a continuous water source and a food supply for the fungi, which accelerates disease development.
Grass Height
The height of the grass blades going into winter is also a significant contributor to the problem. Taller grass lays over when compressed by snow, creating a dense, matted layer that traps moisture and restricts air circulation. This matted canopy provides an ideal microclimate for the fungi to grow and spread. Mowing the lawn to a lower height for the final cut of the season reduces the amount of plant material available to become matted.
Late-Season Fertilization
Fertilization practices in the late fall influence turf vulnerability. Applying high-nitrogen fertilizer too late in the season can promote lush, succulent new grass growth that is not fully hardened against the cold. This tender tissue is particularly susceptible to infection before winter dormancy sets in. Selecting a fertilizer with a lower nitrogen ratio or delaying the application until the turf is dormant helps reduce the risk of creating this vulnerable plant material.
Drainage and Debris
Areas of the lawn with poor drainage or compacted soil are also highly prone to snow mold development. When water pools or snow melts slowly, these saturated spots maintain the wet, cold conditions required by the fungi. Other forms of debris, such as unraked leaves or large snow piles, mimic these poor conditions by trapping moisture and light, further exacerbating the localized risk of infection.