Low-temperature turfgrass diseases pose a significant challenge to turf managers, especially during the winter months. Among these diseases, gray snow mold caused by Typhula incarnata and speckled snow mold caused by Typhula ishikariensis are of particular concern. These pathogens thrive in cold, snowy conditions and can cause extensive damage to turfgrass. Understanding the symptoms, signs, biology, and control measures associated with gray snow mold is essential for effective disease management.

Causal agents

Typhula incarnata & Typhula ishikariensis

Primary Hosts

All grasses species are susceptible to snow mold, but grasses grown in areas where prolonged periods of snow cover are particularly affected.

Disease Cycle

The disease cycle of gray snow mold begins with the survival of Typhula spp. as sclerotia during the summer. These sclerotia remain dormant until late autumn when they are exposed to wet and cool conditions that promote germination. The optimal temperature range for sclerotial germination is between 10-18°C (50-64°F). After germination, sporocarps may form depending on the light exposure of the sclerotia. Diffuse light with a high ultraviolet spectrum favors sporocarp formation, while direct light or absence of light prevents it, such as when the sclerotia are buried in soil, covered by deep snow, or mulch.

Germinating sclerotia or basidiospores produced on sporocarps can infect turfgrass plants under an insulating snow cover where conditions are relatively mild and moist. The fungus grows slowly, even under optimal conditions, requiring long periods of snow cover to cause disease. As the snow melts in the spring, sclerotia are produced on infected leaves or within sheaths. These sclerotia can then spread into the thatch as infected leaves decompose. The disease tends to recur in the same areas year after year if conditions are favorable for its development.

Credit: Penn State University

Epidemiology

Gray snow mold, caused by Typhula incarnata and Typhula ishikariensis, thrives in specific environmental conditions during the winter season. The fungus grows at temperatures just above freezing (1-2°C or 34-36°F). These diseases develop under prolonged snow cover, especially in soils that are not frozen. For T. incarnata, approximately 60 days of snow cover is required, while T. ishikariensis necessitates longer snow cover periods of around 75-90 days. These pathogens are most active in areas where snow cover persists for an extended period, creating a moist and cool environment ideal for their growth and development. Turfgrass areas with a history of snow accumulation and slow snowmelt are particularly vulnerable to gray snow mold infections. Understanding these favorable conditions is crucial for implementing appropriate management practices to prevent and control the diseases.

Symptoms

Gray snow mold is characterized by the appearance of circular patches on the turfgrass in the spring after snow melt. These patches typically range in size from a few inches to about one foot in diameter. The color of the patches can vary, ranging from light yellow to reddish-brown, straw-colored, or grayish brown. As the disease progresses, the patches may coalesce, leading to a widespread blighting appearance across the affected area. Symptoms can often be confused with Microdochium patch, so it is important to confirm signs of the pathogen, which can be found within the infected tissues.

Credit: John Kaminski/Penn State University

Signs

The leaves in affected areas are matted together and are often covered with white to gray mycelium with clamp connections. As grass dries, the mycelium disappears and the leaves become grayish to silvery white, brittle and encrusted over the patch. Small (0.2 – 5.0 mm in diameter), hard spherical sclerotia are formed on or in infected leaves. T. incarnata produces large salmon or pink-colored sclerotia while T. ishikariensis produces small brown to black sclerotia. Leaves speckled with sclerotia is a distinguishing characteristic between this disease and pink snow mold which does not produce sclerotia. Although rarely seen, sporocarps developing from oversummering sclerotia produce basidiospores that contribute to new infections in the winter.

Credit: John Kaminski/Penn State University

Management

To effectively manage gray snow mold, a comprehensive approach that combines cultural and chemical control strategies is often necessary. Cultural practices focus on creating unfavorable conditions for pathogen development and promoting a healthy turfgrass environment, while chemical control involves the targeted application of fungicides to suppress the diseases. By implementing a combination of these strategies, turfgrass managers can minimize the impact of gray snow mold and maintain the overall health and aesthetics of the turf.

Cultural

Implementing cultural practices such as mowing late into the season and applying nitrogen in the spring can be beneficial in controlling gray snow mold. Mowing late into the season helps to reduce the height of the turfgrass, preventing lodging of the grass. Late-season nitrogen applications are discouraged to prevent the development of succulent leaf tissues. In areas with many deciduous trees, raking leaf litter can also help to reduce the disease. In the absence of snow, lodged turf or heavy leaf litter can mimic conditions favorable for snow molds. Once disease symptoms are observed in the spring, fungicides are not effective and simply applying nitrogen promotes vigorous turfgrass growth, which can aid in the recovery of damaged areas and discourage the growth of the pathogen.

The use of preventive barriers, such as snow fences and windbreak plantings, can also contribute to the control of gray snow mold. These barriers help to reduce the accumulation of snow in specific areas, preventing prolonged periods of snow cover and creating a less favorable environment for the pathogen. By strategically placing snow fences or windbreak plantings, turfgrass managers can redirect the movement of snow and minimize the risk of disease development.

Facilitating drainage by removing snow and cutting channels is another effective cultural practice for managing gray snow mold. By removing snow from the turf area and cutting channels, excess water is allowed to drain away more efficiently. This reduces the duration of snow cover and helps to create a drier environment that is less conducive to the growth and spread of the pathogen. Improved drainage also aids in minimizing the risk of other turfgrass diseases associated with prolonged moisture, further enhancing the overall health and vigor of the turfgrass.

Chemical

Chemical control is an essential component in managing gray snow mold, and it involves the strategic application of fungicides to suppress the activity of the causal pathogens. To effectively control this disease, it is crucial to implement fall applications of fungicides, ideally with one or two applications before the onset of snow cover.

The choice of fungicides depends on the severity of the disease pressure. In cases of moderate pressure, fungicides such as chloroneb, DMI’s, and PCNB are commonly used, particularly on fairways. For severe pressure, DMI’s have shown to be the most effective, and tank-mix combinations of effective fungicides can be employed to enhance control.