Turfgrass Diseases: Gray Snow Mold (Causal fungus: Typhula incarnata)

This disease is caused by the fungus Typhula incarnata and results in patches of blighted turf on lawns, golf courses, and athletic fields. Gray snow mold affects all cool-season turfgrass species and is prevalent during years where a thick layer of snow cover lasts for long periods during mid to late winter.

Figure 1. Gray snow mold symptoms on a perennial ryegrass lawn in central Pennsylvania. Photo: Peter Landschoot, Penn State

Symptoms and signs

Gray snow mold symptoms and signs are first noticed as snow cover melts away in late winter or early spring. The most notable symptoms are white or tan crusted patches of dead and matted leaf blades, similar in appearance to papier mâché. Patches can range from a few inches to several feet in diameter, often coalescing to cover large areas of turf. Dense gray fungal mycelium can often be observed close to receding snowbanks. Gray snow mold can be distinguished from other snow mold diseases by the presence of tiny (0.5 to 2.5 mm), reddish-brown, rounded fungal structures called sclerotia embedded in the leaves and crowns of the infected grass plants.

Figure 2. Gray snow mold patches of differing sizes and shapes near a building on the Penn State University Park campus (left), and matted, bleached leaves in an individual patch (right). Photos: Peter Landschoot, Penn State

Figure 3. Gray mycelium of T. incarnata covering turfgrass foliage near receding snowbank. Photo: Peter Landschoot, Penn State

Figure 4. Reddish-brown sclerotia of T. incarnata embedded in infected tall fescue leaves. Photo: Peter Landschoot, Penn State

Disease cycle

Gray snow mold begins with the infection of turfgrass leaf tissues by T. incarnata mycelium produced from germinating sclerotia. Sclerotia germinate during cool, wet conditions in late fall or winter, typically under snow cover. Mycelium spreads from plant to plant under snow cover causing infections that result in patches of blighted leaves and stems. Once snow melts and turfgrass is exposed to sunlight, fungal mycelium undergo desiccation and form sclerotia. Sclerotia of T. incarnata remain dormant in leaf litter, thatch, and soil during spring, summer, and fall, then germinate and give rise to new mycelium the following winter. Gray snow mold seldom occurs except under snow cover when the soil is not frozen. In most instances, the fungal pathogen kills leaf blades of susceptible plants but does not kill crowns and roots.

Disease development

Gray snow mold is commonly found in areas of greatest snow accumulation, such as along driveways and sidewalks, or over the brink of a hill where snowdrifts tend to accumulate. Typically, the deeper the snow cover and the longer the snow remains on the turf surface, the greater the extent of symptom development and turfgrass injury. Ideal conditions for the development of gray snow mold include temperatures slightly above freezing at the snow/turf interface, wet leaf tissues, and darkness. Once snow melts and no longer covers the turf, disease activity subsides. This disease is usually most severe on close-cut turf, such as golf course putting greens, tees, and fairways; lush turfgrass maintained with high amounts of N fertilizer; and recently seeded grasses that have not completely matured prior to winter. In most cases, mature turfgrass affected by gray snow mold tends to recover quickly when temperatures warm and leaf growth resumes in spring.

Cultural control

Gray snow mold usually can be managed in lawns without using fungicides. Cultural practices that may lessen the severity of this disease include mowing turf well into the fall, avoiding the creation of long-lasting snowbanks on turf when removing and piling snow from sidewalks and driveways, and using only moderate amounts of nitrogen fertilizer on turf in fall. When symptoms appear at snowmelt in the spring, diseased areas can be raked to break-up the crusted, matted leaves, and nitrogen fertilizer can be applied to enhance turfgrass recovery. Although all turfgrass species are susceptible to gray snow mold, Kentucky bluegrass and the fine fescues are less prone to injury than annual bluegrass, creeping bentgrass, perennial ryegrass, and tall fescue. In cases where susceptible turfgrass cultivars have been planted, gray snow mold may cause extensive foliar injury.

Chemical control

Preventative fungicide applications can be used to control gray snow mold on high-value turf, such as golf course putting greens, tees, and fairways; sports fields; and recently seeded lawns. Combinations of fungicides with different active ingredients are often more effective than the use of a single active ingredient. To obtain successful control, fungicides should be applied prior to the first permanent snow cover in the fall, typically in late November. Applying fungicides after gray snow mold symptoms appear in the spring is of no value.

Some penetrant fungicides labeled for control of gray snow mold disease.

Some multisite contact fungicides labeled for control of gray snow mold disease.

Some combination product fungicides labeled for control of gray snow mold disease.

*FRAC is an abbreviation for Fungicide Resistance Action Committee. The FRAC code/resistance group system consists of numbers indicating classes or groups of fungicides based on the mode of action, and letters that refer to broad classifications of fungicides (P = host plant defense inducers; M = multi-site fungicides; and U = unknown mode of action and unknown resistance risk). Due to the risk of fungicide resistance, turf managers should avoid excessive use of fungicides within the same FRAC code/resistance group and alternate products among different FRAC codes/resistance groups.

**Plant mobility classification refers to a fungicide's ability to penetrate plant surfaces or remain on a plant leaf or stem surfaces without penetration. Fungicides that penetrate plant surfaces and are translocated mostly upwards through plant xylem tissues are called acropetal penetrants (acropetal = toward the apex). Fungicides that enter plant cuticles or move limited distances in internal plant spaces, but do not translocate through vascular tissues (xylem and/or phloem) are called local penetrants. Contact fungicides do not penetrate plant surfaces and only inhibit fungal pathogens residing on leaf and stem surfaces.

***Follow label precautionary statements, use restrictions, and directions regarding tolerant turfgrass species, rates, and timing of applications.

References

Buhler, W. Fungicide spraying by the numbers.

Clarke, B.B., P. Koch, and G. Munshaw. Chemical control of turfgrass diseases 2020. University of Kentucky, Rutgers University, and University of Wisconsin.

Latin, R. 2011. A practical guide to turfgrass fungicides. American Phytopathological Society Press, St. Paul, MN.