There's No Mold Like Snow Mold

THERE’S NO MOLD LIKE SNOW MOLD

Written by Jim Janks

Gunnison County Advanced Master Gardener

This winter has brought heavy snow to many mountain areas. As a result, one of the things that we can expect in our turf grass areas is a fungus known as Snow Mold. (See figures 1, 2, 3, & 4) Snow mold affects all cool season grasses, such as Tall Fescue and Kentucky Bluegrass. They appear primarily in two forms, Gray Snow Mold, Typhula incarnata and Typhula ishikariensis or Pink Snow Mold, Microdochium nivale. It is possible to have both types of snow mold in one area. 

 
Typhula ishikariensis infections may progress down to the root crown and may cause more severe and lasting damage. T. incarnata infections are normally less severe. Patches of T. incarnata also tend to recover more quickly in the spring.

Figure 1

Figure 2

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Figure 4

Depending on the height of the mown grass, the shape of the infections may vary. On grass cut ¾” or less, the size and shape of gray snow mold is roughly round and 6” to 12” in diameter. On grass mown taller, the size and shape of the infections may not be as well defined. These patches enlarge by radial expansion of mycelium (the mass of fine branching tubes, know as hyphae, that forms the main growing structure of the fungus) under the snow. The Typhula spp. survive in the form of sclerotia (skil – ro – she – a). These are survival structures often found embedded in dead leaf tissue or in thatch. (See Figures 5 And 6) These sclerotia vary in size and color, becoming smaller and darker as they dry.

Formation of gray snow mold requires snow cover for infection and patch development. Another condition which is favorable for gray snow molds are temperatures just above freezing, 32 degrees F - 36 degrees F. The optimal conditions for snow mold activity occur when snow falls suddenly and remains on ground that has not been frozen.

Figure 5

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CULTURAL MANAGEMENT

The most important means of preventing or reducing snow mold problems is the care of the grass at the end of the summer season. Fall fertilizer programs should be timed so that there is no burst of succulent green growth which makes the grass prone to winter injury. Fertilizers should be timed when the leaf blade growth has stopped, yet is still green. Because snow mold activity is greatest beneath covers that maintain moist conditions, all leaves or other materials should be removed from the lawn. In the spring, rake away dead and matted areas of grass to allow the new growth to begin and to allow air movement within the turf canopy.

CHEMICAL MANAGEMENT

Fungicide applications for snow mold are not recommended for home lawns except in extreme cases. Fungicides are most effective if applied just before the first snow fall.

PINK SNOW MOLD

Pink Snow Mold, Microdochium nivale, is the other snow mold we are likely to run into in this type of season. The common name of this disease, however, is deceiving. Pink snow mold is only pink in certain environmental conditions, so do not be too quick to diagnose. The sure way to diagnose which snow mold you have is by the kind of spores they produce. In gray snow mold look for the sclerotia (Figures 5 & 6) and in pink snow mold look for mycelial threads (Figures 7 & 8). The pink snow mold produces large numbers of microscopic crescent shaped spores in sticky masses (Figure 9). This snow mold blights the leaves, but is not reported to affect roots or crowns. Pink snow mold is most prevalent on turf maintained at heights of three inches or more. This pathogen, unlike grey snow mold, may be active over a broader temperature range, from 30 degrees F to 60 degrees F. Because of this, pink snow mold does not require snow cover for infection and may be active for longer periods of time.

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Figure 9

With pink snow mold, the spores are carried primarily by splashing water, flowing water, air currents, or turf grass equipment. Pink snow mold also favors alkaline turf surfaces. Home lawn care for pink snow mold is the same for as for gray snow mold.

References: Snow mold fact sheets from Purdue University Extension, University of Rhode Island Extension, Utah State University Extension and University of Wisconsin at Madison.