Most plant pathogens shut down during cold winter temperatures, but there are exceptions. There are two cold-loving “snow mold” winter diseases of turfgrass that can develop. The two diseases are called gray snow mold (Typhula blight) and pink snow mold (sometimes called Microdochium Patch or Fusarium Patch). Snow molds are most likely to occur when snow falls on ground that has not yet frozen, and remains for weeks or months. The lush turf under the snow is an excellent nutrient source for the fungi. The diseases cause damage on home lawns as well as golf courses, with putting greens as a particularly vulnerable site. Both diseases can occur together at the same site.
Gray snow mold is a “true snow mold” because it primarily grows under the snow at temperatures just above freezing, and requires snow cover of at least about 2 months, which is pretty rare in Kansas. It is caused by several closely-related fungi in the genus Typhula. It causes bleached areas about 2 feet across. After the snow melts the diseased turf is matted down, sometimes with white to gray fungal mycelium if turf is wet. Unlike pink snow mold, gray snow mold usually produces sclerotia (light-brown to black, hard, pea-sized structures) in the turf . The sclerotia are the survival structures where the fungus spends the summers. Gray snow mold often occurs repeatedly at the same site.
Sclerotia of gray snow mold. Photo from http://urbanext.illinois.edu/hortanswers/detailproblem.cfm?PathogenID=178
Nasty gray snow mold in Alaska. Photo from http://www.apsnet.org/publications/apsnetfeatures/Pages/snowmold.aspx
Pink snow mold is caused by the fungus Microdochium nivale. Like gray snow mold it causes circular, bleached patches in the turf that are up to about 2 feet across. When the turf is wet, the fungal mycelium is sometimes visible and is whitish but sometimes has a pinkish color (which is the source of the name of the disease).
The fungus survives in plant tissue and debris. It becomes active during cold, wet weather (32-60 F) and goes dormant when conditions are warm and dry. Unlike gray snow mold, pink snow mold does not require snow cover. It can develop if conditions are simply cold and wet, like in foggy or drizzly weather, or alternating periods of repeating frosts. This snow-free phase of the disease is sometimes called Microdochium Patch (formerly called Fusarium Patch)
Pink snow mold, image from http://www.ppdl.org/dd/id/snow_mold_grass.html
For both diseases, cultural practices in the fall can reduce disease pressure.
• Keep mowing the grass as long as it is growing. Overly tall turf (more than 3 inches) will be matted down under the snow, making a favorable environment for the fungi.
• Don’t fertilize too late in the fall. Late applications of N will overstimulate the turf, making a lush growth that is a susceptible to snow molds. Late growth will also interfere with winter hardiness.
•Rake leaves in the fall. A layer of fallen leaves will contribute to the matting down and moisture build-up.
• Golf Courses: Use snow fences and windbreaks to prevent deep buildup of snow on golf greens
• Rake lawns to air out the turf and promote drying.
• Shovel the snow off of susceptible areas like golf course putting greens. Alternatively, you can spread something dark (like a humate product) on the snow surface to enhance melting.
Home lawns: Fungicides are not recommended for home lawns except in extreme situations. Though symptoms look dramatic, the turf will recover eventually. Use cultural practices described above.
Golf course: For gray snow mold, fungicides must be applied in the fall before the first significant snow. Fungicides in the spring will not have an effect, because the disease has already been active all winter. For pink snow mold, spring fungicides can sometimes have an effect. Remember, unlike gray snow mold, pink snow mold does not require snow and can grow if conditions are simply cold and wet. Spring fungicides may reduce such development.
For pink snow mold, fungicides with the active ingredients fludioxanil, iprodione, PCNB, propiconazole, pyraclostrobin, thiophanate methyl, and trifloxystrobin have been reported to provide good to excellent control. PCNB can cause phytotoxicity, though, especially on creeping bentgrass.
For gray snow mold, some available products include those with PCNB, triadimefon, iprodione, flutolanil, or fludioxanil as active ingredients.
More photos and information for commercial turf are available here:
Gray snow mold: http://www.ces.purdue.edu/extmedia/BP/BP-101-W.pdf
Pink snow mold: http://www.ces.purdue.edu/extmedia/BP/BP-102-W.pdf