Believe it or not, turf disease can occur even when old man winter sends the snow flying. The snow molds, coming in hues of gray, speckled, and pink, actively infect turfgrasses at very low temperatures. As the name implies, these pathogens inflict damage when a nice comfy layer of snow covers the turf surface and provides a nice wet microenvironment for the fungus to thrive. The term for these type of organisms is psychrophilic, which doesn't mean they are scared of anything, but instead denotes them as cold-loving organisms. For most areas in Missouri, the winters are not harsh enough for home lawn owners to be scared of the snow molds. In some years though, managers of high amenity turfgrasses can have problems with snow mold damage.
Gray Snow Mold & Cottony Snow Mold
Gray snow mold/Typhula blight and cottony snow mold/Coprinus snow mold were observed in the late winter of 2010 on several higher cut Kentucky bluegrass and mix bluegrass/fescue lawns and sports fields. Gray snow mold appears as light yellow, or straw colored patches from 1 inch to 3 feet in diameter (Fig. 1A). Cottony snow mold was being observed as conspicuous tufts of mycelium 2-4 inches in diameter (Fig. 1B). Of the areas seen last winter, only the turf leaves were affected, meaning regrowth from crowns and stolons healed these areas quickly when the temperatures rose.
Speckled Snow Mold
Plant symptoms are very similar to gray snow mold, but fortunately we leave this type of snow mold for our neighbors in the Great North of Wisconsin, Minnesota, Michigan, and Canada to deal with.
Pink Snow Mold/Microdochium Patch
Of the varying types, pink snow mold is much more prevalent in Missouri, with gray or cottony snow molds occurring in unique winters (like 2010-11) with more extended snow cover. Pink snow mold is the phase of the disease that occurs under snow cover, but the disease can also be very active in cool, wet weather in the spring when it is referred to as Microdochium patch. These two disease phases, caused by the fungus Microdochium nivale, are often more severe than other snow molds because the pathogen often infects the crowns of the plant making regrowth difficult.
The disease can occur and damage nearly all cool-season grass species, but is most severe on Poa annua and creeping bentgrass putting greens. Pink snow mold symptoms often occur as patches that can be 4-8 inches in diameter or larger. Microdochium patch symptoms are more often observed as smaller reddish-brown spots or patches that may resemble Pythium streaking due to the dispersal of conidia by traffic or water. In conducive environments, pink-salmon colored sporodochia (which produce the conidia) can be observed on leaf sheaths (see Figure 2).
Snow Mold Control
For turf maintained at heights over one inch, it is important in the fall to mow as long as the grass is actively growing to avoid a matted turf and a conducive microenvironment. Nitrogen applications too late in the fall have been shown to increase disease severity, but this should not be a deterrent to fall fertilization for home lawns. Fall fertilization is very important to cool-season turf, and should already be completed. In areas affected by snow mold, resume spring maintenance as soon as possible and lightly rake affected turf areas to break apart the matted layer and allow turf drying.
Chemical control is unnecessary on home lawns in Missouri, with the possible exception of pure Kentucky bluegrass stands in the far northern reaches of the state. Conversely, chemical control may be necessary in lower mown, high amenity areas that have a history of the disease, such as sports fields (Kentucky bluegrass baseball infields in particular) or golf putting greens. In these cases, preventive fall fungicide applications are needed with a specific target of pink snow mold, with possible re-application necessary in curative situations or during periods of snowmelt. Timing of application should be as close to anticipated snowfall as possible to shorten the time that the fungicide needs to persist through the season. For our relatively low pressure climate, thiophanate-methyl and the dicarboximides (vinclozolin or iprodione) mixed with chlorothalonil have been shown as a good economical solution for gray and pink snow mold treatment. If the site has not had prior history of snow mold though, I would suggest saving your resources and applying curatively if the need arises.
REVISED: September 29, 2015