Snow Molds in the Winter of 2013-14

From a turf disease perspective, evidence of the severity of this winter is apparent in the extent of snow mold development.  Symptoms of Typhula snow molds (gray snow mold and speckled snow mold), and/or pink snow mold are obvious throughout Indiana and the Midwest.

Typhula snow mold pathogens are adapted to grow and infect at low temperature (32°F to 36°F), and infection occurs only under extended periods (45 days or more) of snow cover.  Snow molds are primarily leaf blighting diseases.  Crowns and roots are not affected, and symptoms usually fade in spring with turf recovery in spring.  Recovery may be delayed after severe outbreaks, sometimes allowing damaged areas to be colonized by annual bluegrass. 

On turf maintained at heights of 3/4 inch or less, Typhula snow mold infections result in nearly circular patches that are typically 6 to 12 inches in diameter (Figure 1). On taller mown turf, patches may be larger, but they may not be as well defined.  Snow mold patches enlarge by radial expansion of mycelium under the snow. The mycelium may be evident in the days during and after snowmelts. Small survival structures called sclerotia often are found embedded in dead leaf tissue within circular patches.  Gray snow mold fungi produce reddish brown sclerotia—speckled snow mold fungi produce dark brown or black sclerotia (Figure 2).  For the purpose of this article, it is not important to distinguish between the two.

Figure 1.  Well-defined circular patches characteristic of Typhula snow molds.

Figure 2.  Sclerotia are produced within infected leaf tissues.

Since outbreaks occur only under snow cover, fungicides applied to Typhula snow mold patches after snow melts will not lessen disease severity or hasten turf recovery.  At this time best results will be achieved by raking the damaged and matted areas to hasten turf recovery.  

Pink snow mold and Microdochium patch (formerly Fusarium patch) are two phases of a disease caused by a single fungal pathogen, Microdochium nivale. The phase that occurs under snow cover is referred to as pink snow mold.  When outbreaks occur in cold wet weather in the absence of snow it is referred to as “Microdochium patch.”  For both phases, infection and colonization result in a general leaf blight—crowns and roots reportedly are not affected.  All cool-season turf species are affected, but outbreaks tend to be most severe on creeping bentgrass and annual bluegrass on golf courses.  Moreover, juvenile creeping bentgrass (less than 1 year after seeding) is most susceptible to the disease and likely to suffer the most lasting effects.  Patches thinned by disease are more vulnerable to colonization by annual bluegrass. On mature golf greens, annual bluegrass appears to suffer greater damage.

On short-mown golf turf, pink snow mold symptoms include well-defined, circular patch clusters (Figure 3). On creeping bentgrass and annual bluegrass putting greens, streaks of off-colored turf that follow the green’s surface drainage patterns may appear during the spring. During cool, wet conditions, white-pink mycelium may be observed on infected leaf blades. Patches of dead, matted leaf blades also occur on taller mown turf, but often lack a clearly circular pattern.

Figure 3.  Pink snow mold infection results in 6”-12” circular patches.

The pink snow mold pathogen is active over a broad temperature range (30°F to 60°F), so infection may continue throughout the spring. Unlike Typhula diseases, snow cover is NOT necessary for extensive pink snow mold infection. However, the insulating effect of snow cover extends the duration of temperature favorable for disease development. The pink snow mold pathogen survives with infested leaf residue in the thatch and decaying turf debris—it produces NO sclerotia.

Spring maintenance also is important to hasten the recovery of turf damaged during the winter. Any practice such as raking and/or mowing that disturbs and aerates affected turf will limit further disease development in the spring and allow the turf to recover as temperature increases during spring.  Fungicides can be effective in preventing Microdochium patch increase on putting greens where infection may continue during periods of cold wet weather.

More detailed information about snow mold diseases is presented at the following links.
http://www.extension.purdue.edu/extmedia/BP/BP-101-W.pdf (gray snow mold) http://www.extension.purdue.edu/extmedia/BP/BP-102-W.pdf (pink snow mold)

Rick Latin, Professor of Plant Pathology