Myriosclerotinia borealis ( syn. Sclerotinia borealis Bubak & Vleugel)

Menace in the SNOW!


Amanda Kaye
A project for PP728: Soilborne Plant Pathogens
North Carolina State University


Introduction: The Ascomycete Myriosclerotinia borealiscauses snow scald, an economically important disease in winter cereals and turf grasses in areas with prolonged snow cover. This disease causes patches in fields and dramatically reduces yields in winter wheat. The damage caused by M. borealis can vary greatly from year to year, but during the 1960's, it was so destructive that wheat farmers in Finland had to be compensated by the government when they were unable to harvest (2). This fungus is a psychrophilic organism, which means it prefers cold temperatures and can grow at 0 C and below.


Snow scald on grass--Photo Courtesy of Gerard Vaillancourt


Host range and distribution:  M. borealis attacks winter cereals and grasses in boreal and sub-arctic zones, such as Finland, Norway and Alaska (1, 2, 3).


Patches of mycelia on winter wheat--Photo Courtesy of JH McBeath


Isolation:  M. borealis can be grown on potato dextrose agar (PDA).  The sclerotia should be surface sterilized and thinly sliced.  Mycelia will grow out of the slices and can be transferred to a fresh PDA plate.  It is also possible to isolate M. borealis from young lesions on the infected plant, which are usually found near the soil line on the sheath.  The optimal temperature for isolating and culturing M. borealis is 4 C or lower (6).


Identification:  M. borealis is a Discomycete with no anamorph. The mycelium is grayish and the sclerotia are tan and variable in size and shape.  The sclerotia can be oval, flat or spherical and are 2-4 mm in diameter (2,3).


Sclerotia on grass--Photo courtesy of Gerard Vaillancourt

Symptoms:  Patches of grayish fungal mass will appear as the snow recedes in spring.  Leaves are water-soaked in appearance.  The leaves are also covered with gray mycelium and tan or black sclerotia.  Later, the leaves turn white and are desiccated.  M. borealis produces pectolytic extracellular enzymes, which decompose leaves, sheaths, and crowns of the plants (2,3).

Lesions at the soil line on the sheath
Photo courtesy of JH McBeath

Maceration on the sheath
Photo courtesy of JH McBeath

Advanced disease on wheat
Photo courtesy of JH McBeath


Ecology and life cycle:  Disease development is favored by prolonged periods of deep snow cover up to 200 days on frozen soil.  The ideal temperature for the development of the fungus is -2 C.  If conditions remain unfavorable, the sclerotia can survive in the soil for many years.  If humid conditions occur, the sclerotia germinate to form apothecia, cup-shaped structures which produce wind-borne ascospores as the primary source of inoculum.  This fungus can also produce mycelia directly under arid conditions from cracked sclerotia.  Infection from ascospores is through leaf blades and sheath of plants near the soil surface; however, the mycelia occasionally can infect through stomata and direct penetration of the cuticle (1, 2, 3)


Mat on winter wheat under the snow--Photo courtesy of JH McBeath

Links to other sites:    A Golf Course Managing Website with some great articles on Myriosclerotinia borealis   -- An APS Snow Mold Article  -- The University of Alaska Extension Service


Selected references:


1. Gaudet, D.A; Laroche, A and Yoshida, M. 1999. Low temperature-wheat-fungal interactions: A carbohydrate connection. Physiologia Plantarum 106: 437-444.


2. Jamalainen, E.A. 1974. Resistance in winter cereals and grasses to low-temperature parasitic fungi. Annual Review of Phytopathology 12: 281-302.


3. McBeath, J.H. 2003. Snow Mold: Winter turfgrass nemesis. Golf  Course Management February 2003.


4. McBeath, J.H. 2003. Biological management of snow mold. Golf Course Management March 2003.


5. Robinson, C. H. 2001. Cold adaptation in Arctic and Antarctic fungi. New Phytologist 151: 341-353.


6. McBeath, J. H. 2005. Personal communication.