Ecological Strategies of Snow Molds to Tolerate Freezing Stress

Snow molds attack dormant plants, such as forage crops, winter cereals, and conifer seedlings, at low temperatures under snow cover. These fungi belong to various taxa (oomycetes, ascomycetes, and basidiomycetes). Oomycetes, Pythium spp. were reported to be less tolerant to chilling and freezing temperatures than other fungal taxa. Free mycelia and hyphal swellings, structures for survival of P. iwayamai lost viability after cycles of freezing–thawing. However, mycelia in host plants survived the treatment. Basidiomycetous snow molds produce extracellular antifreeze proteins, which attach to the surface of ice crystal to inhibit ice crystal growth. An ascomycete, Sclerotinia borealis prevails where soil freezing is severe. Mycelial growth rate on frozen plates at −1 °C was faster than that on unfrozen medium at the optimal growth temperature of 4−10 °C. S. borealis can grow at low water potential on media and an increase in intracellular osmosis enhances mycelial growth at low temperature. Though, as a whole, snow mold fungi can tolerate low temperatures to prevail under snow, strategies to adapt cryoshpere differ from fungus to fungus according to the adaptability in their particular habitats.