Snow molds of wheat (pink snow mold, snow rot, and speckled snow mold)
Snow molds occur primarily where snowfall accumulates and are caused by several pathogens including Microdochium nivale, Globisporangium spp., and Typhula spp. Symptoms are typically patchy and appear after snow melts in spring. Severe infection can damage the growing point, causing plant death. With pink snow mold, infected portions of the plant turn pink, and pinkish-orange fungal structures may develop. Plant tissue remains intact and later appears bleached. In contrast, speckled snow mold symptoms include gray, brittle tissue which can crumble when touched. A gray-white fungal growth can be observed immediately after snow melt. Within a few days, small, dark fungal structures form on leaves, causing infected tissue to appear speckled. Snow rot appears as dark green blotches on leaves of plants in low areas with cold water accumulation. Infected plants die and appear bleached as they dry.
The pathogens causing snow mold survive in soil and residue. Snow molds are prevalent where persistent snow accumulation occurs over ground that does not freeze or does not freeze deeply prior to snowfall. Snow provides insulation allowing pathogen growth on plants throughout winter. Damp, cool weather favors continued plant damage after snow melt.
Plant snow mold resistant or tolerant varieties. Planting very early or late can reduce the impact of snow mold diseases.Crop rotation is not practical since all these fungi can survive for long periods of time in the soil.Fungicide seed treatments and foliar fungicides are labeled for some snow molds, but efficacy is inconsistent. In addition, foliar fungicides must be applied prior to snow cover and it is difficult to predict when disease will be severe enough to justify the application costs.
Gallery images: M. Burrows and J. Marshall.