Snow mould
Microdochium nivalis (Fusarium nivalis)

Description
Monographella nivalis is an ascomycetous fungus belonging to the order Xylariales. Oval to globose, papillate perithecia (100 - 260 x 300 µm) may develop in cereal leaf sheaths or the stem base. They have gold to dark-brown plectenchymatous walls (10 - 30 µm thick). Asci are clavate (60 - 70 x 6 - 9 µm), thin-walled, and contain 6 - 8 hyaline ascospores (10 - 17 x 3.5 - 4.5 µm).

Damage
M. nivalis can attack cereals during all stages causing various diseases:
Seedling blight: Infection of seedlings can cause pre- and post-emergence damage; death early after germination, discoloration of coleoptiles, superficial lesions on plants. Sometimes lens-shaped, pale-brown lesions appear on the first and second leaves.

Cause
M. nivalis is active over a wide range of temperature conditions favoring its wide geographical distribution on the Gramineae. With an optimum between 18 - 20 °C (64.4 - 68 °F) for most isolates, some isolates are able to grow at temperatures as low as -6 °C (21.2 °F) and as high as 32 °C (89.6 °F). Infections of roots, coleorhizas and coleoptiles generally occur under cold (0 - 5 °C; 32 - 41 °F)), dry soil conditions. The pathogen spreads to the ears via splash dispersal of conidia or wind dispersal of ascospores. Infection of ears occurs during anthesis at 12 - 18 °C (53.6 - 64.4 °F) and is promoted by prolonged periods of wetness.