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).
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.
Snow mold: Bleached patches covered with abundant white mycelium, which may mat together the leaf blades, appear after prolonged snow cover of crops. Patches gradually turn pink due to color changes in the mycelium and the development of sporodochia.
Foot rot: At tillering stage, brown decay of the lowest leaf sheaths is visible, sometimes a shredded first leaf blade lies on the soil surface. The oldest two leaves may bear red/brown spots. Following stem elongation, dark perithecia may appear in the brown areas of the lowest leaf sheaths. Later on a grayish-brown discoloration, frequently girdling the first internode above crown roots, sometimes vertically as dark brown stripes appear as first foot rot symptoms. After milk ripening, brown to black streaks or blotches may occur on higher internodes and nodes are discolored dark-brown. Severely infected stems may break at ground level and may bear orange fungal mycelium. Premature death of tillers causes whiteheads.
Ear blight: Early symptoms consist of small, brown, water-soaked lesions on the outer glumes of florets. Under humid / wet conditions, florets or whole spikelets become infected associated with premature bleaching of ears. Orange sporodochia become visible on the rachis at the base of infected spikelets under humid conditions. Purplish-brown perithecia may also develop on the bracts.
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.
Conidia are curved, broadly falcate with a pointed apex and a flattened, wedge shaped foot-cell. Depending on the number of septae (0 to >5) conidial size varies from 8 - 18 x 2 - 3 µm to 19 - 30 x 2.5 - 4 µm with M. majus producing larger conidia.
As M. nivalis does not produce chlamydospores, soil-borne saprophytic mycelium on infected straw is central to the life cycle for seedling blight, foot rot and ear blight. Later in the growing season, air-borne inoculum (conidia or ascospores) can infect ears and results in ear blight and the development of seed-borne inoculum. Although M. nivalis colonizes all tissues of cereal grain - epidermis, pericarp, testa, endosperm and embryo- the fungus has been isolated preferentially from the epidermis, pericarp and testa. Seed-borne inoculum is the predominant source of M. nivalis inoculum. Seed severely infected by M. nivalis may be small and shrivelled and is associated with reduced germination and reduced emergence of seedlings. However, heavy infected seed results in more severe symptoms of seedling blight than light seed.