Loose smut of wheat is caused by the basidiomyceteous fungus Ustilago tritici (Teliomycetes, Ustilaginales).
Loose smut causes no detectable symptoms until heading of plants. Infected plants often produce heads earlier and are taller than non-infected ones. Usually all spikelets of the ear are transformed into a mass of olive-green teliospores forming smut masses. Smutted kernels are initially covered by a thin grayish membrane, which bursts and releases the powdery spores when mature. Spores are dispersed within a few days and a naked, blackish and erect rachis remains visible above the healthy crop at maturity.
In general all tillers produced by an infected kernel are affected, however, damage may vary with the cultivar. Loose smut destroys the kernels of infected plants and contaminates the grain of the non-infected plants during anthesis thus reducing the quality of the grain. No definitive symptoms of seed infection are evident on infected seeds.
The fungus is strictly seed-borne and enters the field within the embryos of seed in which it was established earlier by infecting the seed crop during anthesis.
The pathogen restarts activity with seed germination, grows upward with hyaline mycelium in the plant within the apical meristem and enters the seed primordia when the plant forms the head. Typically all young spikelets are colonized by intracellularly growing mycelium which destroys most of the ear tissue, except the rachis, and converts the spikelets into sori. The mycelium in sori differentiates into brown, spherical teliospores forming a loose mass of spores in place of the kernels. The formation of these spores coincides with the flowering of healthy neighbouring plants; the teliospores are dispersed onto the open flowers of these plants, where they invade the ovaries to repeat the cycle. Within one day, teliospores germinate and form 4-celled monocaryotic promycelia (basidia). After fusion of sexually compatible (+ and -) haploid hyphae produced by the promycelium, the resulting dikaryotic infective hyphae penetrate the flower through the stigma or the ovary wall. The mycelium becomes established in the pericarp, scutellum and embryonic tissue before the kernels are mature. During maturation of the caryopsis, the mycelium is changed to a thick-walled, swollen, dormant mycelium.