Loose smut

Ustilago nuda
Scientific classification
Kingdom:	Fungi
Division:	Basidiomycota
Class:	Ustilaginomycetes
Order:	Ustilaginales
Family:	Ustilaginaceae
Genus:	Ustilago
Species:	U. nuda
Binomial name
Ustilago nuda (C.N. Jensen) Rostr. (1889)

Loose smut of barley is caused by Ustilago nuda. It is a disease that can destroy a large proportion of a barley crop. Loose smut replaces grain heads with smut, or masses of spores which infect the open flowers of healthy plants and grow into the seed, without showing any symptoms. Seeds appear healthy and only when they reach maturity the following season is it clear that they were infected. Systemic fungicides are the major control method for loose smut.

The major symptom of loose smut is the "smutted" grain heads, which contain masses of black or brown spores where the grain would normally be. The spores completely replace the grain head so that there is no grain to be harvested on infected plants. It may be possible to identify infected plants in the field before they reach the flowering stage by looking for plants which are taller and more mature than the rest of the field. The fungus causes infected plants to grow slightly taller and mature slightly sooner than the uninfected plants in the field. Since it must infect through the open florets, this gives the fungus a competitive advantage by allowing it to fall down to the healthy plants and insuring that the fungus has a little extra time to produce and disperse spores before the florets of the healthy plants open.

Ustilago nuda infects barley 'Hordeum vulgare L.' but there are many strains of Ustilago which infect many different cereal crops in a similar manner. Ustilago tritici, for example, is loose smut of wheat.

The disease cycle of loose smut begins when teliospores are blown to open flowers and infect the ovary either through the stigma or directly through the ovary wall. There are multiple mating types for Ustilago spp. so infection will only occur if two compatible mating types are present in the same flower.

After landing in an open floret, the teliospores give rise to basidiospores. Without dispersing to any alternate host plant, the basidiospores germinate right where they are. The hyphae of two compatible basidiospores then fuse to establish a dikarytic stage.

After germination inside the ovary, the fungal mycelia invade the developing embryo in the seed. The fungus stays alive in the seed until the next growing season, when it is planted along with the seed. As the developing plant grows, the fungus grows with it. Once it's time for the flowers to form, teliospores are produced in place of the flowers and develop where the grain would be.

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Wikipedia