Ustilago avenae is a plant pathogen. Semiloose smut of oats can be found wherever oats are grown, as they are seldom treated with seed treatments. The level of treatment is low because oats command a lower sale price than other cereals and hence it is thought that treatment is uneconomic. Semiloose smut of oats, unlike loose smut of wheat and barley, can thus infect up to 80 per cent of a crop.