Ustilago avenae

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. Smuts, unlike rusts, infect plants in arid climates, often infecting up to 20% of land and over half of the panicles in each field. As with wheat and barley the grains of semiloose smut infected oat plants are completely replaced by the smut fungus. It is not evident until heads emerge and is very difficult to distinguish from covered smut. Like rust fungi, the smut fungus has multiple strains. Treatment may involve resistant varieties and chemical agents.

from Wikipedia