Fusarium is a common fungal disease of cereals causing head blight and leading to significant crop losses. Beer brewed with Fusarium-affected grain can contain dangerous mycotoxins and may be prone to gushing. Fusarium primarily infests the seed head of the barley plant, digesting the seed tissues and resulting in shrunken and discolored grains.

The genus is classified in the Ascomyces group of fungi and cultures produce distinctive banana-shaped spores. The mycelia are often pink- or purple-colored and infected barley grains may appear dark.

A range of Fusarium species infect barley, primarily F avenaceum, F culmorum, F graminearum, and F poae, but their prevalence depends on location and climate, with F graminearum most common worldwide. Infection is most likely vectored by plant residues in fields or spread by wind from nearby plants and it affects plants from the flowering head stage onward. In wheat the fungus grows rapidly through the flowering head but in barley it attacks grains separately, causing a patchy appearance.

Mycotoxins, particularly trichothecenes, are often produced by Fusarium infections as part of the fungus’s virulence and are highly toxic even at low concentrations—so toxic as to have been considered for possible use as a biological weapon. Fusarium species can also produce hydrophobin proteins that initiate gushing in beer.

Commercial barley is tested for Fusarium infection to prevent contamination of grain stocks, both by visual observation and by antibody or DNA analysis.

See also barley diseases and gushing.