Virulent and fungicide-tolerant races of loose smut (Ustilago nuda and U. tritici) in Ireland

Following the severe outbreaks of loose smut in the early 1980s, a survey was carried out over the period l984-86of the Ustilago nuda and U. tritici populations in the Republic of Ireland. U. tritici was very rare, only one infected wheat plant being reported in the 3 years of the survey. This fungus was represented by a virulent race distinct from those identified in Britain in the 1950s. U. nuda was widespread, and two races were identified in 1984. A virulent race, capable of overcoming the recessive resistance gene present in differential variety Missouri Early Beardless, was isolated from a single crop grown from imported seed. Several crops grown from seed treated with Vitavax (active ingredient carboxin), which had been imported from France, contained a carboxin-tolerant race of U. nuda. Subsequent surveys in 1985 and 1986 failed to recover these U. nuda races. The role of imported infected seed in the introduction of new races of the loose smut fungi is discussed.