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The class Ustilaginomycetes comprises more than 1400 species of basidiomycetous plant parasites, which are distributed in approximately 70 genera. They occur throughout the world, although many species are restricted to tropical, temperate or arctic regions. Some species of Ustilago and Tilletia, e.g. the barley, wheat or maize smut fungi, are well known because they are of economic importance. For example, from 1983 to 1988 the barley smut fungi reduced annual yields by 0.7% to 1.6% in the prairie provinces in central Canada, causing average annual losses of about U.S. $8,000,000 (Thomas 1989). Tilletia contraversa is important in the international wheat trade (Trione 1982), and 2-5% of the plants in a corn field are generally infected by Ustilago maydis, while up to 80% of a field can be infected if conditions are good for the smut fungus. On the other hand, the galls of U. maydis are considered a delicacy in the Mesoamerican tradition. They are known in Mexico as "Huitlacoche" and in the U.S.A. as "maize mushroom", "Mexican truffles" or "caviar azteca" (Valverde et al. 1995).