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Urediniomycetes (rust fungi; subdivision Basidiomycotina) A class of fungi that are obligate parasites (see PARASITISM) of a wide variety of plants including angiosperms, gymnosperms, and pteridophytes. Many have complex life cycles that involve the sequential formation of up to 5 different types of spore. Some rusts can complete their life cycle in a single host plant, while others require two different host species.