The rustPuccinia xanthii forma specialisambrosia-trifidae attacks leaves of giant ragweed,Ambrosia trifida L., significantly reducing seed and pollen production, seed weight and seedling vigor. In tests, teliospores did not infectA. artemisiifolia L.,Xanthium strumarium L., and cultivated varieties of chrysanthemum, safflower, sunflower, cosmos, marigold, lettuce, endive, dahlia, zinnia, gloriosa daisy, aster, scabiosa, and gallardia. This fungus commonly attacksA. trifida in eastern and central North America, and it should be of value for the biological control ofA. trifida where introduced in Eurasia.