- flag smut fungus - a smut fungus causing a smut in cereals and other grasses that chiefly affects leaves and stems and is characterized chains of sori within the plant tissue that later rupture releasing black masses of spores
--1 is a kind of smut, smut fungus
--1 is a member of Urocystis, genus Urocystis
--1 has particulars: wheat flag smut, Urocystis tritici