| Noun | 1. | 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 smut fungus, smut - any fungus of the order Ustilaginales genus Urocystis, Urocystis - a genus of smut fungi belonging to the family Tilletiaceae Urocystis tritici, wheat flag smut - fungus affecting leaves and stems of wheat |