Urocystis agropyri is a smut fungus in the order Urocystidales, causing flag smut disease in wheat and related grasses, recognized as an economically damaging pathogen of cereal crops globally. The fungus infects wheat (Triticum aestivum) and other gramineous hosts including Agropyron species, penetrating seedling tissues systemically and eventually producing dark, sooty spore masses (sori) on flag leaves and stems, which rupture to release teliospores and dramatically reduce grain yield. Infected plants often show characteristic streaking and shredding of leaf tissue before spore release. Urocystis agropyri survives in soil and on seed surfaces, making seed-borne transmission a primary infection route. Historic outbreaks devastated wheat harvests before the widespread adoption of seed treatment fungicides and the development of resistant cultivars. Modern control relies on certified disease-free seed, seed dressing with systemic fungicides, and cultivation of resistant varieties. The pathogen remains a concern in regions where resistant varieties are not widely deployed or where fungicide resistance may emerge. Its distribution mirrors global wheat cultivation zones across temperate regions of Europe, Asia, North America, and Australia.