Masses of black teliospores are produced by Urocystis agropyri in narrow strips just beneath the epidermis of leaves, leaf sheaths and occasionally the culms. Diseased plants often are stunted, tiller profusely and the spikes may not emerge. This subset identifies materials that may be resistant to flag smut.

A severe infection usually induces the leaves to roll, producing an onion-type leaf appearance. The epidermis of older diseased plants tends to shred, releasing the teliospores. Germinating kernels or very young seedlings are infected by germinating spores on the seed or in the soil. The disease continues to develop systemically, and the black subepidermal strips of teliospores become visible near heading. Infection is favored by low soil moisture and cool soil temperatures.

Source: CIMMYT Wheat Germplasm Bank