Which fungus is the main causal agent of wheat stem rust?

The main causal agent of wheat stem rust is Puccinia graminis.

Stem rust, also called black rust or cereal rust, infects wheat and other grasses, including barley and triticale. The fungus attacks stems, leaf sheaths and sometimes heads, producing reddish-brown pustules that later darken as spores mature.

Puccinia graminis has a complex life cycle involving two unrelated host plants. Many forms use wheat or another grass as one host and barberry as the alternate host. This unusual cycle helped scientists understand how rust fungi spread and reproduce.

A major modern concern was race TTKSK, commonly called Ug99, first identified in Uganda in 1999. It overcame resistance in many wheat varieties and threatened crops across Africa, the Middle East and Asia. Stem-rust resistance breeding and fungicides remain important defenses.