Puccinia graminis var. tritici is the fungus that causes wheat stem rust. It is an obligate parasite that infects wheat and barberry hosts. It has a complex life cycle involving five spore stages on the two hosts. On wheat, it produces urediospores and teliospores which lead to recurring infections. On barberry, it produces pycniospores and aeciospores which serve in the sexual reproduction and allow the disease cycle to continue across seasons.