Variation in the gene determines resistance against wheat stem rust race Ug99.

f. sp. () causes wheat stem rust, a devastating fungal disease. The resistance gene confers immunity against this pathogen's most virulent races, including Ug99. We used comparative whole-genome sequencing of chemically mutagenized and natural isolates to identify a fungal gene named that is required for avirulence. The gene encodes a secreted protein capable of interacting with Sr35 and triggering the immune response. We show that the origin of isolates virulent on is associated with the nonfunctionalization of the gene by the insertion of a mobile element. The discovery of provides a new tool for surveillance, identification of host susceptibility targets, and characterization of the molecular determinants of immunity in wheat.