Mitchell Fetch, J. and Fetch Jr., T. 2011. Inheritance of resistance to oat stem rust in the cultivars Ronald and AC Gwen. Can. J. Plant Sci. 91: 419-423. Oat stem rust, caused by Puccinia graminis Pers. f. sp. avenae Eriks. and E. Henn., causes sporadic epidemics and yield losses in Manitoba and eastern Saskatchewan. Oat (Avena sativa L.) cultivars registered for production in this area possess resistance to stem rust, but their genetic composition is unknown. Race TJJ (NA67), first detected in 1998, is virulent on most Pg genes; thus, it is desirable to know the gene(s) that are present in currently grown oat varieties. The hulled cultivar Ronald and the hulless cultivar AC Gwen were selected for characterization of inheritance of stem rust resistance, and crossed to the susceptible cultivar Triple Crown. Race BLD (NA1) was used to evaluate F₁ plants and F₂ populations. All F₁ plants were resistant, indicating the presence of at least one dominant gene. Pooled F₂ populations fit a 13:3 (resistant:susceptible) ratio for both Ronald (χ² =0.31, P =0.58) and AC Gwen (χ² =3.32 and P =0.07), indicating the presence of one dominant and one recessive gene. Segregation ratios in pooled F₃ families fit a 7:8:1 (homozygous resistant:segregating:homozygous susceptible) ratio with race BLD (χ² =0.84, P =0.66 for Ronald; χ² =1.99, P =0.37 for AC Gwen), a 1:2:1 ratio (segregating families with 1:3 ratio) with race TJD (χ² =2.87, P =0.24 for Ronald; χ² =3.45, P =0.18 for AC Gwen), and a 1:2:1 ratio (segregating families with 3:1 ratio) with race FDJ (χ² =1.33, P =0.51 for Ronald; χ² =2.04, P =0.36 for AC Gwen). These results and reactions to known races of Puccinia graminis f. sp. avenae indicate that both Ronald and AC Gwen possess the dominant gene Pg2 and recessive gene Pg13. This genetic study characterized the stem rust resistance in Ronald and AC Gwen, and confirmed the limited base of stem rust resistance in Canadian oat cultivars.