Resistance to stem rust in barley and inheritance of resistance to race QCC

Twenty-two barley (Hordeum vulgare L.) cultivars and lines were evaluated for resistance to stem rust (Puccinia graminis Pers. f. sp. tritici Eriks. & Hem.) race QCC in field trials over a 3-yr period, and the inheritance of resistance of several genotypes to race QCC was investigated using replicated F-3 lines. Rust severities ranged from 17 to 58% between 14 different cultivars and lines which carried the stem rust resistance gene Rpg1, indicating that factors other than Rpg1 conferred resistance. Hietpas-5 (Rpg2) had moderate resistance while PI 382313 (Rpg3) had good resistance to race QCC. In the cross Hiproly-15/Peatland-9, F-2 adult plants segregated 3R : 1S to race MCC, confirming the presence of gene Rpg1 in Peatland-9. Some F-3 lines derived from F-2 plants with Rpg1 were susceptible to race QCC, however, indicating that Rpg1 was ineffective against race QCC. A second gene in Peatland-9, designated for the interim as RpgU and independent of Rpg1, was shown to confer a moderate level of resistance to race QCC. Similar segregation patterns in the presence of race QCC and pedigree analysis suggested that the cultivars Husky and Diamond contain this gene. This gene is likely present in other related cultivars such as Vantage and Bonanza. Segregation in progeny of the cross Hiproly-15/Q21861 indicated that resistance to race QCC was controlled by two recessive genes in Q21861. Gene Rpg2, tested in F-3 lines, was found to provide a moderate level of resistance to race QCC, similar to RpgU, but was less effective in the heterozygous condition. The phenotypic expression of RpgU differed from Rpg1, Rpg2, and Rpg3, suggesting that RpgU is a previously unreported gene for stem rust resistance in cultivated barley.