Since the mid-1940's, barley cultivars grown in the northern Great Plains of the USA and Canada have been resistant to stem rust caused by Puccinia graminis f. sp. tritici. This durable resistance is largely conferred by a single gene, Rpg1, derived from a single plant selection of the cultivar Wisconsin 37 and an unimproved Swiss cultivar. At the seedling stage, barley genotypes with Rpg1 generally exhibit low mesothetic reactions at 16-20-degrees-C and slightly higher mesothetic reactions at 24-28-degrees-C to many stem rust pathotypes. This resistance is manifested by a low level of rust infection and mostly incompatible type uredia on adult plants. Rpg1 reacts in a pathotype-specific manner since some genotypes of P. g. f. sp. tritici are virulent on cultivars carrying this gene in the field. Several factors may have contributed to the longevity of stem rust resistance in barley, a) since barley is planted early and matures early, it can sometimes escape damage from stem rust inoculum carried from the south; b) one or more minor genes may augment the level of resistance already provided by Rpg1; c) the cultivation of resistant wheat cultivars and eradication of barberry have reduced the effective population size and number of potential new pathotypes of P. g. f. sp. tritici, respectively; and d) virulent pathotypes of P. g. f. sp. tritici and P. g. f. sp. secalis have not become established. This situation changed in 1989 when a virulent pathotype (Pgt-QCC) of P. g. f. sp. tritici became widely distributed over the Great Plains. However, Rpg1 may still confer some degree of resistance to pathotype QCC because stem rust severities have been low to moderate and yield losses light on barley cultivars carrying the gene during the last four seasons (1989-1992). Several sources of incomplete resistance to pathotype QCC have been identified in barley. To facilitate the transfer of resistance genes from these sources into advanced breeding lines, molecular marker assisted selection is being employed.
