DETECTION OF THE STEM RUST RESISTANCE GENE RPG1 IN BARLEY SEEDLINGS

The stem rust resistance gene Rpg1 has conferred durable resistance in barley (Hordeum vulgare) to Puccinia graminis f. sp. tritici (wheat stem rust pathogen) for nearly 50 yr in the northern Great Plains. Detection of Rpg1 at the seedling stage can be difficult because most barley genotypes show mesothetic reactions to the stem rust pathogen. Genotypes with and without genes for stem rust resistance were inoculated with pathotypes Pgt-TPM, -MCC, and -HPH of P. g. tritici in the seedling stage. After 12 days of incubation at 25-28 C, infection types were assessed on plants. With pathotype TPM, differences between barley genotypes with and without resistance genes were not distinct, since similar infection types were frequently observed on genotypes in both groups. In contrast, marked differences were observed among barleys to pathotypes MCC and HPH; genotypes with Rpg1 exhibited low infection types (0;, 0;1, 10;, occasionally 12 with pathotype HPH), whereas those with rpg1 (the recessive allele for susceptibility) exhibited distinctly higher infection types (mostly 3-3, 3-2, 33-, 33+, occasionally 23- with pathotype MCC). Genotypes carrying resistance gene Rpg2, Rpg3, or rpgBH displayed mostly intermediate (23-) to high (3-2, 3-3) infection types to these pathotypes. These data demonstrate the usefulness of pathotypes MCC and HPH in detecting barley genotypes with Rpg1. The reliability of this screening method, coupled with the short cycle (19 days) of seedling tests, will hasten the development of barley germ plasm with stem rust resistance and aid in subsequent genetic studies in this pathosystem.