Genetics of Stem Rust Resistance in ‘Gage' Hard Red Winter Wheat.

Wheat stem rust, caused by Puccinia graminis f. sp. tritici, re-emerged as a devastating disease of wheat (Triticum aestivum L.) because of virulent race Ug99 (TTKSK). Most modern wheat cultivars grown in Nebraska are susceptible to Ug99 and derivatives with additional virulence. Gage, a Nebraska hard red winter wheat cultivar, was released in 1963 mainly for its excellent field resistance to stem rust conferred in part by adult plant stem rust resistance gene Sr2. Preliminary studies indicated that Gage had seedling resistance to Ug99 and derivatives, which is conferred independently of Sr2 since this gene is effective only at the adult plant stage. Gage could provide much needed Ug99 resistance if resistance were transferred to modern breeding germplasm. To facilitate the efficient selection of stem rust resistance derived from Gage in breeding populations, we derived 245 F_2:3 families from a cross made between Gage and susceptible cultivar ‘Bill Brown’ with the objective of characterizing the genetics of resistance and identifying linked molecular markers. Infection phenotypes at the seedling stage to race TTKSK and both seedling and adult plant stages to North American race QFCSC were recorded. The Sr2 marker phenotype from F₂ plants further validated the involvement of adult plant resistance gene. Adult plant stem rust resistance to race QFCSC in the greenhouse suggested the involvement of other major gene than Sr2. Segregation of seedling resistance to races QFCSC and TTKSK among F_2:3 families did not agree with simple gene models suggesting that the seedling resistance is conferred by multiple genes. We are currently performing genome-wide marker analyses in order to map resistance genes. Though selection of polygenic resistance is difficult from a breeding perspective, such resistance is likely to remain durable to pathogen adaptation and would be beneficial for long-term resistance to stem rust.