Pyramiding adult-plant powdery mildew resistance QTLs in bread wheat

Pyramiding of quantitative trait loci (QTLs) can be an effective approach for developing durable resistance to powdery mildew in wheat (Triticum aestivum L.). The Chinese wheat cultivars Bainong 64 and Lumai 21, with outstanding agronomic traits, possess four and three QTLs, respectively, for adult-plant resistance (APR) to powdery mildew. To achieve optimal durable resistance, 21 F6 lines combining two–five powdery mildew APR QTLs were developed from the cross Bainong 64/Lumai 21 using a modified pedigree selection. These lines were planted in a randomised complete block design with two replicates in Beijing during the 2009–10 and 2010–11 cropping seasons, and were evaluated for powdery mildew response using the highly virulent Blumeria graminis f. sp. tritici isolate E20. Based on the phenotypic data of both maximum disease severity (MDS) and area under the disease progress curve (AUDPC), analysis of variance indicated that there were highly significant effects of QTL combinations on reducing powdery mildew MDS and AUDPC. Six pyramided QTL combinations possessing QPm.caas-1A and QPm.caas-4DL in common along with one or more of the others expressed better APR to powdery mildew than the more resistant parent, Bainong 64. Thus, pyramiding these two QTLs with one or more of QPm.caas-2BS, QPm.caas-2BL, and QPm.caas-2DL from Lumai 21 could be a desirable strategy to breed cultivars with high levels of durable resistance to powdery mildew. Experienced breeders with a good knowledge of minor genes can achieve APR by phenotypic selection, and selection by molecular markers will still require uniform field testing for powdery mildew and disease phenotype to validate the resistance. These results provided very useful information for pyramiding APR QTLs in wheat breeding programs.