Background selection was accomplished with the GoldenGate 6KSNP chip (Illumina, San Diego, CA, USA). Three BC5F1 plants with the shortest donor ZmCCT segment of each family were used for background selection. Prior to genotyping, DNA quality was carefully checked for each sample. The SNP genotyping and allele scoring were performed as described (Fan et al. 2006; Yan et al. 2010).Phenotypic selection was performed in BC3F1 and BC5F1families after foreground selection. In BC3F1 family, three most resistant plants (scored 0) with the left-shortest donor ZmCCT segment were selected to produce next generation. In BC5F1 family, we selected eight resistant plants for recombination selection to identify three BC5F1 plants with the shortest donor ZmCCT segment for background selection.