Greenbug (Schizaphis graminum, Rondani) is a serious insect pest in many wheat growing regions and has been infesting cereal crops in the USA for over a century. Continuous occurrence of new greenbug biotypes makes it essential to explore all greenbug resistant sources available to manage this pest. Gb1, a recessive greenbug resistance gene in DS28A, confers resistance to several economically important greenbug biotypes and is the only gene found to be resistant to greenbug biotype F. A set of 174 F_2:3 lines from the cross DS28A × Custer was evaluated for resistance to greenbug biotype F in 2020 and 2022. Selective genotyping of the corresponding F₂ population using single nucleotide polymorphism (SNP) markers generated by genotyping-by-sequencing (GBS) led to the identification of a candidate genomic region for Gb1. Thus, SSR markers previously mapped in this region were used to genotype the entire F₂ population, and kompetitive allele specific PCR (KASP) markers were also developed from SNPs in the target region. Gb1 was placed in the terminal region of the short arm of chromosome 1A, and its location was confirmed in a second population derived from the cross DS28A × PI 697274. The combined data analysis from the two mapping populations delimited Gb1 to a < 1 Mb interval between 13,328,200 and 14,241,426 bp on 1AS.

Greenbug (Schizaphis graminum Rondani) is a destructive insect pest that not only damages plants, but also serves as a vector for many viruses. Host plant resistance is the preferred strategy for managing greenbug. Two greenbug resistance genes, Rsg1 and Rsg2, have been reported in barley. To breed cultivars with effective resistance against various greenbug biotypes, additional resistance genes are urgently needed to sustain barley production. Wild barley accession WBDC053 (PI 681777) was previously found to be resistant to several greenbug biotypes. In this study, a recombinant inbred line (RIL) population derived from Weskan × WBDC053 was evaluated for response to two greenbug biotypes (E and TX1) and genotyped using genotyping by sequencing (GBS). A set of 3347 high quality GBS-derived single nucleotide polymorphisms (SNPs) were then used to map the greenbug resistance gene in this wild barley accession. Linkage analysis placed the greenbug resistance gene in a 2.35 Mb interval (0–2,354,645 bp) in the terminal region of the short arm of chromosome 2H. This interval harbors 15 genes with leucine-rich-repeat (LRR) protein domains. An allelism test indicated that the greenbug resistance gene in WBDC053, designated Rsg2.a3, is likely allelic or closely linked to Rsg2. GBS-SNPs 2H_1318811 and 2H_1839499 co-segregating with Rsg2.a3 in the RIL population were converted to Kompetitive allele specific PCR (KASP) markers KASP-Rsg2.a3-1 and KASP-Rsg2.a3-2, respectively. The two KASP markers can be used to select Rsg2.a3 and have the potential to tag Rsg2 in barley improvement programs.