From 94 accessions genotyped 47 (50%) were cultivars and lines developed in breeding centers located in Siberia. Other 47 accessions were cultivars and lines maintained in the Siberian spring barley collection, but originating from other regions and countries. Cluster analysis revealed 4 major groups of accessions (Additional file 2). Two big groups consisting of 38 (group I) and 30 (group IV) genotypes noticeably differed by percentage of Siberian cultivars and lines (73% and 17% respectively). Groups II and III were “half-Siberian” (Additional file 2). Group I had the highest percentage of varieties resistant to both isolates of C. sativus (18% vs 0, 3 and 5% in other three groups) and the lowest percentage of susceptible varieties (32% vs 47, 48 and 50% in other groups) to both isolates (Additional file 2). This may suggest essential contribution of Siberian varieties to resistant germplasm of spring barleys studied. Indeed, we noticed that among accessions resistant to both isolates 67% had Siberian origin.
GWAS revealed three genomic regions (on chromosomes 1H, 2H and 3H) associated with resistance to one or both isolates (Tables 1 and 2, Fig. 2). In spite of small sample size analyzed in the current study, we did not reveal false positive loci. Comparison of three genomic regions identified in our study with locations of C. sativus resistance QTLs revealed previously, showed that each of the three regions contains known spot blotch resistance loci.
SNP associated with resistance to C. sativus isolate Kr2 was revealed between 50 and 60 cM of Chromosome 1H genetic map. The presence of QTL for spot blotch resistance in barley chromosome 1H within 50 and 60 cM was reported in several studies. In 1996, Steffenson et al.  reported QTL for adult resistance to C. sativus on chromosome 1H between markers ABG500–ABG494 (53.6–61.2 cM, according consensus map ). In 2005, Bilgic et al.  has found major locus for seedling resistance in the same region using Steptoe / Morex (R) mapping population. In 2010, Roy et al.  has revealed seedling resistance locus at 59,7 cM by association mapping.
Later, spot blotch resistance locus was found in barley chromosome 1H within 40 and 50 cM. Association with this region was reported in 2013 by Gutierrez et al. , who used AM-approach exploiting DArT-markers. At the same time, Zhou and Steffenson  have found locus for both adult and seedling resistance (associated with BOPA SNP markers 11_10764, 11_10275 and 12_30336) at 41–43 cM of chromosome 1H. In 2015, Afanasenko et al.  has revealed QTL for seedling resistance (at the SNP-locus BOPA11_10764, syn. BOPA1_5381–1950; position in iSelect map - 41.5 cM) by analysis of biparental mapping population Zernogradsky 85 (R) / Ranny 1. In 2016, Haas et al.  has found QTL for seedling resistance (at the SNP-locus BOPA2_12_30404; position in iSelect map - 48.1 cM) by analysis of biparental population PI 466423 (R) / Rasmusson. Recently, Wang et al.  reported about association of the region 42–44 cM (markers SCRI_RS_193392, SCRI_RS_153785, SCRI_RS_170878, SCRI_RS_170869, SCRI_RS_189483, BOPA1_5381_1950) with seedling resistance.
The resistance QTL found on chromosome 1H in our study is likely coincident with previously identified QTLs in the interval 40–60 cM [1, 2, 6,7,8,9, 11, 12]. Besides chromosome region within 40–60 cM, two more regions of chromosome 1H are associated with resistance to spot blotch: distal part of the long arm (1HL; QTL at the SNP-locus BOPA_11_10433, syn. BOPA1_3201–603; position in iSelect map - 87.0 cM ) and the short arm (1HS; gene Rcs6 at ca. 15 cM [3, 14] and QTL at the DArT-locus bPb–9604; 16.9 cM ).
The region on chromosome 2H (within 57–60 cM) revealed in our study for Ch3 isolate resistance included 16 SNP loci (Table 2). Three SNP loci corresponding to this region had suggestive p-values in case of Kr2 tests (Table 1), so the resistance locus on chromosome 2H may also contribute to resistance to Kr2-isolate (Fig. 2).
The resistance QTL found on chromosome 2H in our study is likely coincident with previously identified QTLs for seedling resistance [2, 7]. In 2005, Bilgic et al.  detected QTL in the interval 27.1–46.9 cM (markers Rbcs–ABG459) using Steptoe / Morex (R) mapping population. Later, Afanasenko et al.  revealed QTL at the SNP-locus BOPA_11_11015 (syn. BOPA1_946–2500; position in iSelect map - 54.2 cM) by analysis of biparental mapping population Zernogradsky 85 (R) / Ranny 1.
Besides this chromosome region another part of chromosome 2H (distal part of the long arm) is associated with resistance to spot blotch. Wang et al.  revealed significant association with SCRI_RS_152664 (138.6 cM).
The region of chromosome 3H associated with Ch3-resistance was expanded between markers SCRI_RS_97417 and JHI-Hv50k-2016-158003 and included 11 SNPs, from which JHI-Hv50k-2016-157070, JHI-Hv50k-2016-156842 had the lowest p-values (Table 2). These two SNPs were also significant with resistance to Kr2 isolate (Table 1). Position of genetically mapped marker from this set is 12.1 cM.
The resistance QTL found on chromosome 3H in our study is likely coincident with some of the previously identified QTLs. Bovill et al.  revealed QTLs for adult plant resistance to spot blotch within 2–28 cM of chromosome 3H, using different mapping populations. Later, Zhou and Steffenson  revealed seedling and adult resistance loci on chromosome 3H in positions 9.6 cM (marker BOPA_12_30818) and 19.2 cM (BOPA_11_20742, BOPA_11_10565), exploiting association mapping approach.
More proximal loci on chromosome 3H also have been found. Bilgic et al. (2005) revealed locus for seedling resistance in the interval 28.7–42.4 (markers ABC171-MWG584) using Steptoe/Morex (R) mapping population. Grewal et al.  revealed seedling resistance QTL in the interval 24.9–31.1 cM (marker bPb-3565), and 2 adult plant resistance QTLs in the intervals 23.0–24.9 cM (marker bPb-6127) and 31.8–43.3 cM (marker E40M61.1), using DH recombinant lines from the cross CDC Bold / TR251 (R). Wang et al.  reported two regions on chromosome 3H, each associated with different isolates: BOPA1_3906_558 at 25.3 cM and BOPA1_5960_1302 at 66.2 cM.