Background: Vitis viniferaL. is the most cultivated grapevine species worldwide.Erysiphe necatorSch., the causal agent of grape powdery mildew, is one of the main pathogens affecting viticulture.V. viniferahas little or no genetic resistances againstE. necatorand the grape industry is highly dependent on agrochemicals. Some CaucasianV. viniferaaccessions have been reported to be resistant toE. necatorand to have no genetic relationships to known sources of resistance to powdery mildew. The main purpose of this work was the study and mapping of the resistance toE. necatorin the Caucasian grapes ‘Shavtsitska’ and ‘Tskhvedianis tetra’.Results: The Caucasian varieties ‘Shavtsitska’ and ‘Tskhvedianis tetra’ showed a strong partial resistance toE. necatorwhich segregated in two cross populations: the resistant genotypes delayed and limited the pathogen mycelium growth, sporulation intensity and number of conidia generated. A total of 184 seedlings of ‘Shavtsitska’ x ‘Glera’ population were genotyped through the Genotyping by Sequencing (GBS) technology and two high-density linkage maps were developed for the cross parents. The QTL analysis revealed a major resistance locus, explaining up to 80.15% of the phenotypic variance, on ‘Shavtsitska’ linkage group 13, which was associated with a reduced pathogen infection as well as an enhanced plant necrotic response. The genotyping of 105 Caucasian accessions with SSR markers flanking the QTL revealed that the resistant haplotype of ‘Shavtsitska’ was shared by ‘Tskhvedianis tetra’ and a total of 25 Caucasian grape varieties, suggesting a widespread presence of this resistance in the surveyed germplasm. The uncovered QTL was mapped in the region where theRen1locus of resistance toE. necator, identified in theV. vinifera‘Kishmish vatkana’ and related grapes of Central Asia, is located. The genetic analysis conducted revealed that the Caucasian grapes in this study exhibit a resistant haplotype different from that of Central Asian grape accessions.Conclusions: The QTL isolated in ‘Shavtsitska’ and present in the CaucasianV. viniferavarieties could be a new candidate gene of resistance toE. necatorto use in breeding programmes. It co-localizes with theRen1locus but shows a different haplotype from that of grapevines of Central Asia. We therefore consider that the Caucasian resistance locus, namedRen1.2, contains a member of a cluster of R-genes, of which the region is rich, and to be linked with, or possibly allelic, toRen1.

Downy mildew, caused by the biotrophic oomycete Plasmopara viticola, is one of the most serious grapevine diseases. The development of new varieties, showing partial resistance to downy mildew, through traditional breeding provides a sustainable and effective solution for disease management. Marker-assisted-selection (MAS) provide fast and cost-effective genotyping methods, but phenotyping remains necessary to characterize the host–pathogen interaction and assess the effective resistance level of new varieties as well as to validate MAS selection. In this study, the Rpv mediated defense responses were investigated in 31 genotypes, encompassing susceptible and resistant varieties and 26 seedlings, following inoculation of leaf discs with P. viticola. The offspring differed in Rpv loci inherited (none, one or two): Rpv3-3 and Rpv10 from Solaris and Rpv3-1 and Rpv12 from Kozma 20-3. To improve the assessment of different resistance responses, pathogen reaction (sporulation) and host reaction (necrosis) were scored separately as independent features. They were differently expressed depending on Rpv locus: offspring carrying Rpv3-1 and Rpv12 loci showed the strongest resistance response (scarce sporulation and necrosis), those carrying Rpv3-3 locus showed the highest levels of necrosis while Rpv10 carrying genotypes showed intermediate levels of both sporulation and necrosis.