Mapping of resistance QTL to powdery mildew (Erysiphe necator Sch.) in the Caucasian grape varieties “Shavtsitska” and “Tskhvedianis tetra” (Vitis vinifera L.)

Vitis vinifera L. is the most cultivated grapevine species worldwide. Erysiphe necator, the causal agent of grape powdery mildew, is one of the main pathogens affecting the viticulture. V. vinifera has usually little genetic resistance against E. necator and grape production is highly dependent from agrochemicals. The main purpose of this work was the study and the mapping of the resistance to E. necator in the Caucasian V. vinifera germplasm. The biparental mapping approach was chosen to investigate the genetic basis of the trait and two F1 populations were developed by crossing Shavtsitska and Tskhvedianis Tetra with two susceptible V. vinifera varieties, Chardonnay and Glera. The phenotypic resistance of parental plants and offsprings was studied by using leaf discs bioassays and evaluating the infection features during the pathogen life cycle. Caucasian cross parents showed a resistance to E. necator and the trait segregated in their populations: the resistant genotypes delayed and limited the development of pathogen mycelium, sporulation and conidia but they did not halt completely the infections. A total of 184 seedlings of Shavtsitska cross population were genotyped through the Genotyping by Sequencing (GBS) technology allowing the development of two high-density linkage maps for the cross parents. QTL analysis revealed a major resistance locus on Shavtsitska linkage group 13. Such a QTL was associated with a reduced pathogen development as well as an enhanced plant necrotic response. The QTL explained up to 80.15% of the observed variability and was restricted in an interval of approx. 2.2 cM. The comparison with grape reference genome PN40024 located the QTL at about 18 Mb from the top of the DNA sequence on chromosome 13 and recombinants analysis restricted the locus in a region of 1.4 Mb. Some SSR located in the genomic region were used for genotyping the cross populations of the study and 103 further Caucasian varieties. Resistance associated SSR alleles were shared among Shavtsitska, Tskhvedianis tetra, resistant seedlings and 22 Caucasian varieties suggesting a widespread presence of the resistance trait in the Caucasian germplasm. The QTL isolated in Shavtsitska located in the region where the Ren1 resistance gene carried by several Central Asia V. vinifera was previously mapped. Further molecular analysis is needed to confirm whether different genes in such region, that in the reference genome PN40024 is rich of resistance motifs, control the resistance to E. necator in grape accessions of different geographic origin. Meanwhile, our findings would provide new V. vinifera genetic sources for grape breeding programs aiming to obtain resistant elite cultivars.