Development of SSR markers linked to QTL reducing leaf hair density and grapevine downy mildew resistance in Vitis vinifera

Dense leaf hairs of grapevines have been known to act as a preexisting defense structure for preventing the incidence of grapevine downy mildew, because the pathogen, Plasmopara viticola, needs water to invade grapevines, and water is repelled by a hydrophobic surface due to dense leaf hairs. In the present study, we performed regression analyses of downy mildew resistance with leaf hair density using hybrids of Vitis labrusca origin and confirmed the effect of leaf hairs. Reducing the repelling effect of leaf hairs by detergent application canceled the effect of leaf hair, which confirmed the hypothesis. Thereafter, based on QTL analyses in the population of V. vinifera ‘Muscat of Alexandria’ × the interspecific hybrid ‘Campbell Early,’ we identified a major locus in linkage group (LG) 5 of ‘Muscat of Alexandria’ controlling leaf hair density. This locus was previously reported as a small effect QTL for downy mildew resistance, however we found that the locus had high LOD scores explaining 71.9%–78.5% of the phenotypic variance of leaf hairs. Moreover, this locus was detected as a QTL for downy mildew resistance. The effect of this locus was confirmed in two other hybrid populations. Finally, we could successfully identify three traditional V. vinifera table grapes ‘Muscat of Alexandria,’ ‘Katta Kurgan,’ and ‘Parkent’ as the origin of the allele linked to hairlessness by defining the SSR haplotypes. The use of this locus for marker-assisted selections would be a promising strategy for producing grapevines with resistance by preexisting defense structure.