The effect of homozygosity of locus Rpv12 on downy mildew resistance of grapevine leaves

Inoculation trials with different pathogenic fungi and oomycetes have not yet shown any significant effect of homozygosity in resistance loci on the level of resistance. However, here a considerable reduction in mycelial growth of the oomycete P. viticola was found in association with the homozygous state of the Rpv12 locus. This effect was detected by aniline blue staining of the mycelium after experimental inoculation of leaf tissue and quantified by image analysis. Genotypes homozygous for the Rpv12-locus as classified by SSR marker data, inhibited mycelial growth considerably more in comparison to genotypes carrying the locus Rpv12 in heterozygous state.