Black spot partial resistance in diploid roses : QTL discovery and linkage map creation

Black spot disease (Diplocarpon rosae) is the most important leaf disease of garden roses in warm humid areas. Although partial resistance to black spot has been shown to be moderately heritable, the responsible quantitative trait loci (QTL) remain unidentified. Because of the interspecific nature and high heterozygosity in commercial roses, as well as the relatively small research input compared to row crops, the genomic resources available for rose are limited. To effectively identify markers associated with QTL controlling black spot resistance, abundant markers across the genome and careful phenotyping are required. Fifteen inter-related diploid rose populations with black spot resistant cultivar R. wichuraiana ‘Basye’s Thornless’ in the genetic background were assessed based on the percent of total foliage covered with lesions in June, October and November of 2016 in College Station. Broad sense heritability was estimated at 0.51 which indicates black spot partial resistance is a moderately heritable trait. Genotyping-by-sequencing technology was used to generate SNP markers for linkage map construction. Previous anchor SSR markers were used to designate the linkage group number. The final consensus map used for black spot QTL detection contained 791 SNP covering 430 cM with the biggest gap being 6.6 cM on LG4. One major black spot QTL was discovered on LG3 explaining ~13% of the total phenotypic variance (equaling approximately 26% of the genetic variation) using pedigree-based analysis among all fifteen populations.