Characterization of Rosa spp. Breeding Populations to Black Spot for Molecular Markers Development and QTL Identification

Black spot disease, caused by the fungus Diplocarpon rosae Wolf, is the most serious disease of landscape roses (Rosa hybrid L.) worldwide. Dominant genes for complete resistance to specific races of the pathogen were identified in roses as Rdrs. From a breeding perspective, a rapid screening of potential hybrid materials by molecular markers is beneficial for identifying the resistant germplasm efficiently. Although partial resistance has also been documented, the responsible QTLs remain unidentified. In this project, responses to D. rosae of 16 genotypes of roses that were used as parents in hybrids were characterized with two inoculation methods: the detached leaf assay (DLA) and the whole plant inoculation (WPI) method. The correlation between and each method’s precision at measuring relative resistance among genotypes was analyzed. Three hybrid populations were selected for partial resistance QTL identification based on field rating of black spot infection: Orange Honey x FF (10018), Sweet Chariot x M4-4 (10043) and J06-28-4-6 x Red Fairy (10061). Samples from each population were measured by DLA for resistance to race 8. The results indicate that no complete resistant to race 8 genotype was identified although in some genotypes not all the inoculated samples formed acervuli. DLA is more sensitive than WPI in measuring relative resistance. However, the correlation among the two methods is high (r² >0.8) which indicates that either can be utilized to characterize D. rosae. The three selected hybrid populations which segregate for partial resistance have been developed for QTL identification. In addition, population Home Run x Gold Garden (10027) segregating for race 8 complete resistances is available as well for associated marker development. For all four of selected hybrid populations, accurate phenotypes of all progenies in the selected populations will be measured by DLA.

Index words: Rosa, Black spot, Resistance, Diplocarpon rosae, QTL