Identification of Partial Resistance to Black Spot in an Incomplete Diallel Diploid Rose Population

Black spot disease, caused by the fungus Diplocarpon rosae Wolf, is the most serious disease of landscape roses (Rosa hybrid L.) worldwide. Partial (horizontal) resistance has been documented in roses, which is non-race specific and a more durable resistance compared to complete (vertical) resistance in wild environment controlled by multiple genes. However, the responsible QTLs remain unidentified. In this project, we identified the responses to race 8 of D. rosaeusing the detached leaf assay (DLA) in sixteen hybrid populations, which were from a partial diallel mating experiment.by crossing diploid parents with a high to moderate level of partial resistance to black spot crossed with several susceptible roses. Results from DLA indicated that the distributions of responses to this pathogen in some populations are skewed toward extreme reactions, i.e. either better resistance or susceptibility. Broad and narrow heritability estimated by genetic variances were high (>0.9) for both leaf area with symptoms (LAS) and lesion length (LL). Similar results were estimated from offspring mid-parents regression, in which narrow sense heritability is 0.86 and 0.74 for LAS and LL, respectively. Hybrid populations that segregate for partial resistance (PR) could be utilized for PR related QTL(s) identification and mapping.

Index words: Rosa, black spot, disease resistance, Diplocarpon rosae, QTL, host plant resistance