Black spot disease of rose (Rosa hybrida L.), incited by the fungus Diplocarpon rosae Wolf, is the most serious disease of garden roses worldwide. Breeding for resistance has been hindered by a lack of knowledge of the genetic inheritance of resistance and insufficient characterization of the pathogen. The objectives of this dissertation were (1) unify the race nomenclature for an international collection of D. rosae isolates and establish a standard set of differential cultivars for determining races, (2) examine the inheritance of resistance in tetraploid rose cultivars to North American races of D. rosae and develop molecular markers linked to major resistance genes and (3) characterize combining ability in diploid and tetraploid roses for partial resistance to black spot disease in the absence of major gene effects.

Fifteen previously characterized single-spore isolates of D. rosae from Germany, the United Kingdom, and North America, as well as Belgium and Italy, were inoculated to a common set of fifteen rose cultivars in replicated, detached leaf trials. A total of eleven pathogenic races were differentiated based on their unique host ranges and were assigned a unified race nomenclature. Nine cultivars were sufficient to differentiate the eleven races and are proposed as the standard host differential set for future characterization of D. rosae isolates.

Resistance to North American D. rosae races 3 and 8 segregated 1:1 in multiple F₁ populations, indicating that both are conferred by dominant alleles at single loci and are present in simplex (Rrrr) configuration. Resistance to race 9 was partial and may be conferred by more than one locus. Analysis of these populations with microsatellite markers previously developed for Rdr1 revealed that the gene conferring race 3 resistance resides within the same R gene cluster as Rdr1. Race 8 resistance segregated independently and is, therefore, a novel locus for black spot resistance in rose which we propose to name Rdr3. NBS and LRR profiling were used in a bulked segregant analysis to identify a marker 9.1cM from Rdr3, which was converted to a high-throughput marker form for marker-assisted breeding.

Six diploid and six tetraploid rose cultivars were crossed in two factorial combining ability arrays in order to conduct the first analysis of combining ability for partial resistance to black spot disease in rose. Whole plant and detached leaf inoculation methods were used under two different disease pressures using a characterized single-spore isolate. Parents from both arrays had significant general combining ability effects across multiple inoculation methods and environments. Parent per se performance was highly correlated with progeny performance on a family mean basis. High positive correlations among whole plant and detached leaf inoculation methods indicate that detached leaf assays can substitute for whole plant assays. Based on these results, a breeding strategy including parental selection and early, among-family selection is proposed.