Development of tools to study rose resistance to black spot

Black spot (caused by the fungal pathogen Diplocarpon rosae) is the most severe disease of roses in the outdoor landscape. Most cultivars are susceptible to this disease and its control requires repeated fungicide treatments. New rules on pesticide use encourage breeders to develop roses with a high resistance level to black spot. Growing genetically resistant rose cultivars is an alternative to chemical control of the disease. Resistance durability depends on the kind of resistance, the plant genetic background, the spatial and temporal deployment of resistances, the integration of resistances in cultural systems, and the dynamic of pathogen adaptation. To date, both dominant resistance genes and partial resistance loci have been described particularly in the genetic background of Rosa multiflora. In this study, a genotype closely related to the wild rose Rosa wichurana was examined to identify new resistance genes. Two progenies, connected by the resistant male parent, were scored for black spot resistance after natural infections in field over three and two years in three and one French locations for HW (Rosa hybrida ‘H190’ × hybrid of Rosa wichurana) and OW (Rosa chinensis ‘Old Blush’ × hybrid of Rosa wichurana) progenies, respectively. Genetic maps based on microsatellite and SNP markers were developed for HW and OW progenies, respectively. One common quantitative trait locus (QTL) was localized on linkage group 3 (LG3) of the male maps; it was revealed in the two progenies for several years and in different locations. Another QTL was identified but only in the HW progeny: it was mapped on LG5. The effectiveness of these QTLs should be confirmed against a wide range of pathogen isolates. To provide tools to study the genetic diversity of D. rosae, we sequenced two strains using Illumina paired-end sequencing technology and developed polymorphic microsatellite markers. The genome size of the two strains was estimated to 31 and 34 Mb. A set of 31 polymorphic markers was obtained. 70 monoconidial strains of D. rosae isolated from leaves of Rosa spp. with black spot symptoms were obtained from various locations in Europe and Asia. The analysis of this Eurasian pathogen collection revealed a strong genetic differentiation between pathogen populations collected from wild roses and those collected from gardens. To screen the resistance level of rose genotypes against pathogen diversity, we inoculated rooted cuttings with monoconidial strains of the pathogen under greenhouse conditions. Disease symptoms were scored 10 and 28 days post inoculation (dpi). This pathological test showed a range of resistance levels in rose against the diverse pathogen isolates collected in France and identified genotypes interesting for breeding.