Institute for Ornamental Plant Breeding, Ahrensburg, Germany

Background and objectives
One of the most severe diseases of field grown roses is blackspot, caused by the host specific facultative fungal parasite Diplocarpon rosae. Succesful infection of susceptible rose cultivars leads to the typical black spots on infected leaves surrounded in many cases by heavy chlorotic areas. This results in a very early defoliation of the whole plant which, after several successive infections, is severely weakend and eventually dies. As the treatment of susceptible cultivars with fungicides as the only control measure against blackspot in the past is more and more restricted due to legal restrictions and consumers concerns, breeding of resistance became one of the major goals of current rose breeding programs. As a prerequisite for more sophisticated breeding strategies for resistance to blackspot in roses, the racial structure of the fungus has to be investigated and resistant rose genotypes have to be identified. To accelerate the breeding process marker-assisted selection procedures are currently being developed.

Results and conclusions
Racial structure: the differentiation into physiological races of D. rosae causing blackspot on wild and cultivated roses was investigated with single conidial isolates of the pathogen. Inoculation experiments with a simple excised leaf assay demonstrated differential interactions between single isolates and a set of 10 test rose genotypes. As a result 5 different physiological races were identified. The detached leaf assay offers the possibility for an efficient and quick way to evaluate the reaction patterns of new plant pathogen genotype combinations and facilitates therefore the identification and genetic analysis of resistance genes in Rosa against blackspot. Identification of resistant rose genotypes. As a first step towards breeding of resistant rose cultivars sources of resistance have to be identified which mainly occur in wild species. After screening three rose collections several genotypes of different wild species with resistance to a broad range of pathogen races have been identified.
Genetic analysis: the hybrid breeding line 91/100-5 exhibits a broad resistance to all isolates tested so far. The resistance of this breeding line originated from the diploid R. multiflora hybrid 88/124-46. It was selfed to produce an F2 population, backcrossed to the susceptible tetraploid variety Caramba and crossed to the susceptible varieties Heckenzauber, Pariser Charme und Elina. Inoculation experiments resulted in segregation ratios consistent with the presence of a single dominant resistance locus in the duplex configuration in the hybrid 91/100-5. This genotype is currently used as a donor to introduce this resistance into the genetic background of cultivated roses in model breeding programs in order to develop improved breeding strategies based on marker assisted selection for blackspot resistance in roses.
Marker assisted selection: to reduce the time consuming process of breeding resistant rose cultivars, molecular markers tightly linked to the resistance gene can be used in marker assisted selection procedures. A progeny of the resistant breeding line 91/100-5 and a susceptible cultivar segregating for blackspot resistance was used for the identification of such markers. By testing 114 AFLP-primer combinations, 4 tightly linked fragments have been identified so far. In the same progeny the AFLP-technique was also applied to select genotypes with the smallest possible genome proportion of the wild donor species. Large differences in the proportion of donor genome could be detected and genotypes with the smallest donor genome proportion were crossed to susceptible rose cultivars.