Powdery mildew is one of the most important fungal diseases on greenhouse and field grown roses. Improvement of disease resistance is a major selection aim in rose breeding, especially for garden roses. The development of molecular markers linked to resistance in merely tetraploid rose cultivars is a tedious task. The development of molecular markers linked to resistance in merely tetraploid rose cultivars is a tedious task. Hence, a diploid segregating population of 94 plants was established. The parent plants of this population ‘Yesterday’ and R. wichurana differ in resistance towards to two monoconidial powdery mildew isolates. R. wichurana shows partial resistance to the pathotypes R-E and R-P; ‘Yesterday’ is resistant to pathotype R-E but susceptibility to R-P. Segregation for resistance to the two powdery mildew isolates was studied in the offspring by a bio-assay and by a microscopy study of specific resistance mechanisms. The bio-assay revealed a considerable quantitative variation in resistance for both pathotypes, indicating the resistance is polygenic. Microscopic observations discriminated in more detail resistance mechanisms in the rose plants at different developmental stages of the fungus. For each parent a genetic linkage map was constructed with AFLP, SSR and morphological anchor markers. These resulted in seven linkage groups putatively corresponding to the seven haploid rose chromosomes. Our results are in accordance with mapping data previously published. Final aim is to link the morphological data to the molecular data to search for QTLs that identify genomic regions involved in powdery mildew resistance for specific pathotypes as a helpful tool for marker- assisted breeding in roses.
