Development of Sequence Characterized Amplified Region Markers Linked to Downy Mildew Resistance in Broccoli

Downy mildew, caused by the fungal parasite Peronospora parasitica (Pers.: Fr.) Fr., is a destructive disease of Brassica oleracea L. crops, including broccoli ( B. oleracea , Italica Group). The development and deployment of downy mildew resistant broccoli cultivars is a priority for breeders and producers. Identification of genetic markers linked to downy mildew resistance genes should facilitate selection for resistance and pyramiding of resistance genes into cultivars. The objectives of this study were to 1) identify RAPD markers linked to a single dominant gene for resistance in broccoli, 2) clone and sequence the linked RAPD markers, and 3) develop and evaluate SCAR markers as screening tools for resistance. Bulked segregant analysis led to the identification of eight linked RAPD markers following a screen of 848 decamers. Two of the linked RAPD fragments, UBC359 620 and OPM16 750 , were converted to dominant SCAR markers linked in coupling to the resistance locus at 6.7 and 3.3 cM, respectively. The SCAR marker based on UBC359 620 sequence exhibited less accuracy (94%) than the original RAPD (96%) in differentiating resistant and susceptible plants, but the accuracy (97%) of the OPM16 750 -SCAR was not different than the original RAPD. These SCAR markers are among the first genetic markers found linked to a gene conferring cotyledon-stage downy mildew resistance in B. oleracea . Results of this work provide breeders with useful information and tools for the systematic development of resistant cultivars.