Marker Validation for Rpf1 Red Stele (Phytophthora fragariae) Resistance in Fragaria for Use in Marker Assisted Breeding

Red stele (Phytophthora fragariae var. fragariae) is a devastating root rot disease in strawberries (Fragaria) in most regions of the world where soils stay cool and moist. Several sources for genetic resistance are exploited in breeding, and several race-specific R-genes have been distinguished. Recently, a tightly linked SSR marker was found for the Rpf1 gene at Wageningen-UR, The Netherlands. As part of the “RosBREED: Enabling Marker-Assisted Breeding in Rosaceae” project, 900 strawberry genotypes representing the breadth of relevant diversity used in breeding the domesticated strawberry have been planted in Oregon, Michigan, California, Florida, and New Hampshire. To carry out the validation and discovery of new sources of resistance, a subset of 155 genotypes from this group with known and unknown response to P. fragariae were chosen to test in response to two races of this disease in bench tests: Canadian race 4 (A-3) isolate ONT-3, and Cdn-5 (A-5) isolates BC-23 and NOV-77. SSR marker genotyping after capillary electrophoresis separation of the PCR products indicated presence of the marker allele associated with resistance to the Rpf1 gene in 52 individuals. This marker allele was absent in 103 individuals of the 155 selected for validation. A majority of genotypes lacking the Rpf1 marker allele are wild species and known to have some resistance to P. fragariae. These are expected to show resistance to both race A-3 and A-5 indicating additional factors for the resistance to this disease. Validating this marker for diagnosing the presence of the Rpf1 gene for resistance will allow this marker to be integrated into marker-assisted breeding and its future use in pyramiding resistance genes.