The 2012 ASHS Annual Conference

Black spot disease, caused by the fungus Diplocarpon rosae Wolf, is the most serious disease of landscape roses (Rosa hybrid L.) worldwide. Dominant genes for complete resistance to specific races of the pathogen were identified in roses as Rdrs. From a breeding perspective, a rapid screening of potential hybrid materials by molecular markers is beneficial for identifying the resistant germplasm. In this project, two microsatellite markers (155 at 0 cM and 69E24 at 0.1 cM distance) linked to Rdr1 (resistance to race 3) were used to screen twenty cultivars for which the phenotypes are known and  a rose collection consisting of the TAMU diploid breeding population, Earth-Kind® collection, Ralph Moore roses and various Rosa species. Up to four alleles of the locus 155 were amplified in 168 rose genotypes. The cultivars for which the phenotypes are known amplified fragments around 160 bp for the locus 155 and around 180 bp for the locus 69E24, indicating the presence of the Rdr1 allele that conditions resistance to race 3. One hundred and twenty of the 147 plants with unknown phenotypes assayed had both the 160 bp band (locus 155) and the 180 bp band (locus 69E24) indicating that these possessed the Rdr1 resistance gene. In addition one SCAR marker (ND5E) (9.1 cM distance) linked to Rdr3 (resistance to race 8) was used to screen 51 rose genotypes of which the disease resistance phenotype with race 8 was known for 18. The one amplification product (around 80 bp) for the SCAR ND5E was only detected in the resistant roses and not the susceptible roses.  Among the 33 roses with unknown resistance phenotypes, only one had the 80 bp band. The accuracy of these marker results among the roses with unknown resistance phenotypes will be verified by detached leaf assays with single spore derived cultures of the race 3 and race 8 of the black spot pathogen.