Characterisation of the late blight resistance in potato differential MaR9 reveals a qualitative resistance gene, R9a, residing in a cluster of Tm-22 homologs on chromosome IX

Key message The durable late blight resistance in potato plant Ma R9 is genetically characterized. A novel R -gene is mapped. The monogenic nature and map positions of R9 are negated and rectified. Late blight of potato ( Solanum tuberosum ), caused by Phytophthora infestans , can effectively be managed by genetic resistance. The Ma R9 differential plant provides durable resistance to a broad spectrum of late blight strains. This resistance is brought about by at least seven genes derived from S. demissum including R1 , Rpi - abpt1 , R3a , R3b , R4 , R8 and, so far uncharacterized resistance gene(s). Here we set out to genetically characterize this additional resistance in Ma R9 . Three BC 1 populations derived from Ma R9 were identified that segregated for IPO-C resistance but that lacked R8 . One BC 1 population showed a continuous scale of resistance phenotypes, suggesting that multiple quantitative resistance genes were segregating. In two other BC 1 populations resistance and susceptibility were segregating in a 1:1 ratio, suggesting a single qualitative resistance gene ( R9a ). A chromosome IX PCR marker, 184-81, fully co-segregated with R9a . The map position of R9a on the distal end of the lower arm of chromosome IX was confirmed using PCR markers GP101 and Stm1021. Successively, cluster-directed profiling (CDP) was carried out, revealing six closely linked markers. CDP Sw 58, CDP Sw 59 and CDP Sw5 10 flanked the R9a gene at the distal end (5.8 cM) and, as expected, were highly homologous to Sw - 5 . CDP Tm2 2 flanked R9a on the proximal side (2.9 cM). CDP Tm2 6 and CDP Tm2 7 fully co-segregated with resistance and had high homology to Tm - 2 2 , showing that R9a resides in a cluster of NBS–LRR genes with homology to Tm - 2 2 . Besides R9a, additional resistance of quantitative nature is found in Ma R9 , which remains to be genetically characterized.