Mapping of the S. demissum late blight resistance gene R8 to a new locus on chromosome IX.

The use of resistant varieties is an important tool in the management of late blight, which threatens potato production worldwide. Clone MaR8 from the Mastenbroek differential set has strong resistance to Phytophthora infestans, the causal agent of late blight. The F1 progeny of a cross between the susceptible cultivar Concurrent and MaR8 were assessed for late blight resistance in field trials inoculated with an incompatible P. infestans isolate. A 1:1 segregation of resistance and susceptibility was observed, indicating that the resistance gene referred to as R8, is present in simplex in the tetraploid MaR8 clone. NBS profiling and successive marker sequence comparison to the potato and tomato genome draft sequences, suggested that the R8 gene is located on the long arm of chromosome IX and not on the short arm of chromosome XI as was suggested previously. Analysis of SSR, CAPS and SCAR markers confirmed that R8 was on the distal end of the long arm of chromosome IX. R gene cluster directed profiling markers CDP(Sw5)4 and CDP(Sw5)5 flanked the R8 gene at the distal end (1 cM). CDP(Tm2)1-1, CDP(Tm2)1-2 and CDP(Tm2)2 flanked the R8 gene on the proximal side (2 cM). An additional co-segregating marker (CDP(Hero)3) was found, which will be useful for marker assisted breeding and map based cloning of R8.