The R(Pi-mcd1) locus from Solanum microdontum involved in resistance to Phytophthora infestans, causing a delay in infection, maps on potato chromosome 4 in a cluster of NBS-LRR genes.

The distinction between field resistance and resistance based on resistance (R) genes has been proven valid for many plant-pathogen interactions. This distinction does not seem to be valid for the interaction between potato and late blight. In this study, a locus involved in late blight resistance, derived from Solanum microdontum, provides additional evidence for this lack of distinction. The resistance is associated with a hypersensitive response and results in a delay of infection of approximately 1 to 2 weeks. Both a quantitative as well as a qualitative genetic approach were used, based on data from a field assay. Quantitative trait locus (QTL) analysis identified a QTL on chromosome 4 after correction of the resistance data for plant maturity. A qualitative genetic analysis resulted in the positioning of this locus on the short arm of chromosome 4 in between amplified fragment length polymorphism marker pCTmACG_310 and cleaved amplified polymorphic sequence markers TG339 and T0703. This position coincides with a conserved Phytophthora R gene cluster which includes R2, R(2-like), R(Pi-blb3), and R(Pi-abpt). This implies that R(Pi-mcd1) is the fifth R gene of this nucleotide-binding site leucine-rich repeat cluster. The implications of our results on R-gene-based and field resistance are discussed.