Two related genotypes of Solanum microdontum carry different segregating alleles for field resistance to Phytophthora infestans

Six F-1 populations produced from crosses between two Solanum microdontum g enotypes and three S. tuberosum genotypes have been assessed for resistance to Phytophthora infestans (late blight; LB) in the field and proved to seg regate for P. infestans resistance. The six populations were subjected to g enetic mapping using AFLP markers in combination with a core map consisting of RFLPs and a few PCR-based markers. The two S. microdontum parents MCD16 7 and MCD178 harbour different alleles contributing to resistance. In the M CD167 set of populations a major QTL contributing to resistance was located on chromosome 4. This locus was not detected in the MCD178 set of populati ons. In the latter set of populations, a major QTL for resistance was detec ted on chromosome 10. This locus for resistance was not detected in the MCD 167 set of populations. A third QTL contributing strongly to resistance was detected on chromosome 5. However, this locus was not consistently detecte d in all populations. For the loci on chromosome 5 and 10, segregation for resistance alleles from the susceptible parents was also observed. The resu lts provide a starting point for future combinations of genes, to predict t he value of each combination and finally to obtain potato germplasm carryin g high levels of durable resistance to LB.