Genetic mapping from field tests of qualitative and quantitative resistance to Phytophthora infestans in a population derived from Solanum tuberosum and Solanum berthaultii

Under controlled field conditions, a Solanum backcross population segregate d for resistance to Phytophthora infestans. The population ('BCT') had been derived previously by crossing the Solanum tuberosum dihaploid USW2230 x S olanum berthaultii PI473331 to obtain the hybrid M200-30, and then backcros sing the hybrid to the S. tuberosum dihaploid HH1-9. Resistance was assesse d from analyses of epidemics in small plots of each individual genotype, an d data were recorded as area under the disease progress curve (AUDPC). The parents of the original cross (USW2230 and a selection from PI473331) were not included in the test, but the hybrid was incompatible and HH1-9 was com patible with the tester strain of P. infestans (US-8 lineage). Somewhat mor e than half of the progeny also were incompatible with the tester strain, i ndicating the presence of an R gene. This gene segregated from the S. berth aultii parent and mapped 4.8 cm from the RFLP marker TG63 on chromosome 10. We deduce that the R gene is not R-1, R-2, R-3, R-6, or R-7 and is probabl y not R-4, R-5, or R-10. Among the remaining, compatible progeny, there was a wide range of quantitative resistance. All were more resistant than the susceptible cultivar Superior, and most individuals were much more resistan t than the moderately resistant cultivar Kennebec. AUDPC values among the s ub-population of compatible genotypes ranged from about 400 to 1500 units t he first year and from 400 to 1760 units the second year. At least five qua ntitative trait loci (QTLs) were detected in this sub-population in both 19 97 and 1998, including one detected through segregation of alleles from bot h the hybrid parent and the recurrent S. tuberosum parent. A model of main and epistatic effects explained 56% and 66% of the variation observed for q uantitative resistance to late blight in 1997 and 1998, respectively. Sever al of the QTLs for late blight resistance were located in regions of the ge nome to which QTLs for late maturity have previously been mapped.