Combining Ability for Phytophthora infestans Quantitative Resistance from Wild Tomato

Alleles conferring quantitative resistance to late blight disease [Phytophthora infestans (Mont.) de Bary] were introgressed into cultivated tomato [Solanum lycopersicum (L.)] from resistant wild tomato (Solanum habrochaites Knapp and Spooner). Two sets of sub-near-isogenic lines (sub-NILs) were marker-selected so that each contained resistance QTL alleles from S. habrochaites chromosome 5 or 11. We conducted a combining ability study with the sub-NILs to evaluate the potential utility of the resistance QTL alleles for breeding F-1 hybrid cultivars and identify sub-NILs with the highest combining ability for late blight disease resistance. The sub-NILs were mated with inbred testers in factorial mating designs (Design II). F-1 hybrid progeny from these two sets of sub-NILs, in which each hybrid was heterozygous for one or more QTL resistance alleles on chromosome 5 or 11, were evaluated for P. infestans resistance in replicated field and growth chamber experiments. Differences in combining ability were found in growth chamber but not field experiments, suggesting that sub-NILs contributed similar levels of resistance to their F-1 hybrid progeny in the field, but different levels in the growth chamber. Significant QTL x environment interactions were detected across field and growth chamber experiments. We identified sub-NILs that expressed P. infestans resistance in F-1 hybrid combinations, suggesting that these sub-NILs could be useful for breeding F-1 hybrid varieties with enhanced resistance. The expression of quantitative resistance to P. infestans in F-1 hybrids derived from each subNIL parent was provided by multiple QTL alleles from S. habrochaites.