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Selection for Quantitative Trait Loci Associated with Resistance to Stewart's Wilt in Sweet Corn

The objectives of this research were to identify quantitative trait loci (QTL) for Stewart's wilt resistance from a mapping population derived from a sweet corn hybrid that is highly resistant to Pantoea stewartii and to determine if marker-based selection for those QTL could substantially improve Stewart's wilt resistance in a population derived from a cross of resistant lines and a highly susceptible sweet corn inbred. Three significant QTL for Stewart's wilt resistance on chromosomes 2 (bin 2.03), 5 (bin 5.03), and 6 (bin 6.06/6.07) explained 31% of the genetic variance in a population of 110 F_3:4 families derived from the sweet corn hybrid Bonus. The three QTL appeared to be additive in their effects on Stewart's wilt ratings. Based on means of families that were either homozygous or heterozygous for marker alleles associated with the resistance QTL, the QTL on chromosomes 2 and 6 appeared to have dominant or partially dominant gene action, while the QTL on chromosome 5 appeared to be recessive. A population of 422 BC₂S₂ families was derived from crosses of a sweet corn inbred highly susceptible to Stewart's wilt, Green Giant Code 88 (GG88), and plants from two F_3:4 families (12465 and 12467) from the Bonus mapping population that were homozygous for marker alleles associated with Stewart's wilt resistance at the three QTL. Mean Stewart's wilt ratings for BC₂S₂ families were significantly (P < 0.05) lower for families that were homozygous for the bnlg1902 marker allele (bin 5.03) from resistant lines 12465 or 12467 than for families that were heterozygous at this marker locus or homozygous for the bnlg1902 marker allele from GG88. Resistance associated with this QTL was expressed only if F_3:5 or BC₂S₂ families were homozygous for marker alleles associated with the resistant inbred parent (P₁). Marker alleles identified in the F_3:5 mapping population that were in proximity to the resistance QTL on chromosomes 2 and 6 were not polymorphic in crosses of GG88 with 12465 and 12467. Selection for other polymorphic marker loci adjacent to these two regions did not improve Stewart's wilt resistance of BC₂S₂ families.