Do Major Quantitative Trait Loci for Resistance to Common Bacterial Blight in Common Bean Interact Additively?.

Common bacterial blight (CBB), caused by Xanthomonas axonopodis pv. Phaseoli, is a damaging widespread disease of common bean (Phaseolus vulgaris L.).  Genetic resistance to CBB in common bean is limited, but has been introgressed by inter-specific crossing with   tepary bean (P. acutifolius). In Canadian common bean germplasm, CBB resistance is derived from two distinct tepary bean sources, i.e., PI440795, from which OAC Rex was developed and PI319443, from which HR67 and HR45 germplasm lines were developed. A recombinant inbred line (RIL) population (n=225) of a cross between the resistant genotypes OAC Rex and HR45, known to carry different resistance QTL, was evaluated for resistance to CBB in the field and genotyped with the SCAR marker UBC420 and the microsatellite marker Pv-ctt001, known to be associated with resistance QTLs on LG B6 and LG B4 in HR45 and OAC Rex, respectively. The average Area Under the Disease Progress Curve (AUDPC) was computed for each RIL. The frequency distribution of AUDPC for the RIL population was continuous and had a population mean shifted towards resistance. The highest levels of disease resistance were measured for RILs containing UBC420, which accounted for 29% of variation in AUDPC. While, in the absence of UBC420, the effect of Pv-ctt001 was significant and accounted for 7% of variation in AUDPC, in the presence of UBC420, the effect of Pv-ctt001 was not significant. This may point to epistatic interaction effect between the two QTL. Overall, both markers accounted for 30% of phenotypic variation in AUDPC.