Abstract
Common bean (Phaseolus vulgaris) is an annual legume crop that is grown worldwide for its edible dry seeds and tender pods. Marsh spot (MS) of the seeds is a physio-genic stress disease affecting seed quality in beans. Studies have suggested that this disease involves a nutritional disorder caused by manganese deficiency, but the inheritance of resistance to this disease has not been reported. A biparental genetic population composed of 138 recombinant inbred lines (RILs) was developed from a cross between an MS resistant cultivar ‘Cran09’ and an MS susceptible cultivar ‘Messina’. The 138 RILs and their two parents were evaluated for MS resistance during five consecutive years from 2015 to 2019 in sandy and heavy clay soils in Morden, Manitoba, Canada. The MS incidence (MSI) and the MS resistance index (MSRI) representing disease severity were shown to be both highly correlated heritable traits that had high broad-sense heritability values (H2) of 86.5% and 83.2%, respectively. No significant differences for MSI and MSRI were observed between the two soil types in all five- (MSI) or four-year (MSRI) data collection, but significant correlations among years were observed despite MS resistance was moderately affected by year. The MSIs and MSRIs displayed a right-skewed distribution, indicating a mixed genetic model involving a few major genes and polygenes. Using the joint segregation analysis method, the same four major genes with additive-epistasis effects showed the best fit for both traits, explaining 84.4% and 85.3% of the phenotypic variance for MSI and MSRI, respectively. For both traits, the M1, M2, M3 and m4 acted as the favorable (resistant) alleles for the four genes where M and m represent two alleles of each gene. However, due to epistatic effects, only the individuals of the M1M2M3M4 haplotype appeared to be highly resistant, whereas those of the m1m2m3M4 haplotype were the most susceptible. The m4 allele significantly suppressed the additive effects of M1M2M3 on resistance, but decreased susceptibility due to the additive effects of m1m2m3. Further quantitative trait locus (QTL) mapping is warranted to identify and validate individual genes and develop molecular markers for marker-assisted selection of resistant cultivars.
