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Genetic Analysis of Resistance to Pythium Root Rot Disease in Common Bean (Phaseolus Vulgaris L.) Genotypes

Bean root rot caused by Pythium spp. is one of the most important diseases affecting common bean (Phaseolus vulgaris L.). A study was carried out to determine the genetics of resistance to Pythium ultimum in two common bean (Phaseolus vulgaris L.) genotypes, identify the allelic relationship of the resistance genes in these genotypes, and evaluate selected genotypes with known tolerance or susceptibility to low soil fertility for reaction to the disease. The resistant genotypes RWR 1946 and RWR 2075 were each crossed to three susceptible genotypes—K 132, Lyamungu, and Rushare to produce F1 seeds. Part of the F1 seeds from each cross was used to produce F2 and backcross seeds. F1 and F2 seeds of the crosses RWR 1946 x RWR 2075, RWR 1946 x RWR 719, and RWR 2075 x RWR 719 were also generated. Genotype RWR 719, a known and previously studied Pythium root rot resistance source was included mainly for the allelism studies. Some of the low soil fertility-tolerant genotypes included those from the Bean Improvement for Low Soil Fertility in Africa (BILFA) nursery, aluminum toxicity, and low soil phosphorous-tolerant and susceptible genotypes from CIAT, Colombia. All parental genotypes, F1s, F2s, backcrosses, and the low soil fertility-tolerant and susceptible genotypes were evaluated for Pythium root rot severity. Resistance to Pythium ultimum was controlled by a single dominant gene present at the same locus in all the genotypes. We conclude that the BILFA nursery is a potential source of Pythium root rot resistance.