A breeding scheme was carried out to introgress resistance genes to bean Pythiumroot rot in various commercial varieties grown in Rwanda. The achieved crosses were performed between three selected susceptible varieties (R617-97A, RWR 1668 and Urugezi) which are adapted to the various ecological production zones of Rwanda and two known sources of resistance to Pythium root rot(RWR719 and AND 1062). Following each inter varietal hybridization generation, a series of 4 successive back-crosses was achieved by using the susceptible parents as the initial parent lines to be improved for their respective behavior to Pythium root rot disease. At each back-cross generation, the PYAA 19800 SCAR marker linked toPythium root rot resistance in the two sources of resistance (varieties RWR 719 and AND 1062) was used to identify and to proceed to early selection of progenies possessing the gene of interest. The target materials serving for the molecular analyses were prepared from young trifoliate leaves of 2-weeks bean plantlets. It was observed that at each back-cross generation, there were variable proportions of plants exhibiting presence of the resistance gene according to the SCAR marker profiles. In addition to that observation mainly based to molecular profile, it was also revealed that the proportion of bean seeds having the same color as the susceptible parent line was increasing progressively. Finally, to assess if the individual plants exhibiting the SCAR marker are effectively resistant to the Pythium root rot disease, inoculation tests were carried out with a Pythium ultimum strain on each of them. This ultimate biological evaluation revealed that all the plants showing the SCAR marker were resistant to development of Pythium root rot symptoms after inoculation, confirming thus real introgression of the resistance characteristics through the breeding scheme adopted in our work.
Key words: Phaseolus, vulgaris, Pythium, molecular, root rot, bean, genotype, resistance.
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