Inheritance of resistance to fusarium wilt (f. Oxysporumf.Sp. Phaseoli) and breeding multiple disease resistant and marketable climbing beans
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Common bean (Phaseolus vulgaris L.) is the primary source of dietary protein In Rwanda. Climbing beans are important for intensified production due to their high productivity (3-fold of the bush type) and adaptation under the acute scarcity of cultivable land conditions in Rwanda. However, the commercial climbing bean cultivars: Umubano (G2333), Vuninkingi (G685) and Ngwinurare (5911-2) lacked multiple resistance to major soil-borne and foliar fungal diseases like, Pythium root rot, Fusarium wilt, angular leaf spot and anthracnose; and/or market-preferred red-mottled or red seed-types. Fusarium wilt (F. oxysporumf sp. phaseoli) in particular caused the abandonment of Umubano cultivar that was the most popularly adopted (over 80%) among the farmers in Rwanda. The objectives of this study were therefore to: i) Determine the nature of inheritance of resistance against Fusarium wilt disease in climbing beans, and ii) Select high yielding climbing bean lines that combined multiple resistance against angular leaf spot, anthracnose and root rots with the marketable red or red-mottled seed-types. To achieve the first objective, two sources of resistance to Fusarium wilt: G685 and Flora were crossed to the susceptible cultivar, G2333, in single and backcross arrangement. The parents, progenies of F], F2 and the backcrosses were evaluated for Fusarium wilt reaction at a hot-spot screening site in Runyinya location in Rwanda. The trial was replicated in a glasshouse at Kabete field station using potted sterile soil. Injured root tips of 10 day old seedlings were inoculated Fusarium isolate (FOP-R W2) at a constration of 106 conidia mrl. The severity of Fusarium wilt was rated at 28 days after inoculation using the CIA T scale of 1 - 9: where 1 - 3 represent resistant, 4 - 6 intermediate and 7 - 9 is susceptible reactions. The Chi-square analysis (p<5%) proved goodness of fit into the Mendelian segregations: the F 1, backcross to the resistant parents and the F2 progenies from the donor parents were all resistant. All the F2 of the susceptible and resistant parents segregated in the ratio of 3 resistant: 1 susceptible, while the backcross progenies to the susceptible parent segregated in the ratio of 1 resistant: 1 susceptible. By applying the ANOV A proposed by Kearsey et al (1996) heritability of resistance against Fusarium wilt was 99.7%. Therefore, the inheritance of the resistance / susceptibility to Fusarium wilt is conditioned by a major dominant gene that is highly heritable. It is thus achievable by backcross selection. By observing and re-isolation of the pathogen from infected vascular portions of the lower stems at 28 and 55 days after inoculation, the rate of vertical spread of the pathogen, was found to be 4 times more in the susceptible than in the resistant plants. Damaged tap roots of some susceptible plants developed new secondary adventitious roots on the hypocotyls. This implies that the resistance mechanism to Fusarium wilt is related with presence of certain natural or induced physical or chemical barriers in the resistant plants. Tolerance is achieved through formation of compensatory roots in some susceptible plants. To attain the second objective, multiple-parent crosses were made to create eleven heterogametic (for resistance and seed types) populations. The FI2 plant families and those of subsequent generations were screened for disease resistance under four natural epiphytotic conditions at Gikongoro (root rots), Ntendezi (root rots/anthracnose), Rwerere (anthracnose) and Rubona (angular leaf spot) sites in rotation. Pedigree was used to select resistant lines at each site, using the CIAT rating scale of 1 - 9. At F7 the 66 lines that had acceptable yields, diseases resistance and seed type attributes with 8 parental, local and improved checks were planted for advanced replicated yield trial at Rwerere (2300 mas I) and Rubona (1700 masl) ISAR research stations. ANOV A revealed significant G; x E interaction and differences in yield and maturity, pods per plant, pod load; and seed mass and size within and across the sites (p < 5%) with higher means observed at Rwerere. Thirty-three elite lines (Fg) that had the most marketable red or red-mottled seed types besides high yield range of 2.5 - 4.5 ton ha-I or 101% to 141% of the improved checks and multiple resistance to at least 2 of the 3 diseases were selected. They offer the opportunity for release to farmers and to replace old climbing bean cultivars in Rwanda in near-future.