| dc.description.abstract | Common bean (Phaseolus vulgaris L.) is the most important legume crop and the main source of dietary protein for urban and rural populations in Eastern and Central Africa. However, its productivity in this region is among the lowest in the world due to the presence of many biotic and abiotic production limiting factors. Diseases and pests are the most economically important biotic stresses. Although several diseases attack common bean, angular leaf spot, anthracnose, common bacterial blight, bean common mosaic virus and the root roots are the most damaging leading to substantial economic losses. Breeding for multiple resistance is the most practical, cost-effective and sustainable approach to cope with production limiting biotic factors since there is no additional investment by farmers. Broadening the genetic base of existing commercial cultivars through inter-racial crosses provides a unique opportunity for effective selection of improved progeny with better agronomic potential. This study was a continuation of a marker-assisted gamete selection programme initiated by the University of Nairobi Legume Breeding Research Programme in 2009 to determine whether this breeding procedure was effective in pyramiding genes for resistance to bean major diseases in Eastern Africa into susceptible, but popular, large- and small-seeded bean varieties. The specific objectives of this study were to: i) Determine the agronomic potential of 16 inter-racial and inter-gene pool populations and select the most promising small- and medium-seeded lines; ii) Determine yield stability and genotype x environment interactions of elite lines, and iii) Validate F1.8 elite lines for multiple resistance to root rots, common bacterial blight, angular leaf spot, bean common mosaic virus and anthracnose using natural epiphytotics and artificial inoculations.
For the first objective, 16 populations were advanced from F1.3 to F1.5 generation at Kabete Field Station of the University of Nairobi between 2013 and 2016. The F1.6 generation was subsequently evaluated at Mwea Research Station of the Kenya Agricultural and Livestock Research Organization (KALRO). Data were collected on seed yield and yield related parameters. The field disease score for target diseases was recorded using the CIAT standard system for bean germplasm evaluation. GenStat 15th edition software was used for analysis of variance (ANOVA) using generalized linear additive model. Fisher’s Least Significant Difference (LSD) test was used for mean separation. Results on agronomic performance revealed significant differences for seed yield among populations, commercial checks and donor parents
(P<0.05). Although the performance of the populations was not consistent over generations and across sites, crosses involving the commercial variety KATB9 were generally high yielding and superior for most agronomic traits. Population KMA13-32 (KATB9 x Mex54 / G2333 // RWR719 / BRB191) with a mean seed yield of 2,844 kg ha-1, out-yielded all the other populations, commercial varieties and donor parents used as checks. The number of pods per plant (r=0.85***) and the seed yield per plant (r=0.97***) were the most positively correlated to seed yield per ha, suggesting that these two traits can be used as indirect selection criteria for grain yield. Inter-racial populations showed low to moderate infection levels in all the generations (1.0 to 5.0) while commercial checks were moderate to highly susceptible to most of the pathogens (3.1 to 9.0). From the F1.6 generation, 92 progeny rows from single plant selections belonging to five market classes (19 small reds, 12 pintos, 13 red kidneys, 16 red mottled and 32 mixed colors) were selected for further testing.
For the second objective, 92 F1.7 lines representing five major market classes were evaluated in three locations (Mwea, Kabete and Tigoni) representing low, medium and high altitude agro-ecological conditions. Data were collected on seedling emergence rate, plant vigor, days to flowering, flower color, growth habit, days to maturity, number of pods per plant, number of seeds per pod, 100-seed mass, seed yield and the harvest index. Prevalent diseases were assessed using the CIAT standard system for bean germplasm evaluation. In addition to ANOVA, the AMMI (additive main-effects and multiplicative interaction) model was used to separate the additive variance from the G x E interaction and to determine the stability of the genotypes across locations using the PCA scores (IPCA1 and IPCA2) and the AMMI stability values (ASV). G x E effects were significant (P<0.05) for all the traits and market classes implying that the tested lines responded differently to variation in agro-ecological conditions, resulting in inconsistent ranking of genotypes across the three sites. The high altitude Tigoni site (2,130 masl) with a mean grain yield of 4,010.2 kg ha-1 was the most favorable environment for common bean cultivation, while the low altitude Mwea site (1,150 masl) with a mean yield of 771.6 kg ha-1 was the lowest yielding environment. AMMI analyses showed that seed yield varied significantly (P<0.001) among genotypes and test sites regardless of the market class. The red kidney market class had the best mean yield (2,299.5 kg ha-1) while the lowest seed yield (1,599.3 kg ha-1) was recorded on mixed color market class. The three major factors (genotype, environment, and G x E) contributed the most to the yield variability (87.8%) regardless of the
market class. Of these, environment factors were largest source of variability (74.2%). The interaction between the genotype and environment was high for the small reds and the mixed colors (17.6% and 26.7%, respectively) suggesting that tested lines were not stable and should, therefore, be selected and recommended to specific environments. From ASV, the higher yielding lines were also the most unstable across sites. Among tested lines, only KMA13-22-21 and KMA13-29-21 combined high yield potential and wider adaptation across the three agro-ecological conditions.
For the third objective on multiple disease resistance validation, pathogens were isolated from diseased bean plants from various parts of central Kenya, multiplied on appropriate media and used to inoculate the elite high yielding lines previously identified using the AMMI model. Data on disease incidence and severity were collected at 14th, 21st and 28th days after inoculation using 1-9 CIAT scale. ANOVA and AUDPC (Area Under Disease Progression Curve) were used for data analysis. Results showed that five of the 26 elite lines possessed multiple resistance to five pathogens; eight genotypes were resistant to four pathogens; nine genotypes were resistant to three pathogens; three possessed resistance to two pathogens and one was resistant to one pathogen. However, there were no significant correlations in the reaction of tested genotypes to the seven diseases (angular leaf spot, bean common mosaic virus, common bacterial blight, anthracnose, Pythium root rot, Fusarium root rot and Rhizoctonia root rot) used in this study, except the significant correlation (P<0.05) existing between the reaction of genotypes to bean common mosaic virus and the angular leaf spot (r=0.39*). This suggested that resistance genes for those pathogens were inherited independently.
The presence of transgressive genotypes combining high yield potential (with a mean grain yield advantage of 17.5% over parental cultivars), stability across locations and high resistance to major diseases confirmed the effectiveness of inter-racial crosses and marker-assisted gamete selection for common bean improvement.
Keywords: Gene pyramiding, market class, Phaseolus vulgaris, stability analysis, yield potential | en_US |
