Morpho-agronomic, Genetic Diversity and Pythium Root Rot Resistance of South Western Kenya Common Beans (Phaseolus Vulgaris L.) Landraces

Abstract

Common bean (Phaseolus vulgaris L.) is one of the most important legume crops used as a source of protein, vitamins, and other beneficial nutrients among resource-poor populations in Kenya. Common bean landraces derived from local tropical germplasm represent an important source of genetic and phenotypic diversity, which is at present under-exploited by Kenyan crop breeding programs besides being threatened by genetic erosion. Besides, common bean cultivation in Kenya is threatened by many biotic stresses, such as Pythium root rot disease. These landraces may also provide important and durable sources of resistance to Pythium root rot disease. The main objective of this study was therefore to evaluate the diversity of local common bean landraces from South Western Kenya for Pythium root rot resistance using morpho-agronomic characters and peroxidase gene-based (POX) molecular markers. A total of 51 common bean landraces were screened for resistance to Pythium root rot in the screenhouse. The leaves of plants established in the creenhouse were used to evaluate genetic diversity using 5 peroxidase gene (POX) markers. Following infection with Pythium spp., 11.77, 54.90 and 33.33% of the landraces were found to be moderate-resistant, susceptible and highly susceptible, respectively. No landrace was found resistance to Pythium root rot. A total of 1119 alleles were amplified by the 5 primers, ranging from 3 to 8 alleles per locus, with an average of 4.8. The polymorphism information content (PIC) of the POX markers varied from 0.10 to 0.47, with an average of 0.28. Using genetic similarity coefficients, un-weighted pair group method with arithmetic (UPGMA) grouped the landraces into two main genetic clusters, and the dendrogram did not reveal any unique groupings according to their reaction to Pythium root rot disease. Population structure analysis using the Bayesian model-based approach separated the germplasm into 3 genetic groups with low admixture. Population structure analysis showed that all the 3 gene pools contained landraces exhibiting both moderate resistance and susceptible to Pythium root rot. For morpho-agronomic characterization, field experiments were conducted using 52 common bean landraces at the Kenya Agricultural and Livestock Research Organization (KALRO). The study was conducted using a randomized complete block design (RCBD) with three replications. Analysis of variance revealed significant differences indicating the existence of genetic variability among the 52 landraces for 14 quantitative traits studied. The genotypic coefficient of variation ranged from 1.00% for biological yield to 84.69% for pod width, while the phenotypic coefficient of variation ranged from 2.34% for biological yield to 84.40% for number of branches. The estimated broad sense heritability ranged from 60.20% for seeds per plant to 87.57% for days to emergence. Estimates of genetic advance as percent of mean ranged from 10.15% for biological yield to 97.45% for number of branches. Positive and highly significant association of plant height, days from planting to 50% flowering, number of pods per plant and biological yield was observed with seed yield per plant hence these traits may be directly attributed for the improvement of seed yield. A survey was also carried out to determine the status genetic erosion of common bean landraces in South Western Kenya. The estimated genetic erosion of the common bean landraces was greater than 50% and the underlying causes were diseases, introduction of new varieties and pests at 100%, 94% and 86%, respectively. The morpho-agronomic and genetic characterization of common bean landraces in this study will give valuable information for breeders and serve as a baseline for efficient development of new cultivars with Pythium root rotresistance.