French bean (Phaseolus vulgaris L.) in Kenya is cultivated predominantly in the Central, Eastern and Rift Valley provinces. Despite its importance as an export commodity, there has been little breeding effort devoted to its improvement and no much focus on gaining an understanding of the genetic diversity of the germplasm used in cultivation. The present objective was to evaluate the genetic diversity of a set of 36 Kenyan French bean accessions, based on a combination of morpho-agronomic and simple sequence repeat markers (SSRs). For the former, 20 morpho-agronomic traits were measured in two growing seasons, while for the latter; the germplasm was genotyped at 26 SSR loci. The accessions varied significantly (P≤ 0.05) with respect to pod diameter, pod length, pod number per plant, pod weight per plant and seed weight, but not with respect to the number of days taken to reach flowering, leaf length, leaf width or plant height. There was a large influence of season over flowering time, pod diameter, pod length, plant height and seed weight, and a significant (P≤ 0.05) interaction between accession and season for pod diameter, pod length, pod weight per plant, number of pods per plant and seed weight. The variation in the morpho-agronomic traits split the 36 accessions into four clusters. Shannon-Weaver diversity indices for qualitative, pseudo-qualitative and four selected quantitative traits ranged from 0.23 to 0.88. Of the 26 SSR primer pairs, which successfully amplified all 36 DNAs, 18 were informative, producing a mean of 2.17 alleles per SSR locus and a PIC value between 0.17 and 0.41. The level of gene diversity ranged from 0.19-0.50 (mean 0.36). The low observed mean heterozygosity reflected the predominantly autogamous habit of French bean. The modest amount of genotypic variation uncovered can be attributed to the effect of intensive selection for pod quality. A phylogeny based on the informative SSR loci revealed three major clusters, each representing a different pod diameter class. Combining the morpho-agronomic and DNA-based markers in this assessment of germplasm provides a way of taking advantage of the best features of both marker types that can be useful for identification of French bean varieties and applications of molecular markers to breeding.
Key words: French beans, morpho-agronomic traits, simple sequence repeat marker (SSR), genotyping, large-sieve beans, fine-sieve beans.