Breeding runner bean for grain yield, disease resistance and short-day adaptation in eastern Africa

Although runner bean (Phaseolus coccineus L.), also known as butter bean, is grown in high altitudes of eastern Africa where common bean (Phaseolus vulgaris L) is adapted, its productivity is poor because no improved short-day varieties are available. Farmers rely on low yielding landraces which are susceptible to diseases. However, the better yielding longday vegetable type varieties are poorly adapted to tropical conditions. Our objectives were to develop breeding populations and select for new short day runner bean lines combining high grain yield potential with resistance to diseases and suitable for cultivation under tropical conditions. Four populations were developed from crosses between a long day variety (White Emergo) and four short-day grain type runner bean landraces at Kabete Field Station. The single crosses were advanced to F5 generation as population bulks at Ol Jorok, Subukia and Kabete. Starting F5, single plants were selected and advanced through single pod descent method. One hundred thirty-nine F6.8 lines were evaluated in a randomised complete block design with three replicates at Kabete (1860 masl) and Ol Jorok (2300 masl) in 2012 and 2013. Scoring for plant vigour and diseases was based on 1 to 9, where 1-3 is resistant/ vigorous, 4-6 intermediate and 7 to 9 susceptible/ poor vigour. Results showed considerable variation for plant vigour, racemes per plant, grain colour, pod set, pod length, reaction to diseases and grain yield. . New lines were significantly more vigorous (P<0.01) compared with check varieties. Plant vigour scores varied from 1.7 to 3.7 among the new lines, compared with 2.7 to 5.7 for five check varieties. White Emergo showed significantly delayed second flowering (P<0.01) but failed to produce the first flush of flowers (50 days after planting), while all short day parents flowered normally within 36 to 53 days. Number of racemes per plant during the first flush of flowers varied from 3 to 13 with a mean of 7. Each raceme had 10 to 30 flowers. The new lines had white, black, purple, purple mottled and speckled grain types, suggesting recombination of grain colours of the parental lines. Major diseases observed were rust, common bacterial blight (CBB) and bean common mosaic virus (BCMV) at Kabete and Ol Jorok, and powdery mildew at Kabete. CBB and BCMV were most severe at both sites. However, the test lines showed resistant reactions to rust, CBB and BCMV. Grain yield varied from 2908 to 10,350 kg ha-1 with a mean of 6531 kg ha-1. The new lines had a yield advantage of up to 67% compared with local short-day checks. The results indicated that new high yielding short-day runner bean varieties with resistance to major diseases and tropical adaptation can be developed from these lines.