Selection for yield potential, disease resistance and canning quality in runner and snap bean lines and populations
Breeding vegetable and grain runner and snap bean (Phaseolus vulgaris L.) varieties with high yield, good pod quality, disease resistance, and suitable for processing industries has received virtually no research attention in Eastern Africa. Available imported vegetable runner bean varieties are photoperiod sensitive and fail to flower and set pods under normal day length. Grain type runner bean landraces grown by smallholders for are low yielding, susceptible to diseases and not suitable for use as a vegetable. Snap bean varieties produced for domestic and export markets by smallholder farmers and multinational companies are low yielding, susceptible to diseases and poorly adapted. Although canned runner and snap beans are becoming a major form of grain and vegetable consumption especially in urban areas because of their convenience and distinctive flavor while providing excellence consumer value, no locally developed varieties are available in eastern Africa. The objective of this study was to evaluate and select for high yield potential, disease resistance, canning quality in advanced lines and populations of runner and snap bean developed at the University of Nairobi. The study materials were 11 F2 populations and F3 families developed from crosses among six climbing and eight bush snap bean lines between 2012 and 2014; 50 vegetable and 50 grain type runner beans lines selected for tropical adaptation, disease resistance and grain yield from 139 F 6.8 lines, and 107 new snap bean lines. In 2014, the F3, F2 and their parents were grown in an irrigated trial at Mwea Research Station and data collected on maturity, pod length, pod diameter, pod per plant and pod yield. F2/F3 regression was used to estimate heritability. Vegetable and grain type runner bean lines were evaluated at Kabete Field Station and Ol Joro Orok in 2012, 2013 and 2014. The best 20 grain and vegetable runner bean lines were further evaluated in an irrigated high input production system at VegPro Farm in Naivasha. One commercial variety and three local varieties were used as checks for comparison. Forty-three runner and 27 snap bean lines, were subsequently evaluated for canning quality and sensory attributes at Njoro Canning Factory, Nakuru in March 2015. Industry references ‗TruFood RB‘, a grain type runner bean, and ‗Julia‘, a processing type snap bean, were used as checks. Results showed significant differences (P< 0.05) for duration to flowering, pod length, pod diameter, pods per plant and pod yield across the six generations (P1, P2, BC2P1, BC2P2, F2 and F3). All the traits showed moderate to high heritability (HNS= give values) but this varied with populations. The new grain and vegetable runner bean lines showed considerable variation for duration to flowering, plant vigour, racemes per plant, reaction to diseases and grain yield. All the test lines flowered and formed pods under short day conditions at the three locations. Duration to flowering varied from 43 to 49 days in Naivasha, 49 to 53 days in Ol Joro Orok, and from 51 to 55 days in Kabete. The crop was extremely vigorous in Naivasha due to adequate and regular supply of water and nutrients. There were no disease incidences in Naivasha since the trial was conducted during dry spell which was not conducive for disease development. Most of the new vegetable type runner had higher pod yield compared with commercial variety, White Emergo. Outstanding lines included KAB-RB13-1-105/2 (18,354kg ha-1) and KAB-RB13-1-105/3(10,114ha-1). White Emergo, the commercial check variety produced a cumulative pod yield of 896 kg ha-1. KAB-RB13-1-105/3 had the highest percentage of grade I pods (93.9%). Twenty six new lines at Ol Joro Orok, 13 in Naivasha and 62 at Kabete were comparable to White Emergo for pod curvature (straight) and met market pod length requirement of 18 cm. Grain yield of the new runner bean lines ranged from 1,888 to 7,414 kg ha-1 in Kabete and Ol Joro Orok under rain-fed conditions, while under irrigation at Naivasha yield varied from 876 to 14, 472 kg ha-1. Compared with best farmers‘ variety, the new lines had yield advantage of 80% at Naivasha, 35% at Ol Joro Orok and 32% in Kabete. Results showed that there were significant differences among snap bean lines for pod yield, pod length, pod diameter, pod per plant and disease resistance. Fifty eight new lines were higher yielding than the checks. For example, KSB15-02 (10,835.4 kg ha-1), KSB15-01 (12, 847.2 kg ha-1), KSB13-11 (9,559.7 kg ha-1) compared to Serengeti (6988.4 kg ha-1) and Samantha (6396.6 kg ha-1). Seventy six lines had round, straight pods with required standards for pod quality and more than 80% proportion of premium grades. Canning quality tests showed that 35 grain type runner bean lines met the industrial canning standards. Among the best performers at Kabete were KAB-RB13-327-92/1, KAB-RB13-326-207/1B and KAB-RB13-326-207/1B. The best performers among lines grown at Ol-Joro-Orok lines were KAB-RB13-471-117/1, SUB-OL-RB13-275-248/3 and KAB-RB13-310-161/5. KAB-RB13-338-41/1 had the highest proportion of clumps (3). The reference variety had low PWDWT (58%) and brine pH before (5.66) and after incubation (5.68). Twenty snap bean lines met the industrial canning standards. Among the best performers were KSB22-147-2M/1, KSB22-147-2M/2 and KSB52-2M. The reference variety, Julia had low HC (1.1) and high fiber content (20%). The results of this study showed that photoperiod sensitivity, duration to flowering, pod length, pod diameter, pods per plant and pod yield are highly heritable and could be transferred to the commercial snap bean varieties via phenotypic selection with good genetic gain. The work described is a milestone in the history of breeding runner and snap bean, not only in Kenya, but also in Africa. The new high yielding grain runner bean lines with resistance to major diseases and tropical adaptation can be used to address food insecurity and poverty alleviation in the country. High pod yield, pod quality and disease resistance of these lines can contribute to increased productivity, reduction in production costs and enhance competitiveness of local products in domestic and export markets. These new grain type runner and snap bean lines will provide the food processing and seed industries with better, readily accessible, high quality raw materials that meet not only producer and consumer‘s preferences, but also will broaden the range of processed products.