Genetic Control of Photoperiod Sensitivity, Selection for Short-day Adaptation in Runner Bean and Validation of Multiple Disease Resistance in Snap Bean in Kenya

Abstract

Runner bean (Phaseolus coccineus L.) and snap bean (Phaseolus vulgaris L.) are the most cultivated species of Phaseolus. These crops offer a great potential for addressing food insecurity, income generation and poverty alleviation in Africa. However, production of runner bean and snap bean (French bean) in Africa is limited by photoperiod sensitivity and diseases. In Kenya, large scale companies produce long-day vegetable runner bean for export using expensive extended artificial light of 4h because preferred varieties do not flower under the natural short-day length (12h). The local grain type runner bean also known as butter bean is well adapted and flowers normally under short-day conditions. However, its productivity is low and not suitable for cultivation as a vegetable crop. There are no locally bred, short-day vegetable or improved grain type runner bean varieties in East Africa because no breeding programs for this crop in this region are carried out. Moreover, breeding short-day tropically adapted runner bean is constrained by lack of information on the mode of inheritance of photoperiod sensitivity and lack of a suitable breeding methodology. Production of snap bean is constrained by reliance on varieties which are susceptible to rust, angular leaf spot and anthracnose which result in yield losses and low product quality. Therefore, the objectives of this study were to: i) determine inheritance of photoperiod sensitivity, ii) select high yielding, disease resistant and market preferred short-day adapted vegetable runner bean, iii) select for high yielding and disease resistant grain type runner bean, iv) involve farmers in selecting improved grain runner bean lines, and v) validate multiple disease resistance and pod quality of new locally bred snap bean lines. To determine the inheritance of photoperiod sensitivity, parental lines, F1, F2 and their backcross progeny developed from crosses between local landraces and long-day variety, White Emergo, were evaluated at Kabete (1820 masl) and Ol Joro-Orok (2300 masl) . Selection for short-day vegetable and grain type runner bean was conducted on F6.7 lines grown at same locations. Snap bean lines selected from F5 bulk populations were evaluated for disease and pod quality at Mwea and Embu respectively. Data was collected on days to 50% flowering, disease occurence, number of racemes per plant for runner bean and pod yield. Pods were graded using export standards of fresh produce commercial companies. Analysis of variance and generation means were used for data analysis. After testing the 3-parameter model (m+a+d) and 6-parameter model (m+a+d+aa+ad+dd) based on the joint scaling test as proposed by Mather and Jinks (1982) genetic analysis showed, that additive-dominant model had the best fit. The gene estimates showed that the additive gene effects accounted for more than 90% of the genetic variability for days to 50 % flowering, number of vi racemes and pods plant-1 in runner bean at both sites. The preponderance of additive gene action than dominance implies that several genes with small additive effects are involved in inheritance of short-day photoperiod in runner bean. This implies that runner bean can be improved through selection procedures like pedigree and single seed/pod descent method where selection of these phenotypic traits will be effective. A modification of these procedures may be necessary because of the insect mediated out-crossing recorded at the trial sites. The F6.8 vegetable bred lines flowered normally under local short-day conditions, and had significantly more racemes (on average 8 racemes plant-1) and high pod yield compared with the long-day check, White Emergo at both sites and seasons. White Emergo had no marketable yield in the first year and had very low yields (25 kg ha-1) in the second year compared to 1,000 kg ha-1 realized in commercial large scale cultivation when extended artificial lighting is used. Numbers of racemes of locally bred runner bean lines was higher during the second flush of flowers, which was cooler at both sites, suggesting better adaptation of runner bean to cooler higher altitudes. Six lines in the first year and four lines in the second year yielded more than 1,000 kg ha-1 per harvest which are the yields realized in commercial large scale companies. The locally developed grain type runner bean showed higher degree of resistance to diseases (scores of 1 to 3) and yield advantage of up to 100% in the first year compared to the local landraces. Mean grain yield of runner bean varied from 2,300 kg ha-1 to 13,300 kg ha-1 in 2013 and from 2,500 kg ha-1 to 7,100 kg ha-1 in 2014. The best 22 lines with high yield at Kabete and Ol Joro-Orok were selected. Yield of the selected lines varied from 5,000 kg ha-1 to 13,300 kg ha-1. Results showed that positive criteria used by both male and female farmers in selecting grain runner bean were earliness, pods per plant, pods with well filled grains, uniform pod distribution, good plant standability and white grain colour. Negative selection criterion was based on late maturity, other grain seed colour apart from white and shorter pods with no grains. Therewere gender differences in selection with male farmers showing preference for plants that retained foliage even after maturity whereas female farmers selected for plants with less foliage. Fifteen new snap bean lines which exhibited multiple disease resistance combined with better pod yield and pod quality compared with existing commercial varieties at both locations were identified. These lines had mean disease score of 1-3 for the three diseases and had fresh pod yield of up to 10,000 kg ha-1, which was higher than average of 4,000 kg ha-1 realized in farmers‘ fields with commercial varieties. None of the commercial check varieties exhibited multiple disease resistance. vii These results indicate the potential of developing snap bean varieties that combine multiple disease resistance as well as high yielding, short-day adapted runner bean with market preferred pod characteristics. New snap and runner bean varieties from these lines can increase smallholder production because they do not require expensive additional artificial light and reliance on costly fungicides. Utilization of the new lines can enhance competitiveness of green bean and grain legume products in domestic and export markets. Key words: Runner bean, photoperiod, yield, disease resistance, French bean