Selection of snap bean genotypes for multiple disease resistance, pod quality and yield
Snap bean (Phaseolus vulgaris L.) is a major vegetable export crop in Kenya, and it is produced mainly by small scale farmers and multinational companies. Angular leaf spot, anthracnose and rust diseases cause crop losses up to 100% on susceptible cultivars of snap bean. Use of chemical is expensive and reduces profitability of snap bean farming and it is not environment and consumer friendly. Therefore, the objective of this study was to select snap bean populations and advanced snap bean lines for multiple disease resistance, pod quality and yield. Snap beans used in this study included populations developed from BelDakMi, BelMiNeb and Beltigrade lines with resistance to rust, G2333 for resistance to anthracnose, L227 with resistance to angular leaf spot and rust, and the popular varieties that have good pod quality. These populations were advanced by bulk method to F4, F5, and F6 generations. Thirty three bush snap bean lines and six climbing lines were also evaluated. The experiments were conducted at KARI- Thika and Mwea for two seasons during 2009 and 2010. The experiments in both locations laid down in split plot design with three replicates. The populations and lines were artificially inoculated with isolates of angular leaf spot, anthracnose and rust at triofoliate stage. Data collected included disease severity, days to flowering and maturity, pod length and width, number of pods per plant, marketable pod yield, pod quality (extra fine, fine and bobby) and seed yield. Analysis of variance showed that disease severity was significantly influenced by cropping season, location, fungicide application and genotype. High disease severity was recorded during long rain season at Thika location when genotypes were grown without application of fungicide. The disease with the highest severity was rust followed by angular leaf spot. Among the advanced lines two bush lines KSB lOW and KSB 10 BR and one climbing line HAV 130 had consistent multiple resistance to angular leaf spot, anthracnose and rust at both locations. Resistant genes in the three lines reduced the mean disease severity by 17%, 16% and 36%, for angular leaf spot, anthracnose and rust respectively when compared with commercial varieties. There were significant differences among the genotypes with respect to marketable pod yield, pod quality and seed yield. Star 2053 had the-highest pod yield of 11.5 t ha-1 among the parents while HAB 428 had the highest pod yield of 8.5 t ha-1 among the evaluated lines. Single plants combining resistance to the three diseases and desirable pod characteristics were selected from the segregating populations. Among the advanced snap bean lines selected for having multiple disease resistance, HAB 501 had the highest pod yield of 10.9 t ha-1 while KSB 10 BR had the highest extra fine pod yield of 2.0 t ha' . All climbing lines had thicker pods of 11 mm compared to bush lines with a mean pod diameter of 8 mm when harvested at a regular interval. Among the parent lines Paulista had the highest seed yield of 1.0 t ha-1 while HAV 135 had the highest seed yield of 2.4 t ha-1 among snap bean lines. Snap bean genotypes with multiple resistance to angular leaf spot, anthracnose and rust were identified. This shows that the parents used to develop the snap bean population and lines have resistance genes that could be exploited in the development of snap bean varieties with disease resistance. Although some of the advanced lines had multiple disease resistance, most did not meet the desired yield and quality of the.bush commercial varieties. Therefore, there is need for continued development of snap bean lines with multiple disease resistance and high yields of acceptable quality from the identified single plant selections.