Selection For Drought Tolerance, Disease Resistance, Canning And Nutritional Quality In Dry Beans
Common bean (Phaseolus vulgaris L.) is the most important legume crop in eastern and central Africa. The crop is low yielding and underutilized due to several factors such as biotic and abiotic stresses and poor seed quality attributes. Therefore, breeders must consider production and utilization constraints when developing new bean varieties. Bean growers want bean varieties to be high yielding and tolerant to production constraints such as drought and diseases. Bean consumers want beans that cook fast and have good sensory qualities. Processors of beans are constrained by the consumer preferences, but they also want beans to meet specific processing standards. In addition, to contribute in fight against micronutrient malnutrition among poor population, new bean varieties should have high nutritional quality. This research was therefore undertaken to identify bean cultivars of different grain types with superior agronomic performance, good culinary, canning and nutritional quality from the available germplasm. Field trials were conducted for two seasons during long-rain 2012 and short-rain 2012/2013. In the first season, 427 lines from seven market classes and local check varieties were planted at Kabete and Thika under irrigated and rainfed conditions in asplit plot design. Selections from the first season were evaluated under rainfed conditions in four sites including Kabete, Nakuru, Thika and Tigoni using 5x5 lattice design. Data on days to flowering, days to maturity, reaction to major diseases under field conditions and grain yield were recorded. Water regimes and site significantly affected days to flowering and maturing of the genotypes. Genotypes matured earliest in Thika and latest in Tigoni. In both seasons, local varieties KATB9, KATB1, GLPx92, GLP1004 and Miezi Mbili were early maturing compared to test lines. During the first season, Kabete experienced severe disease breakout. Anthracnose was the most severe with 24% of the genotypes succumbing to the disease (7-9 score). Red mottled and speckled sugar lines were v affected the most by the disease with over 40% of the genotypes succumbing to the disease. Nearly all local varieties succumbed to the disease. Results showed that Thika site had drier conditions and rainfed plots had 30% less yield compared to irrigated plots. At Kabete, effect of water regime was concealed by the diseases which infected severely the genotypes under both treatments. Lines which performed well under both rainfed and irrigated treatmentsat Thika were selected for drought tolerance during the first season. Most of the local check varieties were low yielding and very susceptible to diseases under different conditions. Mexican142 was the best check in terms of yield and reaction to diseases. One hundred fifty lines were selected and evaluated for cooking time, water absorption and hard-shell defect to identify lines combining good agronomic and culinary traits that are suitable for household preparation and food processing industry. The most agronomically superior variety, Mexican142, was used as control. Mattson bean cooker was used to determine cooking time. Results showed that around 30 lines from different market classes were fast-cooking (<35 minutes), had high water absorptioncapacity (>90%) and zero percent hard-shell defect. Lines combining good agronomic and, fast cooking time, high water absorption and zero hard shell defect traits included: BCB11-324 (Red mottled), BCB11-158 and BCB11-196 (Red kidney), BCB11-386 and BCB11-414 (Speckled sugar), BCB11-108 and BCB11-94 (navy bean), BCB11-184 (small red), BCB11-448, BCB11-274 and BCB11-508 (pinto and carioca) and BCB11-263 from mixed colour market class. Based on results from the first season and visual selections during second season, 29 lines from different market classes were selected for canning quality evaluation to identify lines combining superior agronomic traits and processing qualities. Mexican142 was used as control. Beans were canned in brine. Sensory quality evaluation was done by panelists in two food processing vi factories. Results showed that lines BCB11-182 (small red), BCB11-108 (navy) BCB11-98 (navy), BCB11-162 (red kidney) and BCB11-324 (red mottled) had better canning and sensory qualities than Mexican142, and met the requirements of both processors and consumers. Iron and zinc concentration analysis was done on grain of 31 advanced lines selected during short-rain season and 18 lines from biofort nursery which was previously bred for high iron and zinc. The red mottled line BCB11-145 had the highest iron concentration (136 ppm). However, this line had long cooking time and high percentage of hard-shell seeds (16%). In general, results showed that lines bred for high iron and zinc had higher concentration of these nutrients. Selected lines were ranked using critical weighting factors which included yield, cooking time, hard-shell defect, hydration coefficient and overall acceptability (sensory). Lines BCB11-108 (navy), BCB11-62 (navy),BCB11-344 (small red),BCB11-324 (red mottled),BCB11-80 (navy) and BCB11-303 (speckled sugar) were the best 6 lines in that order.Mexican142 was ranked 15th out of 30 genotypes. Therefore, the crop improvement strategy deployed in this study showed that significant improvement in beans can be made by selection from the available genetic variation.