Validation of effectiveness marker-assisted gamete selection for multiple disease resistance in common bean
Plant breeding is the most cost-effective, practical and environmentally friendly strategy for reducing losses associated with bean diseases, especially in low-input agricultural systems because no additional investment is required from farmers. However, incorporating resistance to one pathogen may not result in a significant change because several diseases co-infection beans at the farm level. Consequently, breeding varieties with multiple disease resistance is a more appropriate, reliable and sustainable approach. In such context, gamete selection is the more appropriate breeding method because it allows simultaneous selection for multiple traits; though as originally proposed and validated, it is largely based on phenotypic evaluation for agronomic traits, which leads to delay in variety development and strong dependence on erratic weather conditions. The objective of this study was to validate 26 F1.8 elite bean lines selected for resistance to angular leaf spot (ALS), anthracnose, root rots, common bacterial blight (CBB) and bean common mosaic virus (BCMV), from inter-racial and inter-gene pool populations developed using molecular markers on the gamete selection method in early generations. Pathogens were isolated from diseased plants collected from various locations in central Kenya, multiplied on appropriate media and used to inoculate the test lines in a greenhouse at Kabete Field Station, University of Nairobi. Data on disease incidence and severity were collected at 14, 21, 28th days after inoculation, using the 1-9 CIAT scale; except for the root rot experiments for which data were recorded once at 21st day after seedling emergence. Results showed that five of the 26 elite lines possessed multiple resistance to five pathogens, eight to four pathogens, nine to three pathogens, three to two pathogens and one was resistant to one pathogen. This implied that markers, used in early generations, were effective in the identification and transfer of resistance genes to susceptible commercial varieties. However, there were no significant correlations in the reaction of tested genotypes to pathogens in this study, except between BCMV and ALS (r=0.3942*). This suggests that resistance genes are in different chromosomes and are assorted independently. The presence of genotypes with multiple disease resistance among test elite lines, confirms the effectiveness of inter-racial crosses and marker-assisted gamete selection to concurrently improve the resistance to common bean major diseases in Eastern Africa.
Key words: Inter-racial elite lines, Kenya, Phaseolus vulgaris