Abstract

To identify indigenous rhizobia with potential as inoculants for increasing cowpea (Vigna unguiculata) yields, we have assessed the host range, symbiotic effectiveness and competitiveness for nodule occupancy among five (AII-2-1, AII-5-2, AI-4-3, AII-3-4 and BIII-2-2) indigenous cowpea bradyrhizobia isolates from the transitional savanna zone of Ghana. ERIC-PCR DNA fingerprinting patterns were used to identify the isolates occupying nodules. All the isolates nodulated cowpea, groundnut (Arachis hypogeae) and mungbean (Vigna radiata), but only AII-2-1, AII-3-4 and BIII-2-2 nodulated soybean (Glycine max).  Apart from cowpea where all the isolates were effective, there were significant differences in the symbiotic effectiveness of the isolates on the other host legumes.  Out of a total of about 250 cowpea nodules that were screened for each inoculum-mix, isolate AII-5-2 was the most competitive for nodule occupancy whilst AII-3-4 was the least. Isolate AII-5-2 occupied 71% of the nodules in an inoculum-mix consisting of equal proportions of AII-2-1, AII-5-2 and AI-4-3 (a 3-isolate-mix) and 60% of nodules in an inoculum-mix consisting of equal proportions of all the five isolates (a 5-isolate-mix). Therefore, among the isolates tested, AII-5-2 has the best potential for use as inoculant for maximizing cowpea yield in N₂- deficient agro-ecological zones of Ghana.

Key words:  Competition, legumes, nitrogen fixation, rhizobia, symbiosis.