Symbiotic and genomic diversity of 'cowpea' bradyrhizobia from soils in Botswana and South Africa

Strain CB756 is usually an effective competitor against indigenous bradyrhizobia for nodulation of peanut in South Africa. Recently, inoculation of peanut and cowpea with CB756 in loamy sand soils of Botswana or a sandy clay loam in South Africa proved unsuccessful, achieving < 2% nodule occupancy. A survey of 'cowpea' bradyrhizobia from five soils in Botswana and one in South Africa showed that many were effective in ability to fix N-2 on peanut and cowpea. However, 15 isolates from Good Hope, Botswana were all effective on cowpea but ineffective on peanut, three failing to nodulate the latter. Selected cowpea isolates were significantly more competitive than CB756 for nodulation of cowpea in Leonard jars, but four were unsuccessful when inoculated at Roodeplaat, South Africa. When strain CB756 and two isolates were inoculated in pots containing Roodeplaat soil, at a 4:1 inoculant to soil bradyrhizobia ratio, their average nodule occupancy was 8% on cowpea compared to 40% on peanut. Significant differences in strain nodule occupancy were not detected on either cowpea or peanut. In contrast, nodule occupancy in loamy sand from Good Hope, Botswana, inoculated at a 40:1 inoculant to soil bradyrhizobia ratio, was 22.4% on cowpea and only 6.8% on peanut. In Good Hope soil, strain CB756 was the weakest competitor on cowpea but strain differences were insignificant on peanut. Whereas the Good Hope soil population was effective on cowpea, it was ineffective on peanut. DNA fingerprinting showed that isolates from Gaborone, Francistown and Roodeplaat contained several different genotypes, whereas those from Good Hope, Rasesa and Maun were more homogeneous. The dominance at Good Hope of genotypes effective on cowpea but ineffective on peanut emphasises the value of assessing the symbiotic capabilities and structures of indigenous populations.