Symbiotic effectiveness of cowpea (Vigna unguiculata (L.) Walp.) nodulating rhizobia isolated from soils of major cowpea producing areas in Ethiopia

Isolation and evaluation of symbiotic effectiveness of native rhizobia isolates are important to develop effective inoculant and achieve maximum legume productivity. This work was initiated to authenticate and evaluate the symbiotic effectiveness of cowpea nodulating rhizobia isolates isolated from major cowpea growing areas in Ethiopia. A total of 28 rhizobia isolates were isolated, purified, authenticated for infectiveness and assessed for their symbiotic effectiveness. The study was conducted under a controlled greenhouse condition and all isolates formed nodules on cowpea upon reinoculation and verified to be true rhizobia infecting their host. The inoculation of native rhizobia isolates had significantly increased nodule number per plant, nodule dry weight per plant and shoot dry weight per plant compared with the uninoculated treatment. The relative symbiotic effectiveness of the isolates ranged from 45.81% to 115.03% whereas the absolute symbiotic efficiency ranged from 32.72 to 233.25%. Besides, symbiotic effectiveness showed positive and highly significant correlation with nodule number, nodule dry weight and shoot dry weight per plant. The study on symbiotic effectiveness of the isolates revealed that 10 isolates (35.72%) were highly effective, 16 isolates (57.14%) were effective and only two isolates (7.14%) were poorly effective. From the study, it was observed that the existence of naturally occurring cowpea nodulating rhizobia varied from poorly to highly effective with a predominance of effective isolates in different agro-ecological zones of Ethiopia where cowpea is under production. Further studies are recommended under varied conditions on the competitiveness, persistence, and potential of the promising isolates.