Effect of Glyphosate and Mancozeb on the Rhizobia Isolated from Nodules of Vicia faba L. and on Their N2-Fixation, North Showa, Amhara Regional State, Ethiopia

This study was designed to assess the effect of glyphosate and mancozeb on growth of Vicia faba rhizobia isolates in vitro and on their N2-fixation performance. Hence, ten isolates were isolated using plant-soil trap method from soil samples collected from farm lands. Those isolates were morphologically characterized using YEMA medium and authenticated as nodulating rhizobia using sand culture. These isolates were treated with 100, 150, and 200 μg a.e. L−1 glyphosate, 100, 150, and 200 mg L−1 mancozeb, and their combinations. The result showed that almost all isolates were affected (only 4–10% survival) at lower (100 mg L−1) concentration of mancozeb. However, 80% of isolates treated with higher concentration (200 μg a.e. L−1) of glyphosate for 72 h formed colonies on YEMA medium. Moderate (40%) isolates also showed better (31–50% and 17–45%) survival within 100 : 100 and 150 : 150 combinations of glyphosate and mancozeb, respectively. For in vivo experiment, faba bean seedlings in sand culture were inoculated with four relatively in vitro test resistant and one sensitive isolates. The inoculated isolates were treated with field recommended concentration of glyphosate, mancozeb, and combinations. Thus, experimental plants almost all showed normal (61–124 nodule plant−1) nodulation and N2-fixation (90–109%) performance as compared to the control.