The effect of rhizobia isolates against black root rot disease of faba bean (Vicia faba L) caused by Fusarium solani

Ethiopia is the largest producer of Faba bean in world next to China. In spite of huge importance, the productivity of Faba bean remains far below the crops potential. Black root rot caused by Fusarium solani is the major treat to Faba bean production in Ethiopia and the World. The use of fungicides against the disease has been shown to induce negative effect on environment and results in the appearance of fungicide resistant pathogen biotypes. It is also unaffordable for farmers in countries like Ethiopia. Thus it is important to look for ecofriendly management strategies of the disease. The main purpose was to evaluate for potential biocontrol agent by assessing isolates for in vitro inhibitory efficacy, probable mechanisms to inhibit fungal pathogen and effect on growth of Faba bean infected with Fusarium solani. In PDA medium, the effect of 41 Rhizobium isolates on the radial mycelium development of F. solani were tested in vitro. The experiments were undertaken by applying the dual culture techniques. Then, 27 isolates that showed remarkable inhibitory effect against Fusarium solani in vitro were tested to assess hydrolytic enzymes and growth promoting traits. Subsequently, the three most inhibitors under in vitro Rhizobium isolates and their combination were tested in vitro to determine their effect on germination of Faba bean seeds against F. solani. These three isolates and combination were again tested in vivo against F. solani root rot on seedling by applying the cell suspension at three different time of application (at the time of inoculation, 7 days before and after) on the pathogen. In dual culture, 27 rhizobium isolates remarkably inhibited the mycelia radial growth of F. solani more than 25% and Rh26(1), Rh15(2) and fb-1WG, inhibited fungal radial growth by 70.58 %, 64.70% and 63.72%, respectively. Among the 27 Rhizobium isolates tested for hydrolytic enzymes 7(26.15%), 12(44.40%), 4(14.81%) were positive to chitinase, protease and lipase production, respectively. chitinase, protease and lipase positive isolates showed significant fungal mycelia inhibition (26.47-70.58%), (25.49-70.58%), (39.21-62.74%), respectively. Eight (29.63%) were positive to hydrogen cyanide production. Also, 24(88.88%) were positive for IAA production and Over 50% were formed visible dissolution haloes on PA. Rh21(4) isolate produced the largest (1.90cm) solublisation index. Concurrent production of protease, lipases, chitinase, IAA and phosphate solubilisation coupled with anti-fungal activity suggests the plant growth promotion and broad spectrum biocontrol potential of these isolates. The highest significant germination percent observed in combination and fb-1WG (79.69%) followed by Rh26(1) (60%) compared to control (45%). Maximum significant radicel length (5.92cm,) plumule length (5.25 cm), and vigor index (889.87), measured in combination, followed by (4.41), (3.77cm) (652.40) in fb-1WG, and minimum (1.28 cm) (0.49 cm) (79.5) was in the control, respectively. Furthermore, combination and fb-1WG consistently reduced disease incidence and severity; and increased growth parameters on seedling in greenhouse at all times of application compared to diseased (control). Maximum disease severity (73.31%) reduction percentage was observed with application of combination before the pathogen, followed by at same time. Combination resulted in the highest (48 cm/plant) shoot height when applied before. This suggested that beneficial traits strongly assist the efficiency of candidate antagonists for desired biocontrol, emphasizing the value of concerted mechanisms of action. The study indicated the possible use of Rhizobial isolates as an alternative means of BRR management but further study is needed to prove the result.