The Effect of Rhizobia Isolates Against Black Root Rot Disease of Faba Bean (Vicia faba L) Caused by Fusarium solani

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 F. solani.

The effect of Rhizobium isolates on the development of radial mycelium of F. solani in PDA medium were tested in vitro. The experiments were carried out using the dual culture technique. Isolates that showed inhibitory effect against F. solani in vitro were tested to assess hydrolytic enzymes and growth promoting traits. Subsequently, the three Rhizobium isolates that showed the greatest inhibitions and their combinations were tested in the greenhouse against F. solani root rot on seedlings by applying cell suspensions at three different times of exposure to the pathogen.

In dual culture, 27 rhizobium isolates inhibited the radial growth of F. solani mycelium more than 25%. Isolates JU26(1), JU15(2) and Ho-1WG, inhibited fungal radial growth by 70.5 %, 64.7% and 63.7%, respectively. Among the 27 Rhizobium isolates tested for hydrolytic enzymes 26.1%, 44.4%, 14.8% were positive for chitinase, protease and lipase production, respectively. Chitinase, protease and lipase positive isolates showed significant fungal mycelia inhibition. Eight (29.6%) were positive for hydrogen cyanide production. Also, 24(88.8%) were positive for IAA production and over 50% formed visible dissolution haloes on PA. Concurrent production of protease, lipases, chitinase, IAA and phosphate solubilization coupled with anti-fungal activity suggests potential plant growth promotion and broad-spectrum bio control of these isolates. Furthermore, combination and Ho-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 (73.3%) reduction was observed with application of combination before the pathogen. The combination formulation provided the highest (48 cm/plant) shoot height when applied before the pathogen.

Beneficial traits strongly assist the efficiency of candidate antagonists for desired biocontrol, emphasizing the value of concerted mechanisms of action. The result indicated the possible use of Rhizobial isolates as an alternative means of BRR management but further study is needed to verify actual use in agricultural production.