Inhibitory Effect of Rhizobia Isolated from Several Leguminous against Phytopathogenic Fungi

Aims: This study was carried out to evaluate the antagonistic effect of ten isolated characterized Rhizobium sp. and three referenced strains against aggressive phytopathogenic fungi Fusarium spp., Aspergillus spp., Penicillium spp., Alternaria spp., Humicola spp. and Cladosporium spp. isolated from infected and wilted plants in vitro and in vivo.

Study Design: First, we have ten strains of rhizobia isolated from leguminous and characterized, after that, some strains of fungi were isolated from infected and wilt plants, such as Fusarium, Aspergillus, Penicillium, Alternaria, Humicola and Cladosporium. Finally, The investigation of the potential of the isolated rhizobia and three referenced strains was evaluated in dual culture, in pots experiments and on seeds.

Place and Duration of Study: The study was carried out at the Department of Microbiology, Faculty of Nature and Life Sciences, Laboratory of Applied Microbiology, between April and November 2016.

Methodology: Rhizobia isolates were identified upon their phenotypic traits as: the morphology of the colony, of the physiological characters (growth temperature, salt tolerance, resistance to antibiotics, metabolism of carbon source, generation time…) and also the production of extracellular metabolites as siderophores and proteases. Fungal isolates were identified by their macroscopic and microscopic characters. The antagonistic effect of rhizobia against phytopathogenic fungi was examined in vitro by: confrontation in Petri dishes and inoculation of seeds; and in vivo by inoculation of plants.

Results: The isolated rhizobia were resistant to penicillin and amoxicillin with generation time ranged from 1.9 to 6.4 h, they were able to grow at temperatures from 4°C to 37°C, tolerate salt concentration (0.5 –2%) and produce siderophores and proteases. The results demonstrated the effectiveness of the rhizobia isolates and the referenced strains against the fungi isolates in vitro and in vivo. In dual culture, all rhizobia isolates inhibited the mycelial growth of the pathogens. The best disease control was obtained with isolate F3 from faba bean which inhibited the fungal growth with more than 70%. The highest inhibition growth was obtained against Alternaria spp.2, Penicillium spp.2, Cladosporium spp. and Humicola spp. with an inhibition rate ranging from 90 to 96%. In pot experiments, Rhizobia isolates from chickpea, lentil and faba bean reduced significantly all disease incidence with more than 75%; where the best fight was observed in lentil plants, while in faba bean no wilted plants were observed. Seeds inoculated with rhizobia and fungi in Petri dishes showed that isolates from faba bean and lentil were the most effective in reducing fungi disease incidence.