Biocontrol of fungal root rot diseases of crop plants by the use of rhizobia and bradyrhizobia

Twenty-one Rhizobium and Bradyrhizobium strains were tested in vitro against the mycelial growth of three pathogenic fungi on solid and liquid media. All tested rhizobia and bradyrhizobia significantly suppressed the growth of the three soil-borne root-infecting fungi (Fusarium solani, Macrophominia phasolina and Rhizoctonia solani) either in the absence or presence of iron. This indicates that the siderophore played a minor role in the biocontrol potential of Rhizobium and Bradyrhizobium against pathogenic fungi. Pot experiments revealed that the numbers of propagules causing disease after 4 weeks of planting varied with species and host plant. The three most active Rhizobium and Bradyrhizobium strains (R. leguminosarum by. phaseoli TAL 182, B. japonicum TAL 377 and Bradyrhizobium sp. (lupin) WPBS 3211 D) tested under greenhouse conditions for their ability to protect one leguminous (soybean) and two non-leguminous (sunflower and okra) seedlings from root rot caused by Fusarium solani, Macrophominia phaseolina and Rhizoctonia solani provided significant suppression of disease severity compared with nonbacterized control in both leguminous and non-leguminous seedlings. Bradyrhizobium sp. (lupin) WPBS 3211 D provided the lowest degree of resistance against all the tested pathogens with all host plants.