Suppression of Pythium ultimum root rot of sorghum by rhizobacterial isolates from Ethiopia and South Africa

Bacteria isolated from the rhizosphere of sorghum in Ethiopia and from different grass species within the Nylsvlei Nature Reserve in South Africa were evaluated for their in-vitro and in-vivo antagonistic activity against Pythium ultimum that causes root rot in sorghum. Root rot caused by this pathogen in sorghum is characterized by blackened discolorations with white mycelial growth on the surface of the roots. Statistically significant disease suppression was achieved by 15 isolates from sorghum rhizosphere in Ethiopia (⩾50% disease suppression), whereas eight isolates from the rhizosphere of grasses at Nylsvlei Nature Reserve in South Africa resulted in ⩾80% disease suppression. The isolates maintained themselves at a level ⩾105 cfu/g indicating that they are also highly competent in sorghum rhizosphere. Isolates which showed the best overall performance in-vitro and in-vivo were identified by means of the API system and sequencing of the bacterial 16S rDNA genes. The majority of these isolates were identified as members of the genus Bacillus, 56% of which were B. cereus. Three other isolates were identified as Brevibacterium laterosporus, Pseudomonas fluorescens and Serratia marcescens. Selected modes of action studies indicated that production of antibiotic substances and siderophores as well as induction of systemic resistance in sorghum were exhibited by some of the most effective isolates. The results of this study offer a significant impetus to the application of plant growth promoting rhizobacteria for use as biological control agents against P. ultimum in sorghum.