Antagonistic Mechanisms of Bacterial Antagonists of Soybean Charcoal Rot in vitro

Charcoal rot caused by Macrophomina phaseolina، is considered to be one of the important diseases of soybean. Two hundred eighty strains of bacteria were recovered from rhizosphere، phylosphere and nodules of soybean and purified on NA media. Eighty nine of these strains displayed antagonistic ability using dual-culture tests against pathogen، of which، nine strains with high antagonistic effect (>50%)، seven strains as Bacillus spp.، one strain each belonging to Pantoea agglomerans and Pseudomonas fluorescens biov. I، were selected for in vitro tests. Results of volatile metabolites assays indicated that P. agglomerans (ENA) isolate with 34. 5%، in antibiotic test، BIA، BL، BDQ and BIN (Bacillus spp.) isolates with 100% and in cell free culture test Bacillus (BL) isolate with 75. 29%، were the most efficient isolates in reducing mycelial growth of the pathogen. Siderophore test showed that ENA isolate produced the highest amount of siderophore (23. 8 mm in diam). P. fluorescens (P2FB) isolate was the only isolate produced IAA. To determine the population dynamics of antagonists during greenhouse experiments، mutants of antibiotic resistant were obtained. In antagonistic assays، mutants and wild types were not significantly different. Using specific primers (PrnAF/PrnAR) in wild types and mutants of Bacillus BIN and P. agglomerans ENA for detecting antibiotic-encoding genes of pyrrolnitrin indicated that the ENA strain carried gene for encoding pyrrolnitrin.