In this study, potential biocontrol agents including Pseudomonas putida, P. aeroginosa and P. fluorescens collected from the roots and rhizosphere of cantaloupe plants (Cucumis mello L.) sabze variety were tested against Phytophthora drechsleri in vitro and in vivo. After in vitro screening of 210 bacterial isolates of Pseudomonas spp, 16 isolates, which were the most effective antagonists, were selected for further studies. In laboratory experiments, most of the isolates inhibited mycelial growth of P. drechsleri by production of antibiotics and extra cellular compounds. Two isolates of P. putida (SCh) and P. aeroginosa (Zp) produced cyanide hydrogen. Seven of the 16 isolates, including P. aeroginosa (SG1 and Zh2), P. putida (SG2 and Zh1) and P. fluorescens (SSh, ZK2 and ZK1) produced siderophore and inhibited spore of Geothricum candidum. Isolates P. fluorescens (ZA, SSh and SU1), P. putida (SCh and SG2) and P. aeroginosa (SG1) produced Protease. Only one isolate of P. putida (SK1) produced cellulase. All tested bacteria produced indole acetic acid. In greenhouse conditions, isolates P. fluorescens (ZK2), P. aeroginosa (SG1) and P. putida (SG2 and ZH1) had the most significant effects on pathogen in seed and soil drenching treatments, respectively. The growth parameters (plant height, shoot and root dry-weight of seedlings) were significantly higher compared to the untreated control. Therefore, the antagonistic strains selected from the screening procedure provided significant protection against P. drechsleri through cantaloupe root colonization and could be applied by either seed or drench treatment as an alternative to chemical treatments of damping off disease of cantaloupe.
