Antagonistic effects of several bacteria on Verticillium dahliae the causal agent of cotton wilt.

Experiments were carried out with 89 bacterial isolates that were collected from cotton rhizosphere in Gorgan province. The antagonistic effects of bacterial isolates on Verticillium dahliae Klebahn were studied using dual culture test. Five highly effective isolates were selected from these antagonists for subsequent studies. According to the biochemical, physiological and morphological tests, isolates 2020 and 3 were identified as Pseudomonas fluorescens and isolate 204, 202 and 309 as Bacillus spp. These isolates were used to investigate their antagonistic mechanisms in vitro and their effects on cotton growth in vivo. Inhibition of V. dahliae by volatile metabolites and antibiotics was studied as described by Fiddamen (1994) and Kraus (1990). Production of hydrogen cyanide was studied qualitatively, using HCN-indicator paper of Castric and Castric (1983). Isolates 204, 202 and 309 inhibited the mycelial growth of the fungus through production of volatile metabolites. Isolates 2020 and 3 produced antibiotic as well as volatile metabolities that inhibited mycelial growth of V. dahliae. They both produced hydrogen cyanide. After four months of greenhouse study, the application of antagonistic bacteria had different effects on growth of cotton plants. Bacterial treatment in soil had better effects on plant growth than that of bacterial seed treatment. In soil treatments containing infested and non-infested soil with V. dahliae, isolates 2020 and 3 caused an increase in plant height in comparison with those in infested and non-infested controls. In non-infested soil, application of isolates 2020, 3 and 202 increased root length and dry weight of cotton plant, but in soil infested with the fungus, only isolate 202 increased root length. Isolate 2020 increased plant dry weight. In conclusion, isolates 2020 and 3 belonging to P. fluorescens and isolate 202 pertaining to genus Bacillus had the greatest effect on increasing the cotton growth.