Pseudomonas and Bacillus isolates showing antagonistic activity
against fungal and oomycete pathogens were noninhibitory to
each other on agar plates, and this compatibility among different
isolates or species of antagonistic bacteria suggests their potential
to be used as a mixture of isolates for disease management. A peat
formulation consisting of a mixture of 2 or 3 bacteria effectively
suppressed damping-off and root rot of cucumber seedlings and
enhanced plant fresh masses in this study. The disease control and
growth promotion effect of combined strains may be due to different
mechanisms employed by those strains and might also be
due to additive or synergistic effects. However, antagonistic interactions
among biocontrol agents are more likely to occur than
synergistic interactions (Xu et al. 2011). The actual mechanisms of
disease suppression and plant growth promotion by Pseudomonas
and Bacillus isolates in this study are unknown and should be
investigated. Our preliminary studies indicate that the strong antagonistic
activity of the isolate P. fluorescens 9A-14 might be due to
the production of siderophores, proteolytic enzymes, indole acetic
acid, and chitinolytic activity. Pseudomonas and Bacillus are good
biocontrol bacteria also known as PGPR. They are capable of
promoting plant growth directly by producing plant growth
regulators or by stimulating nutrient uptake, and indirectly by
protecting plants from soil-borne pathogens or pests through the
production of siderophores or antibiotics (Kloepper and Schroth
1981).