The production of antibiotics and other specific metabolites
or growth factors are more likely mechanisms, as all 3 bacterial
isolates showed growth inhibition of P. ultimum in agar plates.
Beside direct inhibition of Pythium propagules by P. fluorescens
and B. subtilis isolates, induced resistance might also be involved
in damping-off suppression (Van Loon et al. 1998; Powell
et al. 2000; Van Loon 2007). Application of a mixture of 3 PGPR
strains as seed treatment resulted in much more intensive plant
growth and disease reduction compared with strains tested singly
(Raupach and Kloepper 1998). Similarly, bacterization of hydroponic
nutrient solution with plant-growth-promoting Pseudomonas strains
increases root and shoot mass of tomato, cucumber, lettuce, and
potato most likely by suppressing deleterious rhizosphere microflora
(Van Peer and Schippers 1989). Therefore, the level and consistency
of biological control can be improved significantly by
combined application of 2 or more antagonistic isolates having
multiple biocontrol mechanisms (Dunne et al. 1998).