Fusarium oxysporum is an abundant plant pathogen in the soil and is found in the rhizosphere of many plant species. The fungus has numerous formae speciales (f.sp.) that infect and cause wilt diseases of a variety of host plants. F. oxysporum f.sp lycopersici (Fol) and F. oxysporum f.sp lini (Foln) are known to infect tomato and flax, respectively. Fluorescent pseudomonads can suppress various soilborne diseases, and their efficacy has been related both to their antagonistic activities and to their rhizosphere competitiveness. In this study, the biocontrol capacity of plant growth-promoting rhizobacteria (PGPR) strains of Pseudomonas fluorescens against fusarium wilt of tomato and flax was evaluated both in vitro and in planta. The P. fluorescens strains exhibited substantial antagonistic activities against the two fungal isolates on three culture media tested: King B (KB), potato dextrose agar (PDA) and a mixture of both (KB + PDA). As compared to the untreated control, the mycelial growth decreased by 8.33 to 49.33%. Conidial germination and germ tube elongation were significantly inhibited and reduced, respectively, by 6.15 to 47.33% and by 1.63 to 45.45%. In bioassay trials, P. fluorescens strains significantly reduced the disease incidence and severity by 4.86 to 74.49% on tomato plants and 3.93 to 79.19% on flax plants. The different efficacy of the biocontrol agents could be due to the influence of several factors, including the efficiency of the strain, the type of pathogen and the host.
Key words: Plant growth-promoting rhizobacteria (PGPR), Pseudomonas fluorescens, fusarium wilt, tomato, flax, biocontrol.
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