Antagonistic effect of fluorescent Pseudomonas spp. against certain plant pathogens and their potential role as biocontrol agents

Abstract: The antagonistic effect of twenty one fluorescent Pseudomonas spp. belonging to at least three different isolates species, obtained from different Egyptian sites, were tested against five fungal isolates belonging to different pathogens, namely, Alternaria sp., Pythium sp., Phytophthora infestans, Rhizoctonia solani and Sclerotium sp. These isolates were characterised and identified based on the 16S rRNA gene. When the twenty one isolates were tested as biocontrol agents for their antagonistic effect on the in vitro growth of the above fungal pathogens, P. aeruginosa isolate JY21 had more antagonistic effect on Alternaria sp. and P. infestans while P. fluorescens isolate JY8 was the most effective in inhibiting R. solani and Sclerotium sp. In contrast, P. fluorescens isolates JY12 and JY13, compared to other isolates, were more effective in inhibiting the mycelia growth of Pythium sp. Pyocyanin produced by P. aeruginosa isolate JY21 was detected (onto Pseudomonas agar p base medium), extracted with chloroform and characterised by infra-red (IR) spectrum and gas chromatography/mass spectrometry. In a pot experiment, the cell free supernatant of P. aeruginosa isolate JY21 proved to be effective in reducing disease incidence and severity of damping-off of tomato growing in Pythium sp.- or R. solani-infested soil when added to the soil, three days before planting. The supernatant had a stimulatory effect on the growth of tomato plants in terms of fresh and dry weight of both the shoot and the root systems of treated as compared to untreated control plants.