Determination of antibiotic activity on plasmids from fluorescent pseudomonads isolates CW2, WB15 and WB52 against pre-emergence damping-off caused by Pythium ultimum and Rhizoctonia solani in cucumber

Three fluorescent pseudomonads, isolates CW2, WB15 and WB52, were tested for their antagonistic activities against Pythium ultimum and Rhizoctonia solani isolates. In vitro experiments revealed pronounced inhibition zones on PDA agar medium against P. ultimum and R. solani by the isolates CW2 and WB15, respectively. Escherichia coli strains transformed with plasmids from the fluorescent pseudomonads isolates caused larger inhibition zones against both pathogens compared to the wild type isolates. In greenhouse experiments, the most effective isolate against P. ultimum was CW2 and isolate WB52 was the most effective against R. solani. The transformed E. coli strains proved to be significantly (P < 0.05) more effective in reducing pre-emergence damping-off in cucumber caused by P. ultimum than the wild type isolates, while their efficacy against damping-off incited by R. solani was similar to or less than that of the wild type fluorescent pseudomonads isolates. According to the results obtained by HPLC analysis, an isomer of 2,4-diacetylphloroglucinol (DAPG) was detected in culture filtrates of isolate CW2, while in the E. coli strain transformed with the CW2 plasmid, pyoluteorin and other phenazine derivatives were detected. In wild type isolates of WB15 or WB52 and in E. coli isolates transformed with plasmids from the fluorescent pseudomonads isolates, pyoluteorin and phenazine derivatives were detected. These results suggest that the expression of some genes might be blocked in the wild type isolates.