Abstract:In order to investigate the variation in sensitivity of different target pathogens to the biocontrol agent, Pseudomonas fluorescent LT6 and its metabolite, the antagonistic effect of Pseudomonas fluorescent LT6 on 69 isolates of Pythium spp. was tested on a 1/5 TSA plate. Strain LT6 exhibited significantly different antagonistic activities against different species of Pythium or different isolates of the same species. It showed the widest inhibition zone against Pythium ultimum (7.5 mm),while it was lest effective to unknown species. The size of inhibition zones on those eleven tested isolates were all less than 3.0 mm. The sensitivity of Pythium spp. to PCA (phenazine-1-carboxylic acid), produced by Pseudomonas fluorescent LT6 was variant. When the concentration of PCA was 7.0 μg/mL, there were 28 sensitive isolates and 15 highly resistant isolates among the 54 tested isolates. These results demonstrated that the sensitivity of target pathogens to the biocontrol agent varies, and that the metabolite produced in different culture media exhibit distinctly different levels of suppression to pathogens.