INFLUENCE OF THE BIOHERBICIDE PHOSPHINOTHRICIN ON INTERACTIONS BETWEEN PHYTOPATHOGENS AND THEIR ANTAGONISTS

Phosphinothricin is a microbial toxin currently under development as a selective weed killer in the cultivation of transgenic plants engineered to resist its presence. Here it is shown that phosphinothricin is inhibitory to antagonistic soil microorganisms including Bacillus subtilis, Pseudomonas flourescens, and many species of Trichoderma. Phosphinothricin was also inhibitory to the phytopathogenic fungi Rhizoctonia solani and Sclerotinia sclerotiorum. In contrast, phytopathogens Fusarium oxysporum and Pythium aphanidermatum were highly resistant to phosphinothricin. In pathogen-antagonist challenges, species of Trichoderma that parasitize F. oxysporum and P. aphanidermatum were eliminated when phosphinothricin was added to the growth media. Similarly, the ability of Pseudomonas fluorescens to exclude Pythium aphanidermatum was markedly affected by the presence of phosphinothricin in the media. The dominance equilibrium between Trichoderma species and the bacterium Bacillus subtilis was influenced in favor of the former at 0 and 5 mM concentrations of the herbicide and in favor of the latter at 1 mM concentrations. Pseudomonas fluorescens was unable to exclude Trichoderma species regardless of the phosphinothricin concentration in the media.