Abstract
An antagonistic bacterium against Phytophthora capsici, the pathogen of damping-off of cucumber, was isolated from the rhizosphere of scarlet sage (Salvia splendens F. Sellow ex Roem. et Schult.) in Fukui Prefecture. The red-pigmented strain F-1-1 was identified as Serratia marcescens based on its bacteriological properties. The strain F-1-1 strongly inhibited the germination of cystospores and zoosporangia of P. capsici P-9-2 in in vitro tests, and also it suppressed the damping-off of cucumber seedlings inoculated with a cystospore suspension of P. capsici P-9-2 in pot tests. Moreover, all the other red-pigmented S. marcescens strains isolated from the rhizosphere of Japanese holly (Ilex crenata Thunb.) were antagonistic, unlike six non-pigmented Serratia spp. strains. Four mutants defective red pigment biosynthesis were obtained by transposon (Tn7) mutagenesis among 5967 transconjugant clones. These mutants simultaneously lost their antagonistic ability in in vitro and in pot tests. Thus, S. marcescens F-1-1 was found to be a potential biocontrol agent for damping-off of cucumber seedlings and its antagonistic ability was based on the production of antibiotic red pigments.
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