Effect of Benomyl on Seedling Rot of Rice (Pseudomonas glumae) and Microbial Interactions on Germinating Rice Seeds

Abstract

Pseudomonas glumae grew on potato-dextrose agar plates containing benomyl at the concentration of 500 to 1, 000μg/ml depending on the strains, but not at 1, 500μg/ml. The growth inhibition resulted from bacteriostatic action of the chemical. Cultures of the bacterium contained benomyl-resistant mutants at various frequencies depending on the strains. Virulence of these mutants was identical to that of the parent strains. Regardless of the presence of such resistant mutants, seedling rot of rice was effectively controlled by seed-dressing with 0.1% (w/w) benomyl or 0.5 to 1.0% Benlate or Benlate-T. Such suppressive effect was not observed when sterilized rice grains were used. The population of P. glumae was significantly suppressed on benomyl-treated seeds, but rapidly increased on untreated seeds. On the contrary, saprophytic bacteria, particularly fluorescent pseudomonads, substantially increased on benomyl-treated seeds, but not on untreated seeds. The fluorescent pseudomonads inhibited growth of P. glumae in vitro and significantly suppressed development of seedling rot in situ as well. They were identified as P. fluorescens and the production of growth inhibitory substances was confirmed in liquid media. These observations suggested that the efficacy of benomyl against seedling rot of rice resulted mainly from the antagonistic effect of P. fluorescens which preferentially proliferated on the benomyl-treated seeds.