The Significance of Mycoparasitism by Streptomyces sp. MBCN152-1 for Its Biocontrol Activity against Alternaria brassicicola

Streptomyces sp. strain MBCN152-1, isolated from cabbage, has potential as a biocontrol agent for Alternaria brassicicola on cabbage seedlings. The present study examined its mode of action. Light microscopy showed that appressorium formation by A. brassicicola was significantly suppressed on cabbage seedlings bacterized with MBCN152-1. Furthermore, scanning electron microscopy revealed that the mycelia of MBCN152-1, which were epiphytically growing on the cotyledon leaves of cabbage seedlings, intensively coiled around the germinating conidia of A. brassicicola. In vitro co-culture experiments demonstrated that MBCN152-1 is an aggressive mycoparasite of A. brassicicola, but not of A. brassicae or Colletotrichum higginsianum. Biocontrol experiments indicated that MBCN152-1 did not control diseases caused by A. brassicae or C. higginsianum. These results suggest that mycoparasitism is the primary mode of action for MBCN152-1. This is the first study to clearly demonstrate the significance of mycoparasitism in the biocontrol efficacy of endophytic Streptomyces.