Screening of Antifungal Microorganisms with Strong Biological Activity against Oak Wilt Fungus, Raffaelea quercus-mongolicae

Since the mass mortality of Quercus mongolica has been first reported in Gyeonggi province at 2004, the disease spread rapidly over Korean peninsula annually. Ambrosia beetle (Platypus koryoensis) was known as the insect vector of oak wilt fungus, Raffaelea quercus-mongolicae, and control methods of the disease had mainly been focused on eradication of insect vector. However, for the efficient management of the disease, combined control methods for both of the pathogenic fungus and insect vector are strongly required. As one of the efforts to suppress the pathogenic fungus, antifungal activities of Streptomyces isolated from oak forest soil were assayed in this study. Optimum culture condition for the selected isolates was also studied, As a result, Streptomyces blastmyceticus cultured in PDB (Potato Dextrose Broth) at $25^{\circ}C$ for 1 week showed the strongest antifungal activity against oak wilt fungus. Mycelial growth inhibition rates (MGIRs) of Streptomyces isolates were compared on culture media supplemented with heated and unheated culture filtrates of S. blastmyceticus. MGIRs on culture media with unheated culture filtrates were generally higher than those on culture media with heated culture filtrates. Antagonistic mechanism to get involved in the inhibition of hyphal growth and spore formation of the pathogen is due to the antifungal metabolites produced by Streptomyces. This study will provide the fundamental information in developing biocontrol agents for the environment-friendly management of oak wilt disease.