| Summary: | Root rot diseases, caused by phytopathogenic oomycetes, <i>Phytophthora</i> spp. cause devastating losses involving forest seedlings, such as Japanese cypress (<i>Chamaecyparis obtusa</i> Endlicher) in Korea. Plant growth-promoting rhizobacteria (PGPR) are a promising strategy to control root rot diseases and promote growth in seedlings. In this study, the potential of <i>Bacillus velezensis</i> CE 100 in controlling <i>Phytophthora</i> root rot diseases and promoting the growth of <i>C. obtusa</i> seedlings was investigated. <i>B. velezensis</i> CE 100 produced β-1,3-glucanase and protease enzymes, which degrade the β-glucan and protein components of phytopathogenic oomycetes cell-wall, causing mycelial growth inhibition of <i>P. boehmeriae</i>, <i>P. cinnamomi</i>, <i>P. drechsleri</i> and <i>P. erythoroseptica</i> by 54.6%, 62.6%, 74.3%, and 73.7%, respectively. The inhibited phytopathogens showed abnormal growth characterized by swelling and deformation of hyphae. <i>B. velezensis</i> CE 100 increased the survival rate of <i>C. obtusa</i> seedlings 2.0-fold and 1.7-fold compared to control, and fertilizer treatment, respectively. Moreover, <i>B. velezensis</i> CE 100 produced indole-3-acetic acid (IAA) up to 183.7 mg/L, resulting in a significant increase in the growth of <i>C. obtusa</i> seedlings compared to control, or chemical fertilizer treatment, respectively. Therefore, this study demonstrates that <i>B. velezensis</i> CE 100 could simultaneously control <i>Phytophthora</i> root rot diseases and enhance growth of <i>C. obtusa</i> seedlings.
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