Harnessing Rimocidins-Producing <i>Streptomyces</i> sp. JCK-6116 as a Sustainable Fungicide for Biocontrol of Cucumber Soil-Borne Diseases

<i>Fusarium oxysporum</i> f. sp. <i>cucumerinum</i>, the causal agent of cucumber fusarium wilt, along with <i>Rhizoctonia solani</i> AG-4 and <i>Pythium ultimum</i>-that causes cucumber damping-off-are soil-borne fungal and Oomycetes pathogens responsible for significant economic losses in agriculture. In this study, the culture filtrate of <i>Streptomyces</i> sp. JCK-6116, isolated from soil, exhibited strong inhibitory activity against the mycelial growth of multiple phytopathogenic fungi in a 96-well microtiter plate assay. In the dual culture assay, JCK-6116 inhibited the growth of 20 species of plant pathogenic fungi and Oomycetes, suggesting a wide antifungal spectrum. Three active compounds-rimocidin A, B, and C-were isolated from JCK-6116 and identified. These rimocidins exhibited antifungal effects against fungi by binding to ergosterol in the fungal membrane. However, none of the compounds exhibited anti-oomycete activity against the tested Oomycetes strains. Among the three compounds, rimocidin A demonstrated the strongest antifungal activity with minimum inhibitory concentration values ranging from 1.25-10 μg/ml. Furthermore, the culture broth of JCK-6116, at 10-fold dilution, effectively suppressed fusarium wilt and the two damping-off diseases in cucumber. Its butanol extract was also effective against the two fungal diseases but showed no activity against <i>P. ultimum</i> damping-off disease. These findings indicate that the culture broth contains metabolites with anti-oomycete activity. This study demonstrates that <i>Streptomyces</i> sp. JCK-6116 has significant potential as a biological control agent for managing soil-borne diseases caused by fungi and Oomycetes.