Characterization of Rhizosphere <i>Streptomyces hachijoensis</i> JCK-6068 as a Multifunctional Agent for the Control of Soil-Borne Fungal and Oomycete Diseases

Soil-borne fungal and oomycete pathogens endanger global crop production, highlighting the need for sustainable alternatives to chemical fungicides. In this study, an antagonistic actinomycete isolated from agricultural soil was identified as Streptomyces hachijoensis JCK-6068 through 16S rRNA sequencing and phylogenetic analysis. S. hachijoensis JCK-6068 exhibited broad-spectrum antifungal and antioomycete activity in dual-culture and minimum inhibitory concentration assays. Biochemical characterization revealed the production of multiple hydrolytic enzymes, including cellulase, chitinase, protease, and gelatinase, along with the phytohormone indole-3-acetic acid. Volatile organic compounds produced by JCK-6068 strongly inhibited pathogen growth in vitro, and gas chromatography- mass spectrometry identified 2-methyl-2-bornene as the predominant component. In vivo assays demonstrated that curative application of the 10-fold diluted fermentation broth provided effective control of creeping bentgrass dollar spot (74.31%), pepper Phytophthora blight (68.88%), cucumber Fusarium wilt (61.11%), and cucumber damping-off (88.89%). Preventive treatment with 1000-fold dilution of fermentation broth is as effective as chemical fungicides in controlling cucumber damping-off and Fusarium wilt. Furthermore, treatment of PR1::GUS transgenic Arabidopsis thaliana seedlings with JCK-6068-derived materials induced strong PR1 expression, indicating elicitor activity consistent with salicylic acid-responsive defenses signaling. To facilitate practical application, a wettable powder formulation derived from the n-butanol extract of JCK-6068 was evaluated under greenhouse conditions, achieving 91.67% control of cucumber Fusarium wilt at a 500-fold dilution and 77.78% control of cucumber damping-off at a 250-fold dilution, comparable to commercial chemical fungicides. Collectively, these findings highlight the multifaceted biocontrol potential of S. hachijoensis JCK-6068 as a sustainable biocontrol agent for soil-borne fungal disease.