Abstract: Walnut Cytospora canker, a highly destructive and challenging fungal disease to manage, has severely constrained the healthy development of the walnut industry. Screening biocontrol bacteria that can efficiently antagonize walnut Cytospora canker is of great significance for improving control effect of walnut Cytospora canker and developing environment-friendly disease management strategies. In this study, through plate confrontation test, a biocontrol strain HT1 capable of efficiently antagonizing Cytospora chrysosperma was isolated from the bleeding sap of walnuts. The inhibition rate of the fermentation filtrate of strain HT1 cultured for 2 days against C. chrysosperma was as high as 85.71%. Based on the alignment of the 16S rRNA gene sequence, it was identified as Streptomyces sampsonii HT1. Through untargeted metabolomics, 780 differentially upregulated metabolites were detected in the fermentation filtrate of strain HT1. Among them, the concentrations of niflumic acid, triphenylphosphine oxide, and hesperetin, were 17.73 mg/L, 0.24 mg/L, and 0.42 mg/L respectively. These substances exhibited good inhibitory effects on C. chrysosperma, with IC₅₀ values of 2.30 mg/L, 3.18 mg/L, and 5.39 mg/L respectively. In vitro experiment on walnut branches demonstrated that the fermentation broth, fermentation filtrate, and three antibacterial metabolites of strain HT1 could effectively control walnut Cytospora canker, with the efficacy of 33.33%-74.23%. Our findings indicate that the fermentation filtrate of C. chrysosperma HT1 contains antibacterial metabolic components, possessing potential application value in the walnut Cytospora canker control.

Key words: Streptomyces sampsonii HT1, walnut Cytospora canker, Cytospora chrysosperma, biocontrol, untargeted metabolomics