Synergistic effect of ethanol and Streptomyces angustmyceticus NR8-2 cell-free filtrate against Phytophthora palmivora causing durian stem rot

Abstract

Durian (Durio zibethinus L.) production is seriously affected by root and stem rot caused by Phytophthora palmivora, a highly destructive oomycete pathogen. To promote sustainable disease management, this study investigated the synergistic anti-oomycete activity of a low concentration of ethanol (5 %) combined with the cell-free culture filtrate (CF) of the actinobacterium Streptomyces angustmyceticus NR8-2. The agar well diffusion assay revealed that the combination of CF and 5 % ethanol significantly inhibited mycelial growth (approximately 75 %), induced colony lysis, and caused deformation of hyphae and abnormal sporangial formation. Detached leaf assays further demonstrated that P. palmivora pre-treated with CF and 5 % ethanol exhibited reduced infection ability, resulting in significantly smaller necrotic lesions on durian leaves. Moreover, durian leaves inoculated with CF + ethanol-treated P. palmivora showed elevated activities of defense-related enzymes, peroxidase and polyphenol oxidase, suggesting activation of host defense-related enzymes. In vivo experiments indicated that this combined treatment effectively suppressed the development of necrotic symptoms in durian seedlings, outperforming ethanol or the chemical fungicide metalaxyl alone. Overall, these findings suggest that the combination of microbial metabolites and low-concentration ethanol not only directly inhibits P. palmivora but also indirectly activates host defense responses, representing a promising and sustainable alternative to chemical fungicides for durian disease management.