Inhibitory activity of Shikuwasa peel extract against the Fusarium wilt disease caused by Fusarium buharicum

Fusarium wilt, caused by Fusarium buharicum, is an emerging disease of okra in Japan, first reported in 2018. It causes severe damage to seedlings. Current control strategies rely on chemical fungicides, but repeated use may reduce pathogen sensitivity. Plant secondary metabolites are promising sources of natural fungicidal compounds. In this study, we evaluated the inhibitory activity of Shikuwasa peel extracts on F. buharicum. High-pressure extracts of Shikuwasa peel prepared at 200 °C for 1–3 h showed a time-dependent inhibition of conidial germination. Furthermore, volatile compounds from these extracts, tested using a two-compartment Petri dish method, significantly inhibited conidial germination and showed fungicidal activity. A GC–MS analysis identified 16 volatile compounds that increased after 3 h, with eucalyptol showing the largest rise. These findings indicate that Shikuwasa peel extract could serve as a promising natural control agent against Fusarium wilt in okra.