Novel Essential Oil-Based Thiosemicarbazone Compounds as Potential Fungicides in Controlling Postharvest Diseases of Citrus Fruit

To develop novel fungicides for controlling postharvest fungal diseases in citrus fruits, 12 essential oil (EO)-based thiosemicarbazones compounds, termed hydrazine-carbothioamide, were prepared according to the condensation method. <i>In vitro</i> assays showed that compound 13j exhibited the strongest antifungal activity (minimum inhibitory concentration [MIC] = minimum fungicidal concentration [MFC] = 0.0125 mg/mL) against <i>Penicillium digitatum</i>. An <i>in vivo</i> study revealed that 5 × MFC of compound 13j can effectively mitigate the green mold incidence of citrus fruit inoculated with <i>P. digitatum</i>, as well as fruit rot during natural storage, at a level comparable to that of the chemical fungicide prochloraz. Throughout this process, fruit quality was maintained. The hemolysis assay showed that these thiosemicarbazone compounds have good biocompatibility and that their safety is comparable to that of prochloraz. The antifungal activity of compound 13j was attributed to membrane damage, as confirmed using scanning electron microscopy (SEM), Calcofluor white (CFW) staining, propidium iodide (PI) staining, Fourier transform-infrared (FT-IR) spectroscopy, optical density (OD)₂₆₀, and relative conductivity assays. Collectively, our results indicate that compound 13j can be used as an antifungal agent to control the postharvest decay of citrus fruits.