Novel Design of Citral-Thiourea Derivatives for Enhancing Antifungal Potential against Colletotrichum gloeosporioides

Although much progress has been made in developing botanical fungicides to combat fungal diseases in crops, there remains a great need to improve the efficiency and long-term safety of these fungicides. This study proposes a novel strategy for designing citral-thiourea derivatives that feature such desirable properties. The motivation of the work herein was to enhance the antifungal activity of citral against C. gloeosprioides by exploiting the synergistic effect that arises from combining citral and thiourea compounds, thereby producing citral-thiourea derivatives that exhibit good long-term safety. The results revealed that the generated compounds e1, e3, e6, e18, and g showed remarkable antifungal activities against C. gloeosprioides, with corresponding EC50 values reaching 0.16, 1.66, 1.37, 4.76, and 4.60 mg/L, respectively, showing that the compounds significantly outperformed both the positive control kresoxim-methyl and the commercially available fungicide carbendazim. Furthermore, compound g showed stronger protective efficacy against C. gloeosprioides than carbendazim on mango fruit at 25 mg/L. Investigating the preliminary structure–activity relationship (SAR) of the compounds also revealed that the citral-thiourea derivatives exhibited higher antifungal activities against C. gloeosprioides compared to citral and thiourea compounds. This reinforcement of antifungal activity observed in the derivatives was found to be attributable to the two characteristics of low molecular size and the presence of a fluorine atom in the meta-position of the benzene ring. Beyond this, it was determined from QSAR that two molecular descriptors (the Kier–Hall index (order 3) and Tot dipole of the molecules) were negatively related to the antifungal activity of the citral-thiourea derivatives, while one other (the maximum resonance energy of a C–H bond) was positively related to their antifungal activity. More importantly, the citral-thiourea derivatives with high antifungal activities (i.e., compounds e1, e3, e6, e14, e15, e18, and g) exhibited negligible cytotoxicity to LO2 and HEK293T cell lines. The antifungal mechanism of the generated citral-thiourea derivatives was investigated by scanning electron microscopy (SEM) and relative conductivity. The derivatives were found to affect mycelial morphology and increase fungal cell membrane permeability, thereby resulting in the destruction of fungal cell membranes. These promising results provide novel insights into the study and potential application value of citral-thiourea derivatives as high-efficiency antifungal agents against C. gloeosprioides.