Design, Synthesis, and Antifungal Evaluation of Novel Formononetin-Benzothiazole Hybrids with Preliminary Mechanistic Insights

In this study, 22 novel formononetin derivatives bearing benzothiazole were designed and synthesized, and their antifungal activities against nine plant pathogens were evaluated. H2 exhibited promising activity against Phomopsis sp. (P. sp.) (EC50 = 9.85 μg/mL), superior to azoxystrobin (Az, EC50 = 41.1 μg/mL). At 200 μg/mL, H2 showed curative and protective efficacies of 58.4 and 64.5% against kiwifruit soft rot, outperforming Az (41.6 and 48.2%). Preliminary phenotypic investigations suggested that H2 might disrupt cell wall and membrane integrity, causing electrolyte leakage and malondialdehyde (MDA) accumulation, while mitochondrial membrane potential dissipation further hinted at impaired mitochondrial function. Although the precise molecular target remains unknown, H2 shows promise as a lead candidate, and the benzothiazole moiety appears critical for enhancing activity. This study provides a preliminary basis for developing eco-friendly fungicides.