Discovery of Ternary Aromatic (Heterocyclic) Thione/Thioether Derivatives as Phloem-Mobile Fungicidal Candidates

Plant fungal diseases severely threaten global crop production. To develop novel antifungal agents, we designed and synthesized a series of novel ternary aromatic (heterocyclic) thione/thioether derivatives and evaluated their antifungal activity. Most of these compounds exhibited excellent antifungal activity in vitro against Sclerotinia sclerotiorum and Rhizoctonia cerealis . In particular, compounds C3, C7, C8, and C11 exhibited remarkable antifungal activity against S. sclerotiorum, with median effective concentration (EC 50 ) values of 0.479, 0.476, 0.240, and 0.390 μg/mL, respectively, which were more potent than that of the positive control carbendazim (0.586 μg/mL). A further in vivo test confirmed that compound C8 possessed better protective efficacy (78.3%) than carbendazim (58.8%) at 200 μg/mL. The 3D-QSAR model highlighted the dominant role of steric effects on the antifungal activity. Further studies revealed that compound C8 is phloem-mobile, rapidly translocates to roots, and strongly inhibits sclerotial formation. In field trials, compound C8 provided significantly better control efficacy against rape sclerotinia stem rot (64.9%) than carbendazim (45.5%) at 750 g a.i./ha. This study demonstrates that compound C8 has potential as an efficient phloem-mobile fungicide.