Design, Synthesis, and Biological Activity of Novel Thiazole Sulfonamide Derivatives as Potent Fungicide Candidates against <i>Sclerotinia sclerotiorum</i>

<i>Sclerotinia sclerotiorum</i> is one of the most devastating plant pathogens, causing a severe reduction in the quality and yield of oilseed rape. To develop novel fungicide candidates against <i>S. sclerotiorum</i>, 58 novel thiazole sulfonamide derivatives were designed and synthesized via an active substructure splicing strategy. <i>In vitro</i> antifungal bioassays demonstrated that these compounds exhibited significant inhibitory activity against <i>S. sclerotiorum</i>. Notably, compound <b>3d-1</b> showed the highest antifungal potency with an EC₅₀ value of 1.86 mg/L, which was comparable to that of the commercial fungicide thifluzamide (1.84 mg/L). <i>In vivo</i> bioassays further revealed that compounds <b>3d-1</b> and <b>3d-9</b> exerted excellent protective efficacy (95.7, 89.8%) and curative efficacy (80.9, 78.9%) against <i>S. sclerotiorum</i> on oilseed rape seedlings, which were superior to the positive control thifluzamide (89.6, 62.4%) but still inferior to fluxapyroxad (100, 90.0%). Morphological observations revealed that compound <b>3d-1</b> induced severe ultrastructural damage to the mycelia and mitochondria of <i>S. sclerotiorum</i>. Determination of reactive oxygen species (ROS) levels and measurements of mitochondrial membrane potential (MMP), as well as fluorescence quenching experiments, were combined to elucidate the inhibitory effect of compound <b>3d-1</b> on succinate dehydrogenase (SDH). Enzymatic inhibition assays targeting SDH further confirmed that compound <b>3d-1</b> suppressed the activity of <i>Ss</i>SDH, with an IC₅₀ value of 4.14 mg/L. Moreover, molecular docking analysis demonstrated significant interactions between compound <b>3d-1</b> and the target protein <i>Ss</i>SDH, which provides critical insights into the potential mode of action of this compound. This study provides a promising foundation for the development of succinate dehydrogenase inhibitor (SDHI)-based fungicides against <i>S. sclerotiorum</i>.