The Expression of ABC and MFS Transporter Genes, Not Mutations at ScCyp51 , Is Associated with Difenoconazole Resistance in Stagonosporopsis Cucurbitacearum from Melon Fields

Stagonosporopsis cucurbitacearum is a main causal agent of gummy stem blight (GSB) in the melon field. Growers complain about the decreased control efficacy of DMIs for GSB in China recently. Difenoconazole (Dif) resistance frequencies reached 100% and 13.24% in S. cucurbitacearum isolates from Nanjing and Chongqing, respectively. Dif-sensitive isolates exhibited EC₅₀ values of 0.039-0.057 μg/mL and an MIC of 2 μg/mL, whereas Dif-resistant isolates showed EC₅₀ values of 0.494-0.495 μg/mL (RF = 10.75) and an MIC of 250 μg/mL. Although Dif-resistant isolates carried a Y446H substitution in ScCyp51, genetic transformation and molecular docking analyses indicated that these alterations contributed only marginally to resistance. In contrast, the inducible upregulation of ATP-binding cassette (ABC) and major facilitator superfamily (MFS) transporter genes (ScatrA, ScatrF, and ScMdr1p) was associated with Dif resistance. These findings suggest that nontarget-site mechanisms, particularly transporter expression, are important features of DMI resistance in S. cucurbitacearum.