Inhibition, biochemical activities, and control efficacy of fluopyram·tebuconazole against Alternaria spp. causing GALSB: Implications for QoI fungicide resistance management

To tackle the QoI fungicide resistance of Alternaria spp. (the causal agent of ginseng Alternaria leaf and stem blight (GALSB)), this study systematically assessed the antifungal activity and biochemical effects of fluopyram·tebuconazole on Alternaria spp. via the mycelial growth rate method and conidia germination assay. It also evaluated protective and curative efficacy of fluopyram·tebuconazole (FT) 35% suspension concentrate (SC) against GALSB under in vitro and field conditions. Results demonstrated that FT significantly inhibited mycelial growth in three Alternaria species (A. alternata, A. tenuissima, and A. panax) and conidial germination in two Alternaria species (A. alternata and A. tenuissima). Notably, its inhibitory activity against mycelial growth was more potent than that against conidial germination. FT treatment disrupted mycelial growth, ergosterol/DNA/protein biosynthesis, increased cell membrane permeability, damaged membrane integrity, and downregulated resistance-related genes (AOX, P450) and activity of AOX. No positive cross-resistance was observed between FT and other eight commonly used GALSB fungicides (including azoxystrobin). Under in vitro conditions, FT 35% SC at medium-to-high doses achieved protective efficacy (86.75-95.81%) and curative efficacy (88.61-98.35%) against GALSB, outperforming azoxystrobin at equivalent doses. One-year field trials at three sites showed medium-high FT 35% SC doses provided 83.38-85.05% (7 days), 79.59-83.42% (15 days), and 70.44-76.87% (30 days) average disease control after final application. These values exceeded those of control fungicides pyraclostrobin, azoxystrobin, kresoxim-methyl by 10.96-34.38%, 12.51-45.18%, and 19.93-36.29%, respectively. Notably, FT 35% SC retained >70% efficacy 30 days after the final spray, indicating prolonged residual activity. Additionally, FT 35% SC increased ginseng yield by 10.09-16.71%. In summary, FT exhibits potent antifungal and biochemical activity against Alternaria spp., making it a viable alternative to QoI fungicides for GALSB control and resistance management.