Yield Loss Relationships and Fungicide Efficacy for Phomopsis Stem Canker in Sunflower ( <i>Helianthus annuus</i> L.)

Phomopsis stem canker of sunflower (Helianthus annuus L.) is a major disease in the United States. This study quantified the effect of Phomopsis stem canker severity on sunflower yield and evaluated the efficacy of fungicides under field conditions across Minnesota, North Dakota, Nebraska, and South Dakota. To determine the yield-disease severity index (DSI) relationship, non-treated control (NTC) plots from 80 field trials (2009-2025) were analyzed, and yield was found to decrease with increasing DSI, described by the equation: Yield (kg ha⁻¹) = 2552.52 - 11.64 × DSI (R² = 0.14, P &lt; 0.001). Seventeen trials conducted from 2022 to 2025 evaluated fungicides containing Quinone outside Inhibitors (QoI), Demethylation Inhibitors (DMI), and Succinate Dehydrogenase Inhibitors (SDHI) applied at V8 (eight true leaves), R1 (miniature floral head), and/or R6 (flowering complete and the ray flowers wilting) stages. Trials were categorized into low-, moderate-, and high-yield environments using ±1 SD residuals to determine relative differences in yield within the overall DSI-yield relationship. Yield decreased with increasing DSI in low (R²=0.91, P=6.13×10 ⁻5 ) and moderate (R²=0.19, P=0.000359) yielding environments. In low yield environments, fluopyram + tebuconazole at R1 followed by pyraclostrobin at R6 increased yield by up to 55%. In moderate yield environments, fluopyram (SDHI) + tebuconazole (DMI) at V8 or R6, combined with pyraclostrobin (QoI) at R1, increased yield by up to 15% over NTC. These findings indicate that fungicide efficacy differed among yield environments, suggesting that treatment decisions should account for the production environment to maximize sunflower yield.