Impact of Cultivar Architecture and Seeding Rate on Fungicide Coverage, Disease Development, and Yield in Soybean

This study investigates the impact of seeding rate, cultivar architecture, and fungicide application on soybean canopy penetration, disease severity, and yield. Field trials conducted in Iowa in 2022, 2023, and 2025 included two soybean cultivars: Xitavo 2282E, a low-branching cultivar, and Xitavo 2501E, a high-branching cultivar, planted at three seeding rates: low (247,105 seeds ha⁻¹), medium (345,948 seeds ha⁻¹), and high (444,798 seeds ha⁻¹). Fungicide containing mefentrifluconazole, fluxapyroxad, and pyraclostrobin was applied at 584.6 ml ha⁻¹ at the beginning pod (R3) stage using a tractor-mounted sprayer. Spray coverage in the upper canopy was uniform across cultivars and seeding rates, while in the lower canopy, Xitavo 2282E received 41% greater coverage than Xitavo 2501E. Frogeye leaf spot (FLS) and Septoria brown spot (SBS) severity varied among site-years, with Xitavo 2282E exhibiting more disease than Xitavo 2501E. Seeding rate influenced SBS, with higher populations associated with increased disease. Low seeding rates increased branch and node numbers, and Xitavo 2501E had more branches and nodes than Xitavo 2282E. Grain yield increased with seeding rate, while cultivar had no significant effect. Fungicide application increased 100-seed weight and yield modestly (~2%). These results indicate that planting population is the primary driver of soybean productivity, while canopy architecture affects spray penetration, but not yield response under low disease pressure. Integrating cultivar selection, seeding rate, and canopy structure can enhance soybean management and fungicide efficiency.