Effect of Fungicide Application Timing on Gray Leaf Spot and Tar Spot Disease Severity, Standability, and Yield of Corn in Iowa

Foliar diseases such as gray leaf spot (GLS) and tar spot negatively affect corn yields when improperly managed. Field trials done in Iowa from 2021 to 2023 compared the effect of one fungicide application at each of four corn growth stages (V6, V12, R1, and R3) and two applications at multiple growth stages (V6 + R1 and V12 + R3) on GLS and tar spot severity, standability, and yield of corn. In each year of the trials, corn was planted in fields with a history of either continuous corn (CC) or a corn–soybean (CSB) rotation. Disease severity on the ear leaf at R5 in 2021 and 2023 was low (<2%). However, in 2022, GLS and tar spot severity on the ear leaf at R5 were 16.7 and 3.8% for CSB, respectively, and 9.8 and 2.7% for CC, respectively. In 2022, all fungicide application timings reduced GLS and tar spot severity, except the V6 application for GLS, compared with the non-sprayed control. Although no effect of treatment on standability (determined as percent lodging) ( P > 0.05) was detected across the six site years, when harvest occurred later, numerically fewer plants lodged with a fungicide application compared with the non-sprayed control. An effect on yield was observed only in 2022 for CSB, with greater yields ( P = 0.0002) than the non-sprayed control detected for applications made at V12, R3, V6 + R1, and V12 + R3. This research demonstrated that an appropriately timed fungicide application effectively managed GLS and tar spot and protected yield. [Formula: see text] Copyright © 2025 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license .