Tar spot impacts silage corn yield and forage nutritive value

Silage corn (Zea mays L.) in Michigan and the Great Lakes region is prone to an emerging foliar disease called tar spot (caused by Phyllachora maydis). When corn is infected with Phyllochora maydis, stromata develop on the leaves resulting in early senescence and drying. Therefore, to understand the effect of tar spot on forage yield, nutritive value, and predicted milk yield, field trials were conducted at multiple Michigan locations from 2021–2023. Field trials were arranged in randomized complete block design with four replications. Treatments included hybrid resistance (one susceptible and one partially resistant hybrid) and three fungicide treatments using Delaro 325 SC at 8 oz acre⁻¹ (non-treated, one application at silking [R1], and two applications [one at R1 and second at dough stage]). Results showed that tar spot severity increased over time in silage corn. Fungicide application in susceptible hybrid had the lowest tar spot severity across all hybrids and fungicide treatments. Hybrid disease resistance resulted in 50% reduction of tar spot severity and contributed to a lower yield penalty. Reduction of tar spot severity due to hybrid disease resistance also minimized decline in neutral detergent fiber digestibility and predicted milk yield. Fungicide application reduced tar spot severity but did not affect dry yield and forage nutritive value. Overall, our study shows that tar spot reduces forage yield and nutritive value and requires an integrated approach to disease management.