Statewide evaluation of foliar fungicides on corn in Iowa in 2025

In 2025, the effect of commercial fungicides on foliar diseases of corn were evaluated at six ISU Research and Demonstration Farms: Northwest Research Farm (NWRF), Northern Research Farm (NRF), Northeast Research Farm (NERF), Armstrong Memorial Research Farm (lost due to wind damage before tasseling), Ag Engineering and Agronomy Farm (AEA), and Southeast Research Farm (SERF). Fungicide products evaluated and timing of application are shown in Table 1. A generic product applied at silking or at silking + ~21 days later (milk stage) was included to compare return on investment with name-brand products that are more expensive. 

Southern rust was the most prevalent disease observed at all farms (Table 1), although tar spot, northern corn leaf blight, and bacterial leaf streak were also present at various locations (data not shown). Southern rust severity varied across locations and was least severe in central Iowa (7.8%) and most severe in southeast Iowa (50.0%).

Effect of fungicides on foliar disease. 

All fungicides reduced southern rust. Among name brand products, efficacy varied slightly and no one product was consistently better than other products across locations. Two applications of the generic product, Cover XL, consistently reduced southern rust the best, although the reduction in southern rust severity was not always significantly different from name brand products. One application of Cover XL was not as effective at reducing southern rust as two applications of Cover XL, but it was not less effective than some name brand products. 

Numbers followed by the same letter are not significantly different from each other using Tukey's Test.   *NWRF, ISU Northwest Research and Demonstration Farm; NRF, ISU Northern Research and Demonstration Farm; NERF, ISU Northeast Research and Demonstration Farm; AEA, ISU Ag Engineering and Agronomy Farm; SERF, ISU Southeast Research and Demonstration Farm.   ^# Southern rust severity on ear+2.  ^ n.d. Treatment not evaluated

Effect of fungicides on yield. 

Statistically greater yields with a fungicide application occurred at all locations except NRF (Table 2). No statistical differences among products applied at R1 was detected (P<0.001). Interestingly, a double application of Cover XL did not necessarily result in greater yields than a single application. Yields among name brand products did not differ from each other statistically.

Numbers followed by the same letter are not significantly different from each other using Tukey's test.  * NWRF, ISU Northwest Research and Demonstration Farm; NRF, ISU Northern Research and Demonstration Farm; NERF, ISU Northeast Research and Demonstration Farm; AEA, ISU Ag Engineering and Agronomy Farm; SERF, ISU Southeast Research and Demonstration Farm.  ^ n.d. Treatment not evaluated

Comparison of ROI. Return on investment (ROI) at each location was calculated for some products using product prices from the Corn Fungicide ROI Calculator and a grain price of $5.00/bushel. For Cover XL, product price ($5, 14 oz/acre) was received from ISU field agronomists and a farmer in northwest Iowa. Average ROI across all locations were: Cover XL 14 oz at R1, $71.90; Cover XL 10.5 oz + 10.5 oz at R1 + R3, $73.28; Veltyma 7 oz at R1, $60.39; Veltyma 7 oz + 7 oz at R1 + R3, $18.07; Miravis Neo 13.7 oz at R1, $54.45 and Delaro Complete 10 oz, $40.51. 

Management recommendations

The data support data from previous years and from surrounding states that the best time to apply a fungicide to reduce disease is at silking (R1). Moreover, depending on the product used, one application can be enough to effectively reduce disease through grain fill and thereby protect grain fill. The 2026 growing season starts with a clean slate, in regards to southern rust, since the pathogen cannot survive Iowa winters. However, other pathogens, such as those that cause tar spot, northern corn leaf blight and gray leaf spot, can overwinter and thus are always present in Iowa. Disease development will depend on weather conditions during the growing season and hybrid genetics.