Southern Rust Dominates Statewide Corn Disease Evaluations
In 2025, Iowa State University conducted statewide trials to evaluate commercial foliar fungicides for corn disease control. The studies were carried out at six research farms, including the Northwest, Northern, Northeast, Ag Engineering and Agronomy, and Southeast Research Farms. One location, Armstrong Memorial Research Farm, was lost due to wind damage before tasseling.
Researchers tested several fungicide products and compared application timings. Treatments included both name brand products and a lower cost generic product, Cover XL.
The generic product was applied either once at silking or twice at silking followed by an additional application about 21 days later at the milk stage. The goal was to compare disease control and potential return on investment between premium and lower cost options.
Southern rust was the most common disease observed across all research locations in 2025. Disease severity varied significantly by region. Central Iowa recorded the lowest levels at 7.8 percent severity, while southeast Iowa experienced the highest levels at 50 percent. Other diseases, including tar spot, northern corn leaf blight, and bacterial leaf streak, were also noted at some sites.
All fungicide treatments reduced southern rust compared to untreated plots. Among name brand products, effectiveness was similar overall, and no single product consistently outperformed the others at every location.
Two applications of the generic product Cover XL often provided the greatest reduction in southern rust severity. However, differences were not always statistically significant when compared to name brand options.
A single application of the generic product was less effective than two applications but performed similarly to some premium fungicides.
These results suggest that growers should consider disease pressure, product cost, and application timing when making management decisions.
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