By Andi Anderson
Southern rust has continued to spread across Iowa corn fields since its first detection in mid-July, increasing in severity due to persistent warm and wet conditions. July brought above-average rainfall and temperatures ideal for the disease’s development, and early August has followed the same pattern.
Southern rust thrives when temperatures are between 77–82°F with at least six hours of leaf wetness from dew or rain. Under these favorable conditions, the disease can progress from infection to visible pustules in just 7 to 10 days, making it one of the fastest-developing corn leaf diseases compared to tar spot or gray leaf spot. While cooler fall weather will slow its spread, current conditions remain conducive for rapid outbreaks.
Fungicide application timing is critical. Recommendations from the Crop Protection Network suggest applications through the R3 (milk) stage are most beneficial. Severe pressure—defined as visible disease on the ear leaf or upper canopy at around 1% severity—may justify treatment at the R4 (dough) stage. Since the ear leaf and upper canopy are crucial for grain fill, protecting them is key for maintaining yield potential.
For farmers who have already applied fungicide, a second application is typically unnecessary unless disease severity in the upper canopy is high and the crop is at a susceptible stage. The return on investment for a second treatment may be low given current grain prices. Fungicide efficacy varies by product, with newer formulations providing up to 35–42 days of protection, while older or generic options may last closer to 21 days.
Southern rust differs from other major corn diseases because it cannot overwinter in Iowa. Instead, spores travel north each summer on southerly winds from warmer regions. This means the risk resets each season, but favorable weather patterns can still trigger outbreaks.
By closely monitoring disease severity and crop stage, Iowa farmers can make informed decisions to manage southern rust and protect their yields during this year’s growing season.
Photo Credit: getty-images-elhenyo
Categories: Iowa, Rural Lifestyle, Farm Safety