Over the past few days, I have received several reports of southern rust from southwest and southeast Iowa. In addition, I and others have observed southern rust in central Iowa (Fig. 1). Incidence (number of plants affected) and severity (area of leaf diseased) is very low (Fig.2).
Fig. 1. Map showing counties where southern rust has been observed in Iowa (July 16, 2025). This map available at CPN is updated daily as reports are received.
Fig. 2. Symptoms of southern rust observed in Marshall County, Iowa. Note the cluster of small (1/16”), circular orange pustules breaking through the epidermis on the upper surface of the leaf (A), and no pustules visible on the underside of the leaf (B).
Southern rust is caused by Puccinia polysora. Unlike most corn pathogens, P. polysora, cannot survive in corn residue or soil. It survives on living plant tissue in Central America. Consequently, it needs to blow up to the Midwest every growing season. In 2024, southern rust was also observed in mid-July in Iowa. In some areas of the state, the disease became very severe and caused yield losses. Warm (80s), wet (high humidity and/or frequent precipitation/irrigation) conditions favor southern rust development.
Southern rust may be managed with a fungicide application. Be aware however, that products vary in their efficacy against southern rust. Check out the Fungicide Efficacy for Control of Corn Diseases publication from the Crop Protection Network (CPN). Applications are best made between tassel (VT) and milk stage (R3).
For more information on southern rust, CPN has the following helpful publication An Overview of Southern Corn Rust.
If you have observed southern rust in your county, please contact myself or your local extension field agronomist with photos and/or symptomatic leaf samples.
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- Professor of Plant Pathology and Microbiology
- Extension Field Crops Pathologist