If you’ve been checking the Crop Outlook Map you will have noticed that southern rust was confirmed in Benton, Mahaska, Muscatine and Louisa Counties on 21 July 2026. In all locations, the disease was found on a few plants at low severity (few pustules per leaf). Similarly, in 2025 and 2024, southern rust was also reported in Iowa in mid-July. In 2024, we estimated 4% yield loss in Iowa due to southern rust, while in 2025, yield losses were estimated at 7.5%.
What might we expect for southern rust in 2026?
The southern rust pathogen blows up to Iowa every growing season from the southern United States. Southern rust has been less prevalent in the south this year, meaning there is less inoculum being windblown to Iowa.
For southern rust to become established, leaf moisture is critical. In a greenhouse experiment, when corn leaves were wet for 8 hours, 18 southern rust pustules per square inch of leaf developed1. The number of pustules increased with the number of hours the leaves were wet: 12 hours of leaf wetness resulted in 72 pustules per square inch, while 16 hours of leaf wetness resulted in 146 pustules per square inch.
Dew points have been generally lower in 2026 compared to 2025 and consequently we have not been seeing heavy dews lasting into mid-morning like we did in 2025. Vapor pressure deficits during July 2025 were negative for much of the month, meaning the atmosphere was already saturated and dew could not evaporate from the leaves. However, so far this July, vapor pressure deficits have been mostly positive and only slightly negative, meaning the atmosphere has a greater capacity for moisture evaporating from the surface of leaves. Vapor pressure deficits from 2025 compared to 2026 suggest there was a greater potential for dew to sit on the leaves during July of 2025 compared to 2026. This suggests there may be less southern rust development in 2026 since corn leaves are not staying wet as long as they did in 2025. However, it remains to be seen what the next six weeks bring.
Managing southern rust
Fungicides may be used to reduce southern rust and protect yield. I reviewed making fungicide decisions in 2026 in a recent blog. I will reiterate that a fungicide application should be considered on fields planted to “race-horse” type hybrids (which tend to be more susceptible to disease) or hybrids that had severe southern rust in 2025. A fungicide application may not be necessary on more defensive hybrids. Consult the Crop Protection Network’s table on fungicide efficacy for corn diseases to see which fungicides performed best for southern rust management.
Other diseases being reported
In addition to southern rust, other diseases being reported are gray leaf spot, tar spot, northern corn leaf blight and Cuvularia leaf spot. A fungicide application at tassel through milk stage is usually effective at managing these diseases. Bacterial leaf streak and Goss’s wilt are also prevalent across Iowa, but fungicides are ineffective against these two bacterial diseases.
Unsure what diseases are stealing corn yields? Submit a sample to the ISU Plant & Insect Diagnostic Clinic. All information on sample submission guidelines is available on the PIDC website.
1 Hollier and King. 1985. Effect of dew period and temperature on infection of seedling maize plants by Puccinia polysora. Plant. Dis. 69:219-220
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- Professor of Plant Pathology and Microbiology
- Extension Field Crops Pathologist
- Director of Diagnostics & Plant Health Specialist
- Plant & Insect Diagnostic Clinic
- Weather and Climate Extension Specialist