Will southern rust be a problem on corn in 2026?

Southern rust is a leaf disease of corn that is caused by the fungal pathogen, Puccinia polysora. The disease is recognized as clusters of small, circular (1/16” diameter) spots that produce thousands of bright orange urediniospores on the upper side of the leaf (Fig. 1). With southern rust, there are multiple disease cycles during the growing season. As the crop matures, the pathogen switches from producing orange spores and starts to produce black teliospores, which cannot infect corn.

Figure 1. Numerous small spots producing thousands of orange spores on the upperside of corn leaves is characteristic of southern rust infection.

Puccinia polysora urediniospores cannot survive on corn residue or in soil. Every growing season, spores are blown up to Iowa from Central America via the southern states. Warm (77-85^oF), very wet conditions favor infection and disease development. As temperature and leaf wetness duration increase, so does southern rust severity (Hollier and King, 1985). Hybrids vary in their resistance to P. polysora, but since southern rust is rarely a problem in Iowa, seed companies may not have published resistance ratings.

What factors contributed to the 2025 southern rust epidemic?

Southern rust is usually observed every growing season in Iowa, but it rarely causes yield loss. In 2025, however, historic levels of southern rust across the state contributed to reduced yields. Factors that contributed to the epidemic include: 

1. More and stronger southerly winds through June and July carried spores of P. polysora to Iowa.
2. Greater levels of inoculum arriving in Iowa because, 
   1. corn acreage in the south increased considerably in 2025 compared to 2024. For example, corn acreage in Louisiana increased by 75%, Arkansas by 42% and Missouri by 10%. 
   2. many farmers in the south chose not to spray their corn because of low corn prices.
3. Earlier than normal observation of the disease (Mid-July compared to end of July through mid-August in other years). Southern rust was also observed in mid-July in 2024, but weather conditions were less desirable for spread.
4. Very wet conditions in July through mid-August. Precipitation across much of Iowa over this period was two to three times more than 2024.
5. Warmer than normal conditions, particularly with regards to daily minimum temperatures. Mean daily high and low air temperatures in July at the ISU Ag Engineering and Agronomy Farm were greater than average for 21 days in 2025 compared to 15 days in 2024.
6. Corn leaves were wetter for 1 to 3 hours longer each day (12.5 to 14 hours) in 2025 compared to 2024 (10.5 to 11.5 hours).
7. Many hybrids were very susceptible to P. polysora infection.

Looking ahead to 2026

Since southern rust cannot survive Iowa winters, the 2026 growing season starts with a clean slate. For southern rust to be of concern in 2026, several factors will need to come into play. Many of those factors are listed above. In my experience, the importance of the environment in disease development is considerably underestimated. While increased corn acreage in the south will provide greater amounts of inoculum, that inoculum needs to reach Iowa (southerly winds) and once it arrives, extended leaf wetness (greater than 12 hours) and warm (greater than 77 F) conditions are required for successful infection and disease development. In 2026, we will be keeping an eye on southern rust development in the south (https://cropprotectionnetwork.org/maps/southern-corn-rust), and on prevalent weather conditions in July and August. Southern rust can be managed with foliar fungicides; read more about statewide foliar fungicide evaluations in this recent blog. Applications at silking usually reduce disease the best.