Nitrogen Management and Disease – Is There a Connection?

By Bob Gunzenhauser, Director of Agronomy

During the summer of 2025, the Midwestern Corn Belt was hit with both higher-than-average temperatures and precipitation, primarily in July. While this was welcome news to keep the corn growing well during this period of greater water usage and trans evaporation, it also brought Southern Rust (Puccinia polysora) spores from the Gulf northward. By mid-July, pustules from the rust were starting to develop on corn leaves, and by the first week of August agronomists and farmers were finding Southern Rust in about every corn field, even those that had fungicide sprayed previously.

First discovery of Southern Rust in 2025 (Top) and Later Development (Bottom)

In my scouting rounds for local fields in south-central Iowa, I found the prevalence of rust to be defined by A) fungicide application and B) variety, but also C) nitrogen rate and D) soil potassium levels. Certainly, if a field had a fungicide application, primarily in the first or second week of July, Southern Rust pressure was not apparent or very low in August. However, I started to see fields with developing or heavier Rust pressure based on variety and general soil potassium levels (the lower the worse).

I conducted a trial with nitrogen rates of 0 to 240 lbs N/ac applied as UAN-28% at V5 directed at the corn row. At a field day I held in late August, a guest noticed that there was much more Southern Rust pressure on the nearby 180 lbs N/ac buffer plot vs the 0 N/ac treatment plot next to it. Based on this I went out and took example ear leaves from 0, 60, 120, 180 and 240 N/ac plots for both varieties that were planted in the trial. While not exhaustive or quantifiable, it does appear that 0 and possibly 60 lbs N/ac treatments had lower incidence of Southern Rust than the higher rates.

Ear Leaves from Various Nitrogen Rates and Varieties, R5 growth stage

While I wouldn’t suggest to anyone to apply zero nitrogen to get away from diseases, the observation above makes one question the relationship between nitrogen and disease. This field did have a fungicide application at R4 growth stage, which slowed the advance of the disease but didn’t prevent it from occurring.

What are some aspects of nitrogen management that could have affected the prevalence of Southern Rust (or any other disease for that matter)? It has been well-studied that high nitrogen rates applied in a single application can cause fast and lush growth during vegetative stages, not just for corn but small grains, cotton and sunflowers. Through this fast growth, nutrient allocation may be affected; fields that are already lacking in nutrient availability, particularly potassium, may be negatively affected. Likewise, plants that grew through potassium deficiency during vegetative stages will attempt to remove the remaining K from the stalk to fulfill grain fill needs, thereby weakening the stalks even further. This can be measured through a whole-plant tissue analysis and examining the Nitrogen to Potassium (N:K) ratio. A ratio of around 1 is ideal; anything above 1.5 indicates either excessive nitrogen or lacking potassium.

Most nitrogen is taken up by the corn plant in the nitrate form; it can handle the ammonium form as well, but often any applied ammonium nitrogen fertilizer, as well as mineralized nitrogen from organic matter, will nitrify into nitrate by the time the plant needs it.

Applying nitrogen in multiple applications would seem to be a good approach to reducing over-application and lush vegetation development. Peak nitrogen requirements by the plant start around V8 and run through R2, decreasing towards physiological maturity – about 65% of the corn plant’s N uptake is before flowering. Applying some nitrogen at planting, more at or prior V6-8, and an additional amount during early grain fill would be ideal if operations allow for it. This in turn ensures that nitrogen is not in excess to the plant’s needs for too long and creates luxury consumption.

Fungicide application is and was certainly the standard practice to managing disease in corn, but applying a balanced amount of nitrogen, as well as ensuring soil potassium levels are maintained at optimum or higher levels, may reduce the need for fungicide. Likewise, selecting varieties that are resistant to diseases may also be part of the overall program. Fungicides work well, but if used excessively without proper guidance, can lead to disease resistance and reduction of effectiveness. Let’s ensure we keep fungicides as valuable tools in the management toolbox.

Applying the right nitrogen rates to ensure proper ROI as well as potentially reduce disease impacts requires the right guidance. Sentinel Ag delivers timely and accurate nitrogen recommendations through local Certified Service Providers, trained in the use of the software. Through this, you can be confident that your corn crop is receiving the right nitrogen rate to maximize returns and profitability.

Keys To Reducing Disease Impact

1. Apply the proper nitrogen fertilizer rates to ensure nitrogen is delivered when the crop needs it and not allow for excessive vegetative growth.

2. Maintain soil potassium and check nitrogen to potassium ratios (ideally 1:1).

3. Select varieties that are resistant to common diseases.

4. Utilize scouting and disease monitoring services to determine if problem diseases are heading your way, then plan proper fungicide application(s) as needed.