Current Situation in North Carolina

Corn is currently at the tasseling and silking stages across the state, which is typically the time when fungicides are applied to manage foliar diseases. Gray leaf spot, southern corn leaf blight, and holcus spot are the most commonly observed diseases. In most cases, these occur at low severity and/or have minimal impact on yield. Nevertheless, there have been numerous questions about whether a fungicide application provides a positive return in North Carolina.

Scouting Recommendations

Early to mid-July is a critical period to scout corn fields for foliar disease. Inspect plants for signs of gray leaf spot, southern corn leaf blight, or other major foliar diseases on leaves at or above the third leaf below the ear on at least 50% of sampled plants. A standard scouting approach includes 10 plants per location across 10 random field spots, equating to 100 plants total.

Figure 1. Early gray leaf spot symptoms

Knowledge of the hybrid’s disease resistance is essential, as susceptibility influences both disease progression and potential yield loss. Fields with a history of continuous corn, no-till practices, or high levels of residue are at greater risk due to increased pathogen survival.

Economic Considerations

A comprehensive analysis published by Kiersten Wise (Purdue University) and Daren Mueller (Iowa State University), Are Fungicides No Longer Just for Fungi?, reviewed data from 39 high-quality corn fungicide trials across the Corn Belt between 2000 and 2010. In this study, fungicide applications resulted in a statistically significant positive yield response in only 46% of trials. When statistical significance was not required, 80% of treatments showed a positive yield response; however, a positive yield response does not necessarily equate to profitability. In 2009 and 2010, a yield increase of 6 bushels per acre was needed to cover the cost of a single fungicide application made between the V15 and R2 growth stages. This breakeven threshold was met in only 45% of the trials evaluated.

More recent data from a multi-year analysis of over 400 on-farm trials conducted across the United States and Ontario, Canada, and published by the Crop Protection Network (Impact of Foliar Fungicide Timing and Fungicide Class on Corn Yield Response) showed that fungicide timing and class significantly influence yield response. Applications at tasseling (VT) consistently outperformed those made at early vegetative stages (V6), with yield gains averaging 3.3 bu/A for QoI-only products and 7.2 bu/A for DMI + QoI combinations. A two-pass strategy (V6 + VT) produced the highest yield increase, about 8 bu/A, but the added cost rarely resulted in a profitable return. Most importantly, these trials confirmed that return on investment was only observed when foliar disease was present. In fields with low disease severity (less than 5% leaf area affected), yield gains were minimal and rarely justified the cost of application.

To evaluate fungicide decisions based on your field-specific economics, the Crop Protection Network provides a Corn Fungicide ROI Calculator, which allows you to compare product and application costs against anticipated yield benefit:
https://cropprotectionnetwork.org/fungicide-roi-calculator

Timing and Fungicide Activity

Fungicides are most effective when applied between the V10 and VT growth stages, especially when environmental conditions favor disease development. Most fungicide products offer protection for 14 to 21 days. Since corn typically takes about 60 days from silking to reach physiological maturity, applications made too early may fail to protect the crop during the critical grain fill period.

The Crop Protection Network’s multi-year analysis reinforces this recommendation. Applications at tasseling (VT) produced significantly greater yield responses than those made earlier, supporting the guidance to time applications based on growth stage and disease presence. 

Southern rust is one disease that may warrant fungicide use if it appears early before the R5 (dent) stage. While it can be severe, it has typically developed late in the season and at low severity in recent years, with limited impact on yield. However, late-planted corn, late-maturing hybrids, and irrigated fields are at greater risk. 

Though foliar diseases can develop late in the season, their impact is typically negligible if symptoms arise after R5 (dent). Some fields treated with QoI fungicides (FRAC Code 11) may stay green longer, but once corn reaches black layer, grain fill has concluded and additional greening does not contribute to yield.

If you are managing a field with moderate to high pressure from gray leaf spot, southern corn leaf blight, or rust, consult the 2025 Corn Foliar Fungicide Efficacy Guide. This nationally coordinated publication includes efficacy ratings across the U.S., including data from North Carolina:
https://cropprotectionnetwork.org/publications/fungicide-efficacy-for-control-of-corn-diseases

Key Takeaways

• Only apply fungicides when disease risk is present or early symptoms have been observed.
• Scout fields during early tassel and assess hybrid susceptibility, field history, and tillage practices.
• Consider commodity prices and product costs before making fungicide decisions.
• Use the CPN ROI Calculator to guide field-specific return expectations.
• Time applications between V10 and VT to protect the upper canopy during grain fill.
• Use the 2025 Corn Foliar Fungicide Efficacy Guide to guide fungicide selection for your respective disease concern. 
• Use fungicides as one component of an integrated disease management strategy.

Additional Resources and Supporting State Extension Articles

Numerous university-based extension programs have published guidance on foliar fungicide use in corn:

• Michigan State University – Foliar Fungicide Use in Corn
• University of Kentucky – Fungicide Use in Corn and Soybeans (PDF)
• Iowa State University – Applying Fungicides to Corn: Stop, Look, Consider
• South Dakota State University – Is Fungicide Application at Tasseling Profitable?
• American Phytopathological Society (APS) – Fungicides in Corn Production
• Crop Protection Network – Impact of Fungicide Timing and Class