Fungicide decisions in 2026

In the next week or so, corn fields across Iowa will start to tassel and the buzz of crop dusters will no doubt join the buzz of insects. Research has consistently shown that the best time to apply a fungicide to manage foliar diseases, like tar spot, southern rust, northern corn leaf blight and gray leaf spot, is at tassel (VT) through milk (R3) stage. But do you really need to spray a fungicide? Fungicides protect yield potential by reducing foliar disease within the canopy. With tight margins this year, here are some things to think about before you pull that trigger:

1. Consider disease risk during grain fill.
   1. As of 30 June 2026, tar spot has been reported in 10 counties in Iowa. This is not overly surprising since the Crop Protection Network (CPN) Crop Risk Tool, has shown high risk for tar spot development since the beginning of the growing season. Remember, tar spot is favored by extended periods (30 days) of mild temperatures (64-73°F). However, as temperatures warm up in July and August, tar spot development will considerably slow before increasing in September when temperatures cool down. 

   2. As of 30 June 2026, southern rust has been observed in 4 counties in Texas. On 29 June 2025, southern rust had been reported in several counties in Alabama, Arkansas, Florida, Georgia, Mississippi and Texas. Thus, there is considerably less southern rust inoculum in the south blowing north to Iowa, and consequently risk of southern rust in 2026 remains low in the foreseeable future.

       

2. Consider return on investment (ROI). Since greater yield responses occur when disease severity is high, a positive return on investment (ROI) on a fungicide application is more likely when risk of disease is high. Research from Dr. Darcy Telenko’s lab at Purdue University¹ reported when tar spot severity was high (>5% at dent (R5)), a positive ROI on a fungicide application occurred (average of $110 for several products applied at VT through R3), however when tar spot severity was low (<5% at R5), partial returns were negative (average of -$20 for several products applied at VT through R3). 

    

   The caveat here is what is the likelihood at VT (when a fungicide application is  recommended) that disease severity will be >5% at R5? The CPN Crop Risk Tool can be helpful in determining if tar spot or gray leaf spot risk is high. (Models for northern corn leaf blight and southern rust risk are in development.) The weather outlook can also be considered. Foliar diseases all require moisture (precipitation and/or high relative humidity to develop. Thus, if conditions during grain fill are likely to be dry, risk of disease development will be low. A Fungicide ROI Calculator is available from CPN. 

    

3. Consider fungicide product efficacy. Fungicides vary in their efficacy at reducing disease. The CPN publishes a fungicide efficacy table and has a fungicide efficacy tool which can help determine the best product to apply for a specific disease.

4. Consider hybrid susceptibility. In 2025, the Robertson Lab in collaboration with Wyffels, evaluated five fungicide application timings on two hybrids at three locations in Iowa for disease control. The two hybrids varied in their susceptibility to tar spot and southern rust. Severe disease was observed in 2025. Tar spot severity at Iowa State University (ISU) Muscatine Island Research and Demonstration Farm (MIRF) on the non-sprayed controls of each hybrid was 17% and 8% at R5, while at the ISU Northeast Research and Demonstration Farm, southern rust severity on the non-sprayed control of each hybrid was 39% and 18% at R5 for the susceptible and tolerant hybrid, respectively.  Southern rust was observed at MIRF but at very low severity (<2% on ear leaf at R5). Disease severity data were not collected at the ISU Northwest Research and Demonstration Farm. Better yield responses and consequently ROI occurred in the more susceptible hybrid (Table 1). These data suggest that a fungicide application is not necessary on defensive hybrids. 

Table 1. Yield responses and ROI of two hybrids to fungicide applications at different growth stages at three locations in Iowa in 2025. 

^a Veltyma (7oz/acre) was applied at silking (R1), milk (R3), dent (R5), at R1 and R3, and at R3 and R5

^b MIRF, Iowa State University (ISU) Muscatine Island Research and Demonstration Farm; NERF, ISU Northeast Research and Demonstration Farm; NWRF, ISU Northwest Research and Demonstration Farm

^c Susceptible, hybrid was susceptible to tar spot and southern rust; tolerant, hybrid was tolerant to tar spot and southern rust.

^d Yield response (bu/A) = Yield of treatment – yield of non-sprayed control

^e Return on investment ($) = (Yield response x $4/bu corn)-fungicide product cost. A product cost of $22.50 was used for a single application of fungicide, and $45.00 was used for a double application of fungicide.

5. Consider planting date. In planting date trials done at three locations in 2025, southern rust was more severe on corn planted in mid-late May compared to corn planted in mid-late April (Table 2). Moreover, southern rust severity at R5 on the late planted, fungicide applied at R1 treatment was more severe than the early planted, no fungicide applied treatment at two locations. In his Ph.D. research, Dr. Pierce Paul (The Ohio State University) reported late planted corn had greater gray leaf spot severity than early planted corn². These data suggest fields planted early in the growing season are less likely to develop severe foliar disease that may affect grain fill and consequently may not need a fungicide application.

Table 2. Mean southern rust severity on the ear leaf at R5 for corn planted in mid-late April and mid-late May and sprayed with a fungicide at R1 or not-sprayed with fungicide at three locations in Iowa in 2025. 

^a SERF, Iowa State University (ISU) Southeast Research and Demonstration Farm; AEA, ISU Ag Engineering and Agronomy Farm; NWRF, ISU Northwest Research and Demonstration Farm

Acknowledgement: Thanks to Rebecca Vittetoe and Meaghan Anderson for their constructive feedback on an earlier draft.

References

1. Ross, T.J., Allen, T.W., Shim, S., Thompson, N.M. and Telenko, D.E., 2024. Investigations into economic returns resulting from foliar fungicides and application timing on management of tar spot in Indiana hybrid corn. Plant Disease 108:461-472 https://apsjournals.apsnet.org/doi/full/10.1094/PDIS-05-23-0932-RE 
2. Paul, P.A. and Munkvold, G.P., 2004. A model-based approach to preplanting risk assessment for gray leaf spot of maize. Phytopathology 94: 1350-1357. https://apsjournals.apsnet.org/doi/abs/10.1094/PHYTO.2004.94.12.1350