With corn tassels emerging across the state, there are more questions about foliar fungicide use in corn. Foliar fungicides are valuable tools for management of some important diseases of corn. For over a decade, fungicide use in corn has become increasingly popular for many farmers for a number of reasons, including physiological effects they may provide. But, it becomes a more difficult decision when corn prices are low and some producers are looking for ways to reduce input costs when profit margins are narrow. Here are some considerations to keep in mind and more resources.

Dr. Kiersten Wise, Plant Pathologist at the University of Kentucky led team members to analyze and summarize the results of fungicide trials conducted on dent corn by University plant pathologists across 13 U.S. states (including ours in Nebraska) and Ontario, Canada. This was a daunting task considering the diversity of conditions across all of the locations and testing years. They used contemporary statistical analyses to synthesize and understand trends in the data set of yield results from fungicides applied at various crop stages during 2014-2015 as predictors of whether applications may be profitable. Disease levels in the non-treated controls were reported as <5% or ≥5%. The results were recently published in an open access, peer-reviewed journal article, Meta-analysis of yield response of foliar fungicide-treated hybrid corn in the United States and Ontario, Canada.

Applications of fungicides with active ingredients representing 1 or more of the 3 major fungicide classes (QoI, DMI, and SDHI) currently used in corn were made at V6, VT, or both. A brief summary of some of the main conclusions of those analyses are presented here as probabilities of a return on investment:

• About 68.5% of treatments increased yield (1-70 bushels/acre) compared to the non-treated controls. Increases were not always economical considering corn market values and estimated product plus application costs.
• Application of QoI + DMI fungicides applied at VT (full tassel emergence) had the highest probability of return on investment.
  • When corn prices were at $3.00/bushel and fungicide costs were $20/acre, the probability of return on investment was about 50% across all locations, disease severity levels, and other factors when applying QoI + DMI fungicides at VT.
• Applications of QoI + DMI fungicides at V6 for about $14/acre resulted in estimated probability of return on investment of about 40%. Application of a QoI fungicide alone at V6 for $12/acre resulted in estimated probability of return on investment <20%.
• Disease severity of ≥5% leaf area covered by lesions by R4 (dough)-R5 (dent) increased the probability of return on investment from an earlier fungicide applications at VT. The most common and severe disease in most of these trial locations was gray leaf spot.

Return on investment may be impacted by additional factors and effects of fungicides making it impossible to accurately predict. Circumstances that increase disease severity are especially more likely to make fungicide applications profitable. Consider the following factors that increase your chance of a return on investment of a fungicide application in your own field(s).

• Diseases - especially gray leaf spot and others that can be managed with a fungicide
  • History severe disease in that field(s)
  • Enhanced risk of disease development based on favorable weather forecast (usually wet or high relative humidity)
  • Susceptible hybrid vulnerable to disease(s)
  • Cropping rotation that includes 2 or more consecutive years of corn production
  • Minimum tillage maintaining more infected crop debris from previous seasons
  • Irrigation
• Crop
  • Value – return on investment may be more likely on higher value crops, such as in seed corn, etc.
  • Crop stage

Many additional factors exist, including corn hybrid. Some hybrids appear to react better to fungicide applications than others. One of the physiological factors that have been reported by some growers and observed in some fungicide trials conducted at the University of Nebraska-Lincoln and some other universities, is reduced incidence of stalk lodging, more popularly referred to “increased standability.” We’ve observed this trend in some corn hybrids historically, including some years when disease severity was low, such as in 2012. Results from experiments conducted in Iowa by plant pathologist Alison Robertson and team in 2016 and 2017, showed an average reduction in lodging of 9.3% and increase in corn yield of 4.1 bushels/acre when fungicide was applied at R1.

Currently in Nebraska, we’ve reported southern rust development at a very low level in southeast Nebraska. And, we’re monitoring low level development of gray leaf spot and northern corn leaf blight. Most fields do not immediately need a fungicide application for disease management purposes. Most fungicides provide protection for an average of 21-28 days. Applications made too early may mean their protection has worn off before diseases reach critical levels. We recommend using reports of disease to more vigorously scout and monitor disease levels in your own fields, so that if or when disease increases to a critical level a fungicide application can be made in a timely manner. Some research from our University of Nebraska-Lincoln fungicide experiments indicate that applications delayed up to dough stage did not reduce yields in most years.

Fungicides are diverse pesticides belonging to numerous classes with many functions. A good description of those functions is summarized by Damon Smith, University of Wisconsin in his Field Crops Fungicide Information site. Accurately predicting the outcomes of fungicide applications is difficult to impossible. We generally do not recommend the use of pesticides, including fungicides, for purposes other than pest management to reduce potential risks, such as unintended exposure, increased risk of resistance in pests, and others. Fungicide ratings for diseases of corn are provided by the Corn Disease Working Group in the Crop Protection Network publication: Fungicide Efficacy for Control of Corn Diseases. The suggestions above and resources are intended to provide some guidance and considerations to help producers make informed decisions with the goal of responsible, effective pesticide use and hopes of increased profits.

Additional Resources

Meta-analysis of yield response of foliar fungicide-treated hybrid corn in the United States and Ontario, Canada. (K. Wise, et al., PLOS One.)

Effect of Foliar Fungicides Applied at Silking on Stalk Lodging in Corn (A. Robertson et al., Plant Health Progress)

Field Crops Fungicide Information (D. Smith, University of Wisconsin)

Fungicide Efficacy for Control of Corn Diseases (Crop Protection Network resource)

Corn: Fungicide Efficacy for Control of Foliar Diseases section of the 2020 Guide for Weed, Disease and Insect Management in Nebraska