Deciding whether or not to apply foliar fungicides may not always be a simple decision. As corn and soybean fields reach reproductive growth stages, decisions will need to be made. While a foliar fungicide application might help protect the crop from lost yields, applying fungicides may not boost profits. Targeting fields that have higher yield potential is the best way to improve your chances of recouping additional costs from fungicides. 

Typically, weather conditions and disease risk factors can help determine if applying a fungicide is necessary. However, with tight profit margins in corn and soybean production this year, that decision can be complicated. 

To decide on whether or not to apply a fungicide, two questions should be asked. First, will a disease be severe enough to warrant an application? To determine if a disease will be severe enough or if it is present, we need to take a quick look at the disease triangle. The three parts of the disease triangle that must be considered are: pathogens present, susceptible host plants and favorable weather conditions. If any one of these components is absent or greatly reduced due to management strategies, then the disease will either be absent, or the severity will be low enough that yield is unaffected.  

History of disease pressure 

Start by looking at the past history of pathogens or disease pressure. Different cropping systems or crop rotations may greatly affect disease pressure. Next, look at the level of disease resistance or susceptibility rating of the corn hybrid or soybean variety. Seeing an economic benefit is less likely to occur if the corn hybrid or soybean variety has been shown to have strong resistance to foliar diseases such as gray leaf spot or frogeye leaf spot.  

Keep in mind if weather conditions change or become less favorable for disease development, then the pathogens that cause foliar diseases may not cause disease or disease progression may slow down. This can be especially true of diseases that start in the lower canopy, such as gray leaf spot. 

The second question that needs to be asked is, at what growth stage is the crop when the disease is observed? Disease development prior to grain fill has greater impacts on yield than when diseases develop during the late reproductive stages. Generally, if crops are past the grain fill period when diseases become more dominant, then impacts on yield are minimal. Conversely, if disease development occurs before or during early grain fill, yields may be significantly affected. For these reasons, scouting for early signs of diseases prior to plant reproductive stages is critical when deciding whether or not to apply fungicides. 

Make decisions field by field 

For those farmers who have decided to apply fungicides, some IPM approaches may be helpful when deciding which fields to spray. Targeting fields that have a more susceptible corn hybrid and have the presence of a disease prior to early grain fill should be a priority. Low-lying fields where morning fog occurs, or fields with a history of disease, should also be targeted first. Fields with the most disease present have been shown to have the greatest yield response to foliar fungicide applications. 

To limit the development of resistance and the dependency on fungicides, an integrated method of disease management should be practiced, and farmers should avoid spraying “just because.” Making so-called insurance applications for a crop in the absence of disease has potential consequences, not only from a cost perspective (use of additional inputs in corn or soybean production) but also from the effect on fungal populations. Applying fungicides where they aren’t needed may increase selection pressure that could lead to shifts in sensitivity of the fungal population. Overuse can lead to development of fungicide-resistant disease populations.  

Research-based results indicate that fungicide use in corn and soybeans is most profitable when there is high risk for foliar disease development and disease develops to levels that warrant management. For future longevity and profitability of fungicides in crops, the decision to apply a fungicide should be based on disease factors, and not based on presumed yield enhancements that might occur in the absence of disease.   

Fungicide applications with multiple modes of action can limit the development of fungicide resistance. Other disease management practices such as crop rotation, and planting disease-resistant corn hybrids or soybean varieties should also be used to minimize fungicide dependency. Lastly, scouting fields and keeping up to date with news regarding diseases occurring in your area can also aid in making informed decisions on the risk of diseases and the need for fungicide applications. 

For help in identifying and managing crop diseases, visit the ISU website for various publications. 

Michel is an ISU Extension field agronomist covering counties in southeast and south-central Iowa. Email is [email protected].