Determining if a Soybean Foliar Fungicide Should be Applied?

May 12, 2021

Deciding whether to spray a soybean fungicide is an annual dilemma faced by soybean growers across the country. This decision can be difficult due to the many variables that can impact the profitability of a fungicide application. Some of these variables include:

  1. Soybean product response to a labeled fungicide

  2. Type of foliar disease and disease pressure

  3. Application cost versus soybean market price

  4. Potential yield.

Soybean Product and Fungicide Response

Each soybean product has disease tolerance or resistance ratings that can help determine the potential profitability of a fungicide application. These ratings can greatly improve the decision-making process when deciding how to properly manage in-season applications. When purchasing soybean products, be sure to note the disease resistance ratings for the major soybean diseases in your area. One key example would be the ratings for frogeye leaf spot, which can be very detrimental to yield potential. If a soybean product has poor resistance to frogeye leaf spot, and conditions favor its development, then a fungicide application has the potential to be profitable. Conversely, if the reason to apply a fungicide is solely for protecting the plants against frogeye leaf spot and the soybean product has excellent resistance or tolerance, the application may be unwarranted.

Some of the major soybean diseases that can be controlled by a foliar fungicide include Septoria leaf spot (brown spot), frogeye leaf spot, powdery mildew, Sclerotinia stem rot (white mold), soybean rust, and Cercospora leaf blight.

It is important to understand the degree of potential yield loss for each disease.

  • Septoria leaf spot is a disease that is observed most years to varying levels. If conditions are favorable, it can progress through the canopy and potentially cause 8 to 15% yield loss.1

  • Frogeye leaf spot is a problematic disease, especially in more humid regions that can cause yield loss as high as 30% in severe cases.2

  • Powdery mildew is less likely to cause yield loss but can still be problematic in susceptible products.

  • Sclerotinia stem rot or white mold is a major problem in northern climates and can cause a 2 to 5 bu/acre loss per 10% disease incidence.3

  • Cercospora leaf blight can impact yield potential negatively, especially in the southern United States and can cause additional financial loss from dockage due to purple seed stain which is caused by the same pathogen.

  • Soybean rust is a disease that can cause yield losses ranging from 10 to 80% in areas where the disease is well established.4

With these potential yield losses in mind, it is important to scout fields and determine the presence and pressure of the disease to aid in determining if a fungicide application is warranted. The future forecast is also a critical part of the decision-making process as certain weather conditions can favor disease development. Bayer Crop Science releases disease risk maps for specific geographies and these can be excellent tools to utilize when deciding whether the disease pressure present in your area warrants a fungicide application.

Application Costs / Disease Pressure

The economics of a fungicide application play a very important role in determining if an application should be made. Just because it makes sense agronomically, doesn’t always mean it makes sense economically. Most soybean fungicide application costs can run anywhere between $25.00 to $35.00 an acre (there are a lot of variables in the estimated cost, but these numbers are used for profitability projections).

The application cost and soybean market price go hand in hand. At the time this article is being written, the market price for fall soybean is $12.73/bu. If we use an average application cost of $30.00/acre and the current soybean market price is $12.73/bu, that means the fungicide application needs to produce an extra 2.4 bu/acre to breakeven. In drier years, where less disease pressure is observed and when a resistant product is planted, a fungicide may not breakeven. However, in years where disease pressure is higher, and susceptible products are planted, fungicides can often result in more than a 2.4 bu/acre advantage. When considering the potential yield loss to the diseases discussed in this article, it is very easy for many diseases to cause more than a 2.4 bu/acre yield loss.

Potential Soybean Yields

Potential soybean yield is one piece of the puzzle that becomes more difficult to quantitatively analyze as it is an estimate of future yield. When considering that disease-caused yield loss estimates are based on a percentage of yield, the number of bushels lost can vary. As an example, a 4% yield loss in a field with a potential yield of 85 bu/acre, would be a 3.4 bu/acre loss, whereas a 4% yield loss in a field with a potential yield of 55 bu/acre would only be 2.2 bu/acre. In the first case, based on the economic numbers mentioned, a fungicide application to the 85 bu/acre field would be very profitable whereas an application to the 55 bu/acre field would not. To summarize this point, if other factors like water or fertility are more limiting, then you are less likely to see a response from a fungicide application. If those other factors are all in check and yield potential is high, then the application is more likely to be profitable.

The Decision

If the decision is made to apply a fungicide, be sure to choose one with multiple effective modes of action as there are documented cases of fungicide resistance. As an example, frogeye leaf spot is resistant to the group 11 (strobilurins) fungicides. This means that applying a product that only contains a group 11 mode of action fungicide would not be effective at controlling the disease. Therefore, it is critical to always use fungicide products with multiple modes of action. It is also essential to maximize the application’s effectiveness by applying the fungicide at the right growth stage (Figure 1). For most diseases, this is R3 which is when soybean pods are 3/16th of an inch long at one of the four uppermost nodes on the plant. Ensuring these requirements are met can help increase the likelihood of a profitable fungicide application. In the case of white mold, applications need to be made at R1 growth stage to protect flowers, which are the primary tissue that the pathogen infects.

Talk with your local Channel® Technical Agronomist about the soybean disease management benefits of Bayer Crop Science Delaro® 325 SC Fungicide, Delaro® Complete Fungicide, and Stratego® YLD Fungicide.

Cody Hornaday

Technical Agronomist


1Smith, Damon. Septoria leaf spot (Brown spot). Wisconsin Field Crops Pathology. University of Wisconsin. https://fyi.extension.wisc.edu/.

2Giesler, L. (updated by Mane, A., Everhart, S., and Jackson-Ziems, T.) 2020. Frogeye leaf spot. CROPWATCH. University of Nebraska. https://cropwatch.unl.edu/.

3Esker, P. D. 2017. What to do if I have white mold in Soybean? PennState Extension. The Pennsylvania State University. https://extension.psu.edu/.

4Sweets, L.E., Wrather, J.A., and Wright, S. 2004. Soybean rust. G4442. MU Extension. University of Missouri. https://extension.missouri.edu/.

Websites verified 4/14/21.