Beans close up

Editor’s note: The following was written for the Crop Protection Network website, a collaboration of land grant universities, by Daren Mueller, Alison Robertson, Ethan Stoetzer, Kiersten Wise, Carl Bradley, Albert Tenuta, Marty Chilvers, Damon Smith, Shawn Conley, Emma Matcham, Laura Lindsay, Nathan Kleczewski, and Darcy Telenko.


A record number of corn and soybean acres were planted late in 2019 and have reduced yield potential. Weather conditions have also been favorable for foliar disease development.

In a typical year, weather and disease risk factors would be enough to determine if a fungicide is necessary; however, the lower yield potential in late-planted crops across much of the U.S. has complicated this decision-making process. Many farmers are now asking if fungicide application(s) will provide an adequate return on investment.

The answer to this question will be crop, region, state and field specific. However, there are guidelines that can aid in determining if a fungicide application will be a sound investment in 2019.

How much yield is there to protect?

Knowing plant populations in a given field is critical for estimating how much yield a fungicide can protect. Soybean yield potential is less affected by planting date because soybeans are photoperiod sensitive, meaning that even if planting occurred across several days or weeks, the plant adapts based on day length.

It is still possible to achieve 90% of full yield potential if soybeans were planted by

June 1. However, planting past June 15 limits yield potential to approximately 80%. Iowa State University Extension has developed a Soybean Planting Decision Tool that evaluates planting dates and yield potential for Iowa, Missouri and Minnesota at https://bit.ly/2XMQlDE.

Will disease be severe enough to warrant a fungicide application?

To determine if diseases will be present and/or severe, we need to understand the disease triangle. This concept is the foundation for understanding how plant diseases develop and how to manage them.

In order for a plant disease to occur, a pathogen capable of infecting a given host must be present, a susceptible host plant, and favorable weather conditions must occur at the same time. If any one of these three components are missing (or we implement a management strategy that removes or reduces one or more components) then a plant disease will not occur, or occur at such low levels that yield is not affected.

For example, if you planted a corn hybrid or soybean variety that is resistant to foliar diseases like gray leaf spot in corn or frogeye leaf spot in soybean, it is less likely that you will see an economic benefit from a fungicide application. Also, if weather conditions change as the crop develops and become less favorable for disease development, disease progress may slow, reducing the need for a fungicide application.

What stage is the crop when disease is observed?

Disease development prior to grain fill will have a greater impact on yield reduction than when disease develops later in the cropping season.

In a typical year, crops are usually at a more mature stage of development when diseases become prevalent, and thus there is less impact on yield. This year however, with delayed planting conditions and reduced growing degree day accumulation, corn and soybean development in many areas is two to four weeks behind normal.

Disease development prior to or during early grain fill could significantly affect yield, thus scouting fields for yield limiting diseases prior to reproductive stages will be crucial in determining whether or not to apply a fungicide.

Diseases like gray leaf spot and northern corn leaf blight (NCLB) have already been confirmed on corn prior to tassel in some areas. Gray leaf spot is favored by warm temperatures, high humidity and heavy dews. NCLB is favored by cool temperatures and wet leaves.

Both of these diseases typically start in the lower canopy, and if favorable conditions continue, they can quickly spread to the upper canopy and be very damaging. University research indicates that fungicide applications occurring at tasseling/silking (VT-R1) are most effective at minimizing the impact of gray leaf spot and NCLB and protecting yield in susceptible hybrids.

Southern rust could also be problematic in late-planted corn in 2019. This disease moves northward each year and may impact yields in late-planted corn. Fungicide applications prior to milk stage (R3) may be needed to limit yield impact in certain cases.

Frogeye leaf spot and Septoria brown spot are two foliar diseases in soybeans that are prevalent in wet and humid weather. Septoria brown spot generally is not an economic threat to soybean, but may cause yield reductions if infection reaches the upper canopy.

It is important to begin scouting soybean fields for frogeye leaf spot and Septoria brown spot around beginning flower (R1) to help make a foliar fungicide decision. Generally, fungicide applications for management of frogeye leaf spot on susceptible varieties are made at beginning pod (R3).

It is not likely that foliar fungicide applications to soybean prior to reproductive stages will be economical. If using foliar fungicides to manage these diseases, use products that contain multiple fungicide classes, as resistance to strobilurin (quinone outside inhibitor) fungicides has been observed in multiple states by both the frogeye leaf spot and the Septoria brown spot fungi.

What is the frost risk for late-planted crops?

One last point to consider is the likelihood for late-season frost to impact yield in 2019. Late planted crops may not have completed grain fill or mature fully, thus would be at risk for yield loss due to frost. If corn has not achieved enough growth degree units before frost, this greatly decreases quality of grain.

The dense soybean canopy reduces frost damage by holding heat, unlike open (less developed) canopies. Late planting can reduce canopy coverage. In addition, late-planted soybeans may mature slower, thus be more susceptible to frost damage. Yield can be greatly reduced if frost occurs before soybeans are in the R8 stage.

Should I apply a fungicide?

Application of a fungicide is most effective on hybrids/varieties that are rated susceptible or moderately susceptible to diseases of economic importance. So, when evaluating whether or not to apply a fungicide, first check hybrid/variety susceptibility ratings to foliar diseases.

With this information in mind, scout fields for disease, ideally before plants enter reproductive growth stages, and determine the incidence and severity of disease. After scouting, evaluate how much yield potential you are likely to protect, based off of your planting date. Compare the cost of the fungicide application with the estimated price of grain per bushel, per acre of diseased crop.

The formula for calculating how much yield needs to be protected to break even at a given application cost is: Yield protected (bu/A) = application cost ($/A) / crop price ($/bu).

If the rate of return is positive after this calculation, keep in mind the potential yield lost in response to a late season frost, then compare the costs. After all of these calculations, you should have a good idea at the rate of return, should you apply a foliar fungicide.