Soybean and Foliar Pesticides in 2015

With soybean prices falling from historical highs—and more than a decade of research evaluating “protective” (just in case) applications of foliar fungicides and insecticides—what have we learned? Does this practice pay?

Article Summary

  • The more than 500 trials I’ve summarized from across the Central and Northern Corn-Belt suggests fungicide alone, or fungicide+insecticide applied as a protective application (just in case) increases soybean yields 2 and 4.1 bu/ac (Table 1).
  • With cash soybean prices at $9.78/bu, and the average cost/ac for fungicide and a foliar insecticide plus application costs at $32.40/ac, would provide a net return of $7.70/ac, assuming a 4.1 bu/ac yield increase.
    • Based on these assumptions, applying a fungicide+insecticide without considering disease or insect pressure would, on average, yield a net return. But using “something” to predict when we are more likely to see a response would increase the probability and size of the net return.
  • Using economic injury levels greatly increases the probability of net return. However, economic yield increases are sometimes observed when foliar diseases and insect injury are well below those thresholds, or perhaps even non-existent. What else can we do to “predict” when we are more likely to see a response in the absence of obvious disease?
  • Research conducted by the Iowa Soybean Associations On-Farm Network suggests growing seasons that are wetter than normal result in an increased probability of a yield increase associated with the use of foliar fungicides.
  • The 2015 spring and early summer across much of East-Central Iowa and North-Western Illinois is shaping up to be just that, wetter than normal.
    • This indicates foliar fungicides may more consistently increase soybean yields in these regions.
  • Insurance based applications increase the risk of foliar diseases developing resistance to fungicides. Using fungicides when foliar diseases are expected, avoiding multiple applications within a single season, and using fungicides with dual modes of action will reduce this risk.

Foliar fungicides & physiological response?

Foliar fungicides are promoted to increase “plant health,” but is this true? Well, sort of. However, exactly how and when is poorly understood, and irrational claims about protecting corn or soybean against environmental extremes—drought, frost, hail, floods, tornadoes, etcetera—are largely unproven.

We could discuss what is known, but instead let’s cut to the chase and say that we do sometimes see a yield increase in what appears to be the complete absence of foliar diseases. Is the yield increase associated with these plant health/physiological effects enough to consistently create a profitable yield increase? Probably not, but when combined with low levels of disease or insect pressure, perhaps these protective applications do, on average, result in a profitable net returns.

What does previous research tell us?

This conclusion arose from examining research conducted over a period of about the last decade by organizations from across the North-Central Corn-Belt (Table 1.) In this research, fungicide—or the combinations of a fungicide+insecticide—were applied as protective foliar applications. As in, just in case there were insects present or diseases developing.

What we can, in general, conclude from this large body of research is that, on average, we would expect this practice to pay at today’s economics. However, if we could somehow “predict” when yield increases are more likely, we could, in fact, increase the probability of a net return and the size of that return.

In wet growing seasons, foliar diseases are more likely

The soundest, but most time-consuming, method of predicting a net return is to use economic injury levels developed by university crop scientists.

For those less interested in intensive scouting, there is some evidence that relatively wet years will result in larger yield increases associated with foliar fungicides as opposed to relatively dry ones. These conclusions were drawn from a large on-farm study encompassing 282 trials across 5 years conducted by the Iowa Soybean Associations On-Farm Network. In this study, growing seasons with precipitation above the 30 year average resulted in yield increases 0.5-to-2 bu/ac greater than relatively dry years (Kyveryga et al., 2013).

These observations make some sense given many soybean foliar diseases require extended periods of leaf wetness for infection to occur. Moreover, wind and rain drop impact blow and splash fungal inoculum from the soil and decaying residue onto soybean stems and leaves.

Given this information—and that a large percentage of East-Central Iowa and North-Western Illinois has received well above normal precipitation over the last 60 day period (Figure 1)—we might expect foliar diseases to be more prevalent in these regions, particularly if wet warm weather continues.

2015—A probable year for an economic return

I have noticed an unusual amount of Septoria Brown Spot (Picture 1) so far this season. Some varieties show signs of Brown Spot toward the middle of the crop canopy on R3 soybean. In my experience, this is rare for late June. We almost always have some Septoria Brown Spot, but the incidence is usually low and confined to the lower canopy, rarely effecting yield.

I am not saying Brown Spot will cause wide-spread yield reduction or that foliar fungicides will guarantee a profitable yield increase in 2015, but I am saying that foliar fungicide+insecticide—based on the publicly available information and my educated assumptions—will likely be a profitable business decision in regions with above normal precipitation in 2015.

Fungal resistance & routine use of fungicides

While we believe there is an elevated probability of fungicides increasing soybean yields in 2015, it is also apparent from the information in Table 1 that this practice for the last several years has, on average, been profitable.

If foliar fungicides have been repeatedly used as a protective tool over the last several years, it is a good idea to keep in mind that this practice can result in foliar diseases becoming resistant to foliar fungicides.

To avoid resistance development

  • Use fungicides when we are more likely to expect foliar diseases.
  • Avoid multiple applications within a single season.
  • Use fungicides with multiple modes of action.
    • As an example, if Stratego YLD (FRAC group 3|11) has been used for several years, it would be appropriate to switch to Priaxor (FRAC group 7|11). Although Priaxor and Stratego YLD are both dual mode of action fungicides, they only share one of these modes of action.


Table 1.

A summary of yield increases associated with foliar fungicide and insecticide use on soybean from research conducted in the Central and Northern Corn-Belt.

Table 1


Accumulated Precipitation
Figure 1.

Departure from the 30 year average perception in 2015 for the period of May 1 to June 25.


Soybean Trifoliate Leaf

Picture 1.  

Soybean trifoliate leaves showing signs of Septoria Brown Spot in the lower canopy of R2 (full-flower) soybean.



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fungicide applications produce profitable soybean yield responses? Online. Plant Health
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Jeschke, M. and R. Ahlers. 2012. Foliar Fungicide and Insecticide Use In Soybeans. Crop Insights. DuPont Pioneer. .24E3FA11-09AE- 7198-2128-12A19F93B6A4 (accessed 26 June 2015).

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Bradley, C. 2014. Fungicides Resistance: Coming to a field near you?. 2014 On-Farm Network Conference. Ames, IA. 20 Feb.

Giesler, L. and T. Gustafson. 2012. Effect of Foliar Fungicides and Insecticides on Soybean Disease  Severity and Yield. University of Nebraska-Lincoln. (accessed 26 June 2015).

Muller, D., W. Pierson and S. Wiggs. 2013. Evaluation of Foliar Fungicides and Insecticides on Soybean in 2013. Iowa State University Extension. /0107muellerpiersonwiggs.htm (accessed 26 June 2015).