Soybean growers and their advisors are faced with a bewildering array of information, from many sources, about products and practices that might increase yield and profits. Management of soybean diseases is no exception.

Foliar fungicides are valuable soybean disease management tools, says Bruce Potter. However, when applied in the absence of controllable, yield-limiting plant disease, they are typically not a profitable input. 

The following discussion is not intended to cover early season seed, seedling and root pathogens or Sclerotinia white mold which will be discussed in a subsequent article. 

Each growing season, we receive questions on the benefits of foliar fungicide applications to soybean and whether a pesticide treatment will pay, says the University of Minnesota ICM specialist. Although these questions may seem simple, the correct answers often are not.

Some of the reasons that interpreting and assigning economics to disease management research results include:

• Yield impacts from fungicide applications can change as fungal pathogen populations and disease development vary due to differences in environmental conditions, soybean genetics, and pathogen susceptibility to fungicide treatments.
• Are any yield differences enough to pay for the cost of the fungicide and application? Soybean prices are not constant. Fungicide costs range from approximately $9.00 to $19.00 / acre. Application costs also vary widely among ground and air applications. 
• If yield increases occur following a fungicide application, are they due to reductions in plant disease or another factor?
• Finally, “just in case” insurance applications of fungicide can have hidden costs. When plant pathogenic fungi with genetic resistance to a fungicide are selected for and become common, the fungicide(s) become ineffective and production costs can increase. Fungicide applications have some potential to negatively impact biological control of soybean aphids and other arthropod pests. 

Studies we are doing to answer these questions

Standardized studies are being done over a wide geography and over a number of years to provide soybean farmers and their advisors with data to better understand the odds for a return on fungicide investments. This Minnesota Soybean Research and Promotion Council funded project hopes to add to this knowledge base for Minnesota soybean producers.

Study sites were located at University of Minnesota Research and Outreach Centers at Lamberton, Waseca, and Rosemount in 2016-17 with Morris being added in 2017 and Crookston in 2018.

This project looks at several of the factors involved in the economics of foliar fungicide applications: 

• Which diseases are present?
• Historically, which soybean diseases are reducing the profitability of Minnesota soybean growers and where?
• Can yield increases from fungicide applications be detected and if so, are they economical?
• Are there factors that can help predict a profitable fungicide application in Minnesota? 

By planting varieties susceptible to specific pests at each location, these studies examined the prevalence of soybean diseases as well as some nematode and insect pests over a range of Minnesota environments. 

For three soybean varieties at each location, we compared yields without a fungicide treatment to yields of the same variety treated with two types of foliar fungicide at the R3 stage.

The fungicides used were: a strobilurin fungicide [Headline (BASF)] in 2016-17, a strobilurin + SDHI mix [Priaxor (BASF)] in 2016, a strobilurin + triazole mix [Stratego YLD (Bayer Crop Science) in 2016, and Delaro (Bayer Crop Science)] in 2017. 

This study is intended to indicate whether or not a yield response was obtained at a study site and why the response may have occurred. This study is not meant to be a comparison of all labeled fungicides or the probability of a fungicide response on numerous soybean genetic lines. 

What have we found so far?

There were yield differences among soybean varieties (data not shown). However, varieties responded similarly to fungicide at each site and in both years.

• Yield results and projected benefits comparing fungicide treated- and untreated-plots averaged across varieties for each site-year results are shown in Table 1. Economic values were not calculated when fungicide treated yields were not statistically different. In other words, yields were too variable to determine if treatment means were actually different from each other. These values are labeled as No Significant Yield Benefit (NSYB). Yield responses were generally positive where fungicides were applied in the three 2016 study sites but only at one site in 2017. The fungicide mix provided a significant, positive yield effect more often than strobilurin alone in both 2016 and 2017.
• These first two years of studies suggest that foliar fungicides can maintain soybean yield in some environments and provide an economic benefit when used selectively. However, these data also suggest that soybean farmers should not count on insurance foliar fungicide to overcome weaknesses in variety selection and other agronomic decisions for high yield. 
• Disease and insect pressure varies by year and location (data not shown). In time, data like these may lead to a better understanding of preventable soybean yield loss.
• The results also suggest that further work might focus on separating disease control associated yield differences from potential soybean physiology or harvestability differences produced by a fungicide. 
• The disease and yield data also pose questions on the interactions of multiple diseases, some of which are controlled by fungicides and some not, (e.g. brown stem rot) on the efficacy of foliar fungicide and the predictability of yield benefits.

Table 1. Soybean yield response and projected return for fungicide-treated plots compared to untreated plots during 2016-17 at several soybean prices. The gray shaded boxes indicate a fungicide application that produced No Significant Yield Benefit (NSYB).

What will happen in 2018? 

Results from the 2018 locations will be analyzed and combined with previous year’s results. The data will be examined for patterns in disease, weather, and agronomic practices that might help predict soybean yield responses from foliar fungicide applications.

What about white mold? 

This article primarily refers to soybean diseases other than white mold. In 2017, yield-reducing levels of Sclerotinia white mold occurred some Minnesota soybean fields. As a result, we are receiving many questions on fungicide applications for white mold control.

Few of the study locations used for this study had detectable white mold and fungicide control of white mold requires different products and application timings than used in these studies. Results from additional MN studies, placed in environments with white mold, will be discussed elsewhere.

What about foliar fungicides on corn?

A similar study, funded by the Minnesota Corn Research and Promotion Council, was conducted at the Lamberton, Waseca, and Rosemount locations in 2017. We did not observe yield differences for post-tassel fungicide applications in these three environments.

The corn studies will include Morris and Crookston locations in 2018.