The Effect of Spraying Fungicides at R1 or R3 on Soybean

July 14, 2011
ICM News

By Nathan Bestor, Daren Mueller and Alison Robertson, Department of Plant Pathology and Microbiology

Across Iowa many growers are starting to ask if or when to apply fungicide on soybeans in 2011. There are essentially two options being discussed. The first is to apply fungicides at R1 (flowering). This application could be tank-mixed with the last application of glyphosate. The second option is to apply fungicide at R3 during pod formation. This application could be tank-mixed with an insecticide or applied alone. Which application timing is best? Which products best protect yield?

To answer these questions we conducted soybean fungicide trials at various locations in Iowa from 2008-2010. We evaluated the yield response of soybeans to applications of a strobilurin (Headline ®), a triazole (Flusilazole or Domark ®) and a premix of strobilurin + triazole (Stratego YLD ®). We also assessed foliar disease severity at R5. Finally we calculated the probabilities that an aerial application of a fungicide product would "break even" at a specific site year when prices of beans were estimated at $10, $13, and $16 per bushel. The cost of fungicides was estimated using information gathered from area cooperatives and field agronomists.

Our results are summarized in Table 1. 

  • In all years, disease severity was low. 
  • Brown spot was the most common disease and was found virtually only in the lower canopy. 
  • The yield response of soybeans was greater with an R3 application rather than an R1 application. So, an R3 application of a fungicide was more likely to break even than an R1 application
  • The chance that an R3 application of a fungicide would break even ranged from 7 to 99 percent.
  • Products that contained a strobilurin as one of the active ingredients were more likely to  break even compared to a product containing only a triazole. 

Spray when disease could impact yield

Although these data suggest that nearly 60-70 percent of R3 applications of a fungicide containing a strobilurin at least break even, spraying when disease is present will further increase your chances of getting your money back. If brown spot remains in the lowest third of the soybean canopy — it probably will not impact yield. If the disease moves into the middle third of the canopy, brown spot can start to reduce yield. We saw this at one of our locations in 2010. At that location, the mean yield response due to a fungicide application at R3 was nearly double the responses at other locations (5-6 bu/ac vs 2-4 bu/ac).  

Resistance to fungicides reported in 2010

A caution when spraying fungicides — scout fields after applying the fungicides and look for applications that do not appear to affect disease severity. In 2010, resistance to the strobilurin fungicides (Headline ®, Quadris ®, one of two active ingredients in Stratego YLD ® and Quilt ®) in the fungus that causes frogeye leaf spot was reported in Tennessee, Kentucky and Illinois [University of Illinois].

Funding for this project was provided by the Iowa Soybean Association, Bayer CropScience, DuPont, Valent and BASF.

Nathan Bestor is a graduate assistant in the Department of Plant Pathology and Microbiology. He can be reached at 515-294-1741 or Daren Mueller is an extension specialist with responsibilities in the Corn and Soybean Initiative and ISU's IPM program. Mueller can be reached at 515- 460-8000 or by email at Alison Robertson is an associate professor in the Department of Plant Pathology and Microbiology with extension and research responsibilities; contact at or phone 515-294-6708.

Links to this article are strongly encouraged, and this article may be republished without further permission if published as written and if credit is given to the author, Integrated Crop Management News, and Iowa State University Extension and Outreach. If this article is to be used in any other manner, permission from the author is required. This article was originally published on July 14, 2011. The information contained within may not be the most current and accurate depending on when it is accessed.


Daren Mueller Professor

Daren Mueller is an associate professor and extension plant pathologist at Iowa State University. He is also the coordinator of the Iowa State Integrated Pest Management (IPM) program. Daren received his bachelor's degree from the University of Wisconsin-Madison in 1996, and his master's degree a...

Alison Robertson Professor of Plant Pathology and Microbiology

Dr. Alison Robertson is a professor of plant pathology and microbiology. She provides extension education on the diagnosis and management of corn and soybean diseases. Her research interests include Pythium seedling disease of corn and soybean and Goss's wilt. Dr. Robertson received her bach...