A number of field trials and test plots have been conducted the last several years in Iowa looking at whether or not it pays to use foliar fungicides on soybeans. Iowa State University Extension plant pathologist Alison Robertson and her ISU colleague Daren Mueller who works with ISU's Corn and Soybean Initiative have looked at the results of these tests and have written the following summary. They summarized data from soybean fungicide small plot trials conducted by Iowa State University faculty and staff, and also the on-farm trials conducted by the Iowa Soybean Association On-Farm Network, the ISU Corn and Soybean Initiative and the ISU Northwest On-Farm research program.

Does it pay to use foliar fungicide on soybeans in Iowa?

• For the total 831 observations, the overall mean yield response was 2.18 bu/A. 
• Yield response of small plot trials (282 observations) was 1.67 bu/A, compared with 2.44 bu/A for on-farm strip trials (549 observations).  
• The mean yield response varied among years: 2.11 bu/A (for 8 observations in 2006), 4.42 bu/A (16 observations, 2007), 2.37 bu/A (599 observations, 2008) and 1.46 bu/A (208 observations, 2009).
• Applications at R2 and R3 growth stage of the soybean plant resulted in the highest mean yield response (2.32 bu/A and 2.42 bu/A, respectively).  The mean yield response for a fungicide application at R1 was 1.07 bu/A, while the mean yield response at R4 and R5 was <1 bu/A.
• The mean yield response was greatest for fungicides that contained a strobilurin, either alone (2.52 bu/A) or in a premix (2.13 bu/A).
• Disease ratings were not taken from all plots. Where noted, brown spot, downy mildew, Cercospora leaf blight and frogeye leaf spot were rated. The predominant disease was brown spot. Mean yield response was greater when disease severity in a field at R5 was >5 percent (1.79 bu/A) compared with disease severity <5 percent (0.68 bu/A).
• Based on the price of soybean of $9.48 and $24 product + application, the breakeven yield response is 2.53 bu/A.
 
Considerations for 2010: Should you apply fungicide this year?

"In our research, we have found that foliar disease severity in Iowa seldom reaches high enough levels to impact yield significantly," says Robertson. "The warm, wet start to the 2010 growing season, however, has been favorable for brown spot development in the lower canopy. If this weather continues (warm with frequent rains), we could see brown spot move up into the mid-canopy of soybean plants, and impact yield. Thus, a foliar fungicide application at R3 growth stage of the soybean plant could be a good decision."

Other things to consider before applying a fungicide include economics (e.g., the price of soybeans, and price of the fungicide product plus application cost). "You should scout your fields to determine disease pressure," advises Robertson. "Specifically, check on brown spot severity in the mid canopy. Frogeye leaf spot also may occur in the mid-canopy and Cercospora leaf blight in the top canopy."