Fungicide use on soybeans studied

Although plant health is commonly cited as a reason for applying fungicides to soybeans, ultimately a profitable increase in yield is necessary to justify the application.

For the past three years, MSU Extension educators in the Thumb and Saginaw Valley have studied the use of fungicides on soybeans. The same three fungicides – Headline, Stratego, and Quadris – along with an untreated control, were used each of three years. Soybeans were planted in 30-inch rows. Fungicides were sprayed at labeled rates at the R3 growth stage, or beginning pod, regardless of disease presence. These data represent 29 study replications over three years. See Table 1 for study results.

 Table 1. Three - Year Results of Soybean Fungicide Studies in the Thumb at R3 Growth Stage
Treatment 2008 Yield (Bu/A) 2009 Yield (Bu/A) 2010 Yield (Bu/A) Ave. Yield (Bu/A)
Control 56.2 47.8 47.4 50.5
Headline 57.8 52.5 46.2 52.2
Stratego 58.2 49.9 47.3 51.8
Quadris 59.4 49.2 44.6 51.1
CV 5.3 3.7 6.5
LSD (0.05) 3.0 1.9 2.5
Bolding indicates a statistically significant increase from the  lowest yielding treatment

Results show variable response to applied fungicides over the three years. In 2008, one fungicide, Quadris, showed a statistical increase in yield over the untreated control. In 2009, Headline and Stratego showed statistical increases in yield over the control. In 2010, no fungicide-treated soybeans yielded better than the control, and the Quadris treatment yielded statistically less than the control. These results can be summarized best as inconsistent.Digging a bit deeper, August rainfall may help explain some of the inconsistency in these results. Seasonal rainfall, especially rainfall in August, was variable during the three years this study was conducted. In the Thumb region, 2008 and 2009 were average to wet, and 2010 was dry. See Table 2 for month of August rainfall 2008-2010.

Table 2. Average Area Rainfall in August
Year Rainfall (in.)
2008 2.97
2009 2.46
2010 1.02

As you can see, rainfall in 2010 was about 36 percent of the average of the previous two years.  Moisture is a necessary ingredient for foliar disease development and the shortage in August 2010 was likely a contributing factor to the lack of fungicide response.

Averaged over the three years, there was anywhere from about a half bushel to a bit less than two bushel yield increase from soybeans treated with fungicide at R3 (See the right most column of Table 1.). With such a modest increase, and the inconsistency in results, growers should scout fields and base the decision about spraying fungicides on the presence of diseases on susceptible varieties, previous crop, yield potential, weather and application cost vs. value of the soybeans.

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