Fungicide Application on Soybean

Each blue dot on the graph shows the soybean yield for the non-treated control (without fungicide, horizontal axis) and when a fungicide was applied (vertical axis). The black line in Graph 1 represents the point where the yield with a fungicide treatment and yield of the non-treated control were the same. The dots above the black line represent trial results where the fungicide treatment yielded more that the control, while blue dots below the line indicate that soybean with the fungicide application yielded lower than the non-treated control. The average soybean yield without fungicide was 44.6 bushel per acre while the yield with fungicide was significantly higher, at 45.9 bushel per acre. Across all data points, this represents a 1.3 bushel or 3% increase in yield.

However, the yield increase throughout the study was disproportionately dependent on a few trials. The data analyzed here were extracted from replicated research plots and statistical differences could be calculated in each individual trial. A significant yield increase was only found in eight out of the 89 comparisons (9% of the time). When the remaining 81 comparisons were analyzed (without those eight significant comparisons) the yield without fungicide was 44.7 bushel per acre and with fungicide 45.5 bushel per acre, a 0.8 bushel or 2% increase.

For a fungicide application to be economical, various factors and input costs should be considered:

First - Fungicides are most likely to pay off if it manages an important disease impacting the crop. Until 2020, we didn’t worry too much about foliar diseases robbing yield on soybeans (bacterial blight is not managed by fungicides, fungal diseases were rare), and disease pressure throughout these trials were demonstrated to be low. However, frogeye leaf spot, a disease that causes yield loss in other parts of the country, was found throughout SE North Dakota in 2020. While the disease likely showed up too late in the season to cause yield loss last year, it is something growers should watch for in the future. Notably, if fungicides are being considered for frogeye leaf spot, multiple modes of action should be used.

Second - Without an imminent disease threat, agronomics and economics are critical. We have been unable to predict under which conditions a fungicide application is most likely to increase yields while disease is not a factor, but other considerations are known. The value of the crop, cost of application per acre and the cost of product per acre have to be foremost considered. The price of the crop is higher in 2021 than in recent years, but the current drought conditions are reducing yield potential.

Hans Kandel
Extension Agronomist Broadleaf Crops