Deciding When to Apply Soybean Fungicides

Deciding When to Apply Soybean Fungicides

July 25, 2008
Photo of brown spot disease in soybean.
Trifoliate soybean leaves with brown spot.
Frogeye disease damage in soybean
Soybean leaf with frogeye leaf spot.
U.S. map showing soybean rust occurrence this year.
Current distribution of confirmed soybean rust in the United States (red) and scouted (green) areas as of July 27. (Source: sbrusa.net)

While much of the state's soybean crop approaches or has reached the R3 (pod set) growth stage, many producers are asking whether they should apply a fungicide. This article reviews the parameters to consider when deciding. With current soybean prices, a fungicide application to what would normally be considered a low potential loss could provide a significant return on your investment.

Soybean Foliar Diseases

Your primary reason for using fungicides should be disease control. In Nebraska, two soybean diseases can reduce yields — brown spot and frogeye leaf spot. Both have been observed in Nebraska this year and occurrence of each can vary with the field. Both diseases overwinter in residue and will be more severe in continuous soybeans and no-till.

Brown Spot. This disease is the most common and is the gray leaf spot (most common corn disease) of soybean — I say this as they both have a similar epidemiology. The main difference is that the yield loss with brown spot in soybean is not as great as with grey leaf spot in corn. Yield losses of 8%-15% are typical; in Nebraska, we typically see 10% or less. Brown spot typically begins in the lower canopy during periods of high moisture or humidity. It is characterized by angular lesions with chlorotic margins. Lesions can merge together to form large necrotic spots. Soybean varieties will vary in their susceptibility to this disease but are not evaluated by companies. You will need to go on your experience with the variety.

Frogeye Leaf Spot. This disease is not common in Nebraska yet, but has been increasing over the last three years. Frogeye leaf spot typically forms on younger leaves in the upper canopy. Lesions are circular with purple or reddish margins. Older lesions may develop dark centers where the fruiting bodies of the fungus are visible. Yield losses can be as much as 30% in the U.S. Soybean varieties are evaluated for this disease and resistant varieties are available.

Soybean Rust. Extremely dry and hot conditions across much of the southeastern U.S. this spring and early summer have resulted in very limited development of soybean rust. The sentinel plot system is active this year and soybean rust has not yet been detected outside of the Gulf states. There is no risk of soybean rust for Nebraska's soybean crop this year. Current conditions and sites where soybean rust have been observed can be found at sbrusa.net.

Application Timing

Much research has been done on fungicide timing and applications in the R3 (pod set) growth stage have shown the best results. This may extend into the R4 stage if we see dryer conditions followed by rain later (shift in when the diseases develop).

Other Diseases Observed

This year we have seen bacterial blight in many fields due to the stormy conditions. Fungicides will have no effect on bacterial blight. Similarly, there will be no effect on the root and stem rot diseases (Phytopthora, Sudden Death Syndrome and Brown Stem Rot).

Applications in Absence of Significant Disease Development

So far our research hasn't shown consistent yield increases with fungicide applications for low-level disease development. Over three years, we have observed a positive return (two or more bushels per acre) 38% of the time. The average over 52 comparisons was a 1.2-bushel increase. This increased to 44% and a 1.8-bushel increase when we had a 10% or greater brown spot severity. (Note we did not have a significant brown spot in any of these studies.) As these diseases affect the lower canopy, I would estimate that it would take a canopy damage level of 30%-40% defoliation to see significant yield loss.

Test it On-farm

Each year we talk with individuals who have experienced great returns from fungicide applications to soybean. However, so far I have not observed consistent enough results to recommend treatments across Nebraska. I encourage you to do some on-farm testing if you are interested in seeing what the return is. From our data, some will see good returns and some will see nothing. If the percentages are true, approximately 38% will have two bushels or more. Of course, this will vary with growing conditions and with the rains this year; I would assume the percentage of positive returns will be greater.

For more information on soybean foliar diseases and fungicide use see the Nebguide, Foliar Fungicide Use in Soybean (G1862 PDF 1.1MB).

Loren J. Giesler
Extension Plant Pathologist