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Issue No. 17, Article 3/July 29, 2011

Soybean Fungicide Considerations

In Illinois, the two major foliar fungal diseases of soybean that can be controlled with fungicides are Septoria brown spot and frogeye leaf spot. Their apparent impact on soybean yields are being investigated in fungicide research trials.

Septoria brown spot. Caused by the fungus Septoria glycines, Septoria brown spot is a very common disease of soybean. It generally is observed first in the lower leaves and will move into the upper leaves if there is frequent rainfall. Symptoms first appear as small brown spots on the leaves. Given favorable conditions, the brown spots coalesce and cause yellowing (chlorosis) of the leaf and eventually defoliation of affected leaves. Results of University of Illinois fungicide research trials indicate that the most commonly used foliar fungicides provide control of this disease, but economic yield responses are not always observed as a result. Our research results do indicate that in field trials where frequent rainfall fell in July and August, Septoria brown spot affected leaves in the upper one-third of the soybean canopy and economic yield losses occurred in the nontreated check plots. However, in environments where Septoria brown spot did not occur in the upper one-third of the canopy, economic yield losses were rare.

Symptoms of Septoria brown spot on a soybean leaflet.

Frogeye leaf spot. Caused by the fungus Cercospora sojina, Septoria brown spot is not observed as frequently in Illinois as Septoria brown spot, but it can occur when susceptible varieties are planted and warm and humid conditions prevail. Symptoms appear on leaves as round spots with a tan center surrounded by a purple margin. Soybean varieties can differ greatly in their susceptibility to frogeye leaf spot; varieties that contain the Rcs3 gene are resistant to all known races of the pathogen that occur in Illinois. Results of University of Illinois and Southern Illinois University fungicide research trials indicate that economic yield responses to foliar fungicides are more likely to occur when frogeye leaf spot is present.

Symptoms of frogeye leaf spot on soybean leaves.

Summary of foliar fungicide results conducted by the University of Illinois (C. Bradley and W. Pedersen) and Southern Illinois University (J. Bond) with different levels of frogeye leaf spot pressure.

Fungicide resistance management. In 2010, strains of Cercospora sojina (the frogeye leaf spot pathogen) with resistance to strobilurin fungicides were identified in Gallatin and Pope counties in Illinois, as well as Caldwell County in Kentucky and Gibson and Lauderdale counties in Tennessee. Fungicide active ingredients belonging to the strobilurin class of fungicides that are registered for use on soybean include azoxystrobin (found in Quadris, Quilt, and Quilt Xcel), fluoxastrobin (Evito, Evito T), pyraclostrobin (Headline), and trifloxystrobin (Stratego, Stratego YLD, and Gem). In laboratory tests, these resistant strains of the fungus were found to be up to nearly 1,500-fold less sensitive to strobilurin fungicides, meaning that such fungicides will provide no control of these strains. In light of these findings, the following is recommended for control of frogeye leaf spot:

  • Plant frogeye leaf spot-resistant varieties. This is the best way to manage this disease. Resistant varieties are available for Illinois growers; check with your seed dealer and the Illinois Varietal Information Program for Soybeans (VIPS).
  • If you have planted a frogeye leaf spot-susceptible variety and are considering fungicide, apply an effective triazole fungicide for control. Fungicides in the triazole chemistry class (also known as demethylation inhibitors) have a different site and mode of action on pathogenic fungi than strobilurin fungicides, and strobilurin-resistant isolates should not be cross-resistant to triazole fungicides. Fungicides that contain thiophanate methyl also may be effective in controlling frogeye leaf spot. Thiophanate methyl is a fungicide that belongs to the methyl benzimidazole carbamates class, which have a different site and mode of action than strobilurin and triazole fungicides.
  • In situations where other foliar diseases may be present along with frogeye leaf spot and a strobilurin fungicide may be needed to control the other foliar diseases, do not spray a solo strobilurin product. Apply either a strobilurin-triazole or strobilurin-thiophanate methyl tank-mix or a product that contains both a strobilurin product and a triazole product.
  • Apply a foliar fungicide only to control plant diseases. Every time a fungicide application is made, a "selection pressure" is applied to the pathogen population. This pressure selects out individuals in the pathogen population that may have reduced sensitivity to fungicides. Applying a fungicide only when it is needed (based on disease risk and scouting observations) reduces the selection pressure placed on the pathogen population and slows the development and spread of fungicide-resistant isolates.

The soybean fungicide research trials and the Cercospora sojina fungicide resistance monitoring project were funded by the Illinois Soybean Association.--Carl A. Bradley

Carl A. Bradley

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