Considerations for Applying Foliar Fungicide to Soybeans

The use of foliar fungicide on soybeans during any growing season is a tough decision. There are many factors that enter into the equation like growth stage of the crop, weather, presence of diseases and history of the field.

Years ago, when fungicides were first made available for post-emergent application on soybeans, many farmers took a “shotgun approach.” Rather than scout their fields, they applied fungicide as more of an insurance against disease. We’re seeing the effects of those decisions as many diseases are becoming resistant to fungicides, especially those in the Qoi (Quinone outside inhibitor) family, commonly called the strobilurins.

Before pulling the trigger on using any foliar-applied fungicide on soybeans, I encourage farmers to walk their fields or hire a competent scout. Here are a few ideas and suggestions for the more common soybean fungal diseases that show up in Latham Country:

  • White Mold, or Sclerotinia Stem Rot, is widespread across our territory. Infection usually occurs at or just after the R1 (flowering) stage during periods of high humidity and temperatures below 85° Fahrenheit. Look for leaves that are starting to turn yellow after R1 but well before normal senescence should happen. Once leaves turn brown and the stem starts to develop that white, cottony growth leading to dark sclerotia (like mouse droppings) along the stems and pods, it is too late to save that plant. Research from the University of Wisconsin indicates that applications of foliar fungicides registered for use against White Mold can be helpful if applied in the R1 to R3 stage. If you miss the target window for application and the disease becomes severe, harvest that field last so you don’t spread the sclerotia to your other fields.
  • Septoria Brown Spot is found in most fields in Latham Country but it’s seldom the primary source of lost yield. It can, however, contribute to overall yield loss in fields where Fusarium Root Rot is a problem and when damage from Soybean Cyst Nematode is severe. Severe infestations also can occur in the early part of the season where rainfall is over-abundant. Symptoms are small, irregular-shaped spots on leaves and typically start in the lower canopy and can show up as early as V2 stage and as late as R6. Leaves that are severely infected will turn prematurely yellow and drop from the plant. If warranted, foliar fungicides can be helpful when applied at the R3 to R5 growth stage.
  • Frogeye Leaf Spot is caused by the Cercospora sojina fungus. Symptoms appear as small, round gray spots on the upper leaf surface with dark reddish-brown borders. Infection can occur anytime throughout the season but is most common after flowering and after periods of warm, humid weather. Check the upper canopy of plants, especially in the R3 to R6 growth stage. Iowa State researchers have shown that an application of a foliar fungicide containing two or more different active ingredients at R3 is fairly effective in reducing the severity of Frogeye Leaf Spot damage. Application of a product containing only strobilurin chemistry (Qoi) was not effective at all due to resistance.