Many soybean fields in Kentucky are approaching the R3 (beginning pod) developmental stage, which generally is a time to consider an application of a foliar fungicide to protect against foliar diseases. In areas of the state that have received frequent rainfall, the risk of foliar diseases has increased. Besides rainfall, the risk of foliar diseases also is affected by other factors, such as the soybean variety planted, and the cropping history in a field.
The primary foliar diseases of concern in Kentucky are frogeye leaf spot (Figure 1), Septoria brown spot (Figure 2), Cercospora leaf blight (Figure 3), and target spot (Figure 4). Of these diseases, frogeye leaf spot has been the most important in terms of causing yield reductions in Kentucky. The incidence and severity of frogeye leaf spot is influenced greatly by the soybean variety being grown. Some varieties are highly resistant to frogeye leaf spot, while others may be susceptible; therefore, it is important to be aware of the disease ratings of the varieties planted in your fields. In general, Septoria brown spot often is not an economic concern, as symptoms often are only on leaves in the lower canopy. However, in years with frequent rainfall throughout the season, spores of the Septoria brown spot pathogen may splash up to the upper canopy and cause some upper leaves to prematurely defoliate. When this happens, some yield loss can be attributed to Septoria brown spot. Although target spot and Cercospora leaf blight may occur in Kentucky, the appearance of these diseases generally has been later in the season, which has often been too late to cause yield reductions.
A soybean disease “score card” is available in the resources section of the Take Action website, that is titled, “Know Your Disease Risk in Soybeans: What’s Your Score?”. This score card can be used on a field-by-field basis to help determine what the risk is for foliar disease development and can help make fungicide application decisions.
If the decision is made to apply a foliar fungicide, it is important to choose a product that has efficacy against the spectrum of diseases that might affect your field. It is also important to choose a product that contains multiple modes of action to help manage the potential of fungicide resistance. Isolates of the frogeye leaf spot pathogen and the Septoria brown spot pathogen that are resistant to strobilurin (QoI) fungicides are present in Kentucky, and isolates of the Cercospora leaf blight pathogen that are resistant to strobilurin fungicides have been confirmed elsewhere in the U.S., so fungicide resistance is an important consideration. To help make a decision on which fungicide products might work best for the diseases you intend to manage, the “Fungicide Efficacy for Control of Soybean Foliar Disease” publication on the Crop Protection Network can provide information that will help with that decision.
Carl A. Bradley, Extension Plant Pathologist