As soybean fields in Kentucky approach the R3 (beginning pod) developmental stage, it generally is a time to consider an application of a foliar fungicide to protect against foliar diseases. Rainfall is an important factor to consider when making a foliar fungicide application decision, as high rainfall accumulation is one of the main drivers that can increase the risk of foliar diseases. Risk of foliar disease is likely to be low in some areas of the state this year that have experienced extreme heat and dry weather. 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.
Frog-eye Leaf Spot & Target Spot
The primary foliar diseases of concern that have shown the ability to cause economic yield losses in Kentucky recently are frogeye leaf spot (Figure 1) and target spot (Figure 2). Both of these diseases are 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. Target spot is a relatively new disease to Kentucky. In general, my observations of target spot causing severe disease in Kentucky have been limited, and in most cases that I’ve seen it, symptoms developed too late to be an issue. However, there have been a few fields in western Kentucky over the last few years that had severe target spot, likely due to planting a very susceptible variety.
Septoria Brown Spot & Cercospora Leaf Blight
Other foliar diseases that generally do not have an economic impact on soybean, but can in certain years are Septoria brown spot (Figure 3) and Cercospora leaf blight (Figure 4). In general, symptoms of Septoria brown spot often are only on leaves in the lower canopy, which has little impact on yield. 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 Cercospora leaf blight may occur in Kentucky, the appearance of this disease generally has been later in the season, which often has been too late to cause yield reductions.
Disease Score Card
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, Septoria brown spot, target spot, and Cercospora leaf blight pathogens that are resistant to strobilurin (QoI) fungicides are present in Kentucky, 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.
By Carl Bradley, Plant Pathology Extension Specialist