The continued frequent rainfalls and high relative humidity we have been experiencing have led to conditions favorable for the development of foliar diseases of soybean. Some diseases to look for are listed below.
Septoria Brown Spot
Probably the most commonly observed foliar soybean disease is Septoria brown spot (caused by Septoria glycines) (Figure 1).
Although Septoria brown spot can commonly be observed, its potential to economically reduce soybean yields is not great. This disease tends to spread vertically (from bottom to top) in the soybean canopy. Only in very wet years, does this disease make it to the upper canopy, where it has the potential to reduce yields. This disease has the ability to cause premature defoliation, and when this occurs in the upper third of the soybean canopy, then yield reductions can occur.
Frogeye Leaf Spot
Frogeye leaf spot (caused by Cercospora sojina) is another foliar disease of soybean that can be observed in warm, humid, and wet growing seasons (Figure 2). Of all the commonly observed foliar soybean diseases that occur in this area of the U.S., frogeye leaf spot has the most potential to cause yield reductions. However, some soybean varieties are highly resistant to this disease, and yield reductions due to frogeye leaf spot will not occur on these resistant varieties.
Cercospora Leaf Blight
Cercospora leaf blight (Figure 3) (caused by Cercospora kikuchii) is another disease that can be observed, but usually begins to appear later in the year. Because this disease generally is observed later in the year in Kentucky, yield reductions from this disease likely do not commonly occur.
Foliar Fungicide Considerations
Foliar fungicides can be useful tools in managing some of these foliar soybean diseases; however, a “blanket” application across all acres is not the best approach to take. From my program’s soybean foliar fungicide research in Illinois, we did not always observe a “profitable” yield return with the use of foliar fungicides on soybean; however, when a significant level of foliar diseases were present and were being managed with the foliar fungicides, “profitable” yield returns were more likely.
Of the three diseases listed above, management of frogeye leaf spot with foliar fungicides generally provided the greatest impact on yield. This was also observed in a research trial I conducted in Princeton, KY last year, where effective foliar fungicides applied to a soybean variety very susceptible to frogeye leaf spot resulted into approximately a 15 bu/A yield response compared to the non-treated check, which had severe frogeye leaf spot. It is important to know how susceptible your soybean varieties are to frogeye leaf spot, and to scout for this disease in susceptible varieties.
Resistance to the strobilurin class of fungicides exists in all three of the pathogens listed above. Therefore, if the decision to apply a fungicide is made, be sure that the fungicide product contains a mixture of active ingredients from different chemistry classes. Strobilurin fungicide resistance in the frogeye leaf spot pathogen has been documented in several counties in Kentucky. Strobilurin fungicide resistance in the Septoria brown spot pathogen was observed in northern Illinois in 2013 for the first time, but extensive surveys in other states have not been conducted. Strobilurin fungicide resistance in the Cercospora leaf blight pathogen commonly occurs in Louisiana, but no information is currently available for other states.
For more information about fungicide resistance in soybean pathogens, please check out the “Soybean Fungicide Resistance Hub” online at the Plant Management Network .
By Carl A. Bradley, Extension Plant Pathologist