It’s the time of the year in which symptoms of some foliar soybean diseases are becoming apparent. Variety susceptibility, field history, and weather conditions all play a role in the incidence and severity of foliar diseases of soybean, so scouting is important.
Soybean Foliar Diseases
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 is common, 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. Septoria brown spot can 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). Frogeye leaf spot is the foliar disease that has the most potential to cause yield reductions in this area of the U.S. 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.
Figure 1. Brown lesions and yellowing on the leaf edges caused by the Septoria brown spot pathogen of soybean (Photo: Carl Bradley, UK).
Figure 2. Symptoms of frogeye leaf spot on soybean leaves (Photo: Carl Bradley,UK).
Cercospora Leaf Blight
Cercospora leaf blight (Figure 3) (caused by Cercospora kikuchii) is another disease that can be observed in soybean fields, but usually appears later in the year. Because this disease generally occurs closer to harvest in Kentucky, yield reductions from this disease do not commonly occur.
Target spot has been somewhat rare in Kentucky, although it was observed in some fields during the 2016 growing season. This disease, caused by Corynespora cassiicola, is recognized by round spots that have a zonate type pattern inside, which can resemble a “target” Figure 4). This disease reportedly caused yield losses in nearby states during the 2016 growing season, thus, it is a disease to watch for despite its rare occurrence in Kentucky in past years.
Figure 3. “Purpling” of soybean leaf caused by the Cercospora leaf blight pathogen (Photo: Carl Bradley, UK).
Figure 4. Symptoms of target spot on a soybean leaflet (Photo: Tom Allen, Mississippi State University).
Caused by Phakopsora pachyrhizi, soybean rust has been observed in Kentucky several years, but always has arrived too late in the season to cause observable yield reductions. Symptoms of soybean rust may resemble Septoria brown spot, but leaves affected by soybean rust will have small, raised pustules on the underside of the leaf that are best seen with the aid of magnification (Figure 5). Observations of soybean rust in the U.S. currently are tracked on the IPM-PIPE website.
Figure 5. Soybean rust causes, small, raised pustules, observed with the aid of magnification on the underside of a soybean leaflet. (Photo: Carl Bradley, UK).
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. A profitable yield return with the use of foliar fungicides on soybean will not always occur; however, when a significant level of foliar diseases are present and are being managed with foliar fungicides, profitable yield returns will be more likely.
It is important to note that strains of the frogeye leaf spot pathogen that are resistant to QoI (strobilurin) fungicides are widespread across Kentucky. Fungicide resistance also can be a concern for the other pathogens mentioned above; therefore, it is always important to use products that utilize more than one fungicide class. A new Fungicide Classification poster that shows what active ingredients and fungicide classes are being used in several commercially-available fungicide products is available here. In addition, a soybean fungicide efficacy chart developed by university plant pathologists across the U.S. can be found on-line here. Additional information on fungicide resistance in pathogens of soybean can be found at the Soybean Fungicide Resistance Hub and at the Crop Protection Network’s “Fungicide Resistance in Field Crops FAQs.”
By Carl A. Bradley and Kiersten Wise, Extension Plant Pathologists