Frogeye leaf spot (FLS) has historically been more of an economic issue in southern states, but over the years it has expanded into more northern soybean growing regions.1 As more soybean acres are planted and economics favors continuous soybean, FLS may become more prevalent.
Another disease, Phyllosticta leaf spot (PLS), needs to be mentioned in combination with FLS because it has also increased in incidence in the Corn Belt. Though a relatively minor pathogen, PLS can be easily confused with FLS; therefore, identification is important. Little is known about the environmental conditions that favor PLS, and Monsanto soybean breeders do not currently screen products for PLS resistance. If you are uncertain which disease is present, please contact your local Channel® technical agronomist for assistance.
When dealing with any disease, it is imperative to be familiar with its life cycle for successful management. FLS is caused by the fungus Cercospora sojina, and infections occur via infested crop residue or infected seed.2 The fungus favors warm, humid weather for spore production, and once produced, spores are spread throughout fields via wind and rain. Although it may seem as if older leaves are more prone to infection, the younger leaves are more susceptible. It can just take up to two weeks for the visual symptoms to develop (Figures 1-3). It is also important to note that infections tend to be layered within the canopy because the fungus needs moisture for infection to occur; therefore, dry weather stretches are unfavorable for FLS development.2
Managing FLS can be accomplished using several strategies. First and foremost, if you have a history of FLS in your fields, speak with your Channel Seedsman to select soybean products with strong ratings for FLS tolerance or resistance. This is the easiest and most economical way to manage the disease. Another important consideration is residue management. Since the fungus survives on residue and infected seed, tillage to help bury the residue and speed up the decomposition process can be beneficial. Also, crop rotation to corn, alfalfa or small grains can help to break the disease cycle since they are not hosts for the fungus. If needed, a fungicide application can be used to help control FLS. The best timing for a fungicide application, if environmental conditions are favorable, is between R2 and R5 growth stages. Effective scouting and utilizing multiple modes of action is imperative, as resistance in several states (including Iowa) to Group 11 (QoI-strobilurins) has been identified.3,4
In summary, FLS can be a serious yield robber if not managed properly; however, there are several options to address the disease before it impacts potential profitability. As always, if you have additional questions, feel free to reach out to your local Channel Seedsman or technical agronomist.
1Mian, M., Walker, D.R., Missaoui, A.M., Phillips, D.V., and Boerma, H. 2008. Frogeye leaf spot of soybean: A review and proposed race designations for isolates of Cercospora sojina Hara. Crop Science, 2008. 48(1): p. 14-24.
2Mueller, D., Robertson, A., Sisson, A., Tylka, G.L., Licht, M. 2018. Soybean Diseases. CSI 0004. Iowa State University Extension.
3Zhang, G.R., Newman, M.A., and Bradley, C.A. 2012. First report of the soybean Frogeye leaf spot fungus (Cercospora sojina) resistant to quinone outside inhibitor fungicides in North America. Plant Disease, 2012. 96(5): p. 767-767.
4Mueller, D., Yuba, K., and Bradley, C. 2018. Frogeye Leaf Spot Fungicide Resistance Confirmed in Iowa Soybean. Integrated Crop Management. Iowa State University.https://crops.extension.iastate.edu/cropnews.
Frogeye leaf spot quick facts. 2015. Quick FIELD FACTS. Channel.com.http://www.channel.com/agronomics/Documents/AgronomicContentPDF/FrogeyeLeafSpotQuickFacts_Channel_QuickFacts.pdf.