Soybean fields scouted were found to have brown spot (also called Septoria leaf spot) and frogeye leaf spot developing. However, both diseases were seen at low severity. These diseases develop under wet and warm weather conditions; which are currently, being experienced in several areas in the state. Both diseases are caused by fungal pathogens: brown spot is caused by Septoria gylicines and frogeye leaf spot is caused by Cercospora sojina.
Brown spot is characterized by numerous small irregular dark-brown spots found mainly on the older leaves in the lower canopy (Figure 1). These spots can coalesce to form large blotches and this can lead to infected leaves yellowing and premature leaf drop. Under prolonged leaf wetness and warm conditions, the disease can progress to mid-canopy leaves. Whereas, this disease can typically be found in any soybean field, it is un-likely to reach severe levels. If the field is soybean on soybean and narrow row-spaced soybean (<20 inches) then there is the potential for severe levels of brown spot to develop.
Figure 1. Brown spot symptoms on a soybean leaf.
Frogeye Leaf Spot
Frogeye leaf spot lesions start as dark brown circular-angular spots. These spots progress to form an ash gray center surrounded by thin reddish-brown margins (Figure 2). Frogeye leaf spots tend to develop in the mid to upper canopy leaves. The frogeye leaf spot pathogen survives on soybean residue and also on the soybean seed; however, the soybean residue is the main source of inoculum.
Figure 2. Frogeye leaf spot lesions on a soybean leaf. Notice the gray center and a dark brown reddish margin.
Brown spot and frogeye leaf spot can be best managed through residue management which reduces the source of inoculum. Rotating away from soybeans for at least two years and utilizing tillage where practical will assist in reducing the inoculum. An in-season management practice is to apply a foliar fungicide. Some of the soybeans are approaching beginning pod (R3) growth stage. This stage is recommended for applying a fungicide for foliar disease management. However, both on-farm research and small plots research trials indicated a limited benefit of applying a fungicide to manage foliar diseases in soybeans, except when managing white mold. It is therefore important to scout soybean fields at the start of podding (R3) to determine the need for fungicide application. Most of the fungicides are effective against brown spot and frogeye leaf spot. Efficacy of fungicides against soybean fungal diseases can be found here.
Fungicide Resistance Caution
Frogeye leaf spot pathogen has been found to be resistant to strobilurin fungicides (QoI, Fungicide Resistance Action Committee Group 11) in a few states. Growers are encouraged to scout and apply a fungicide only when it is warranted and to rotate the fungicide mode of action in order to avoid the chance of fungicide resistance developing.