Frogeye Leaf Spot Starting To Show Up In Soybean

 

Over the past week some initial reports of frogeye leaf spot of soybean started coming in. Frogeye leaf spot is a fungal disease caused by Cercospora sojina. It is not common across the state but is becoming more widespread.

 

 

Nationally, yield loss due to frogeye leaf spot with extensive leaf blighting has been estimated as high as 30%; however, in Nebraska I would estimate potential loss in highly susceptible varieties at less than 20%. The disease is most severe when soybean is grown continuously in the same field, particularly in fields where tillage is reduced, since this is a residue-borne disease.

 

The primary sources for this disease are infested residue, infected seed, and airborne spores.  In areas where this disease was observed in past years it will typically show up again if weather conditions are favorable.

 

 

Infection can occur at any stage of soybean development, but most often occurs after flowering and in the upper canopy (Figure 1). Initial symptoms are small, dark spots on the leaves. Spots eventually enlarge to a diameter of about ¼ inch. Lesion centers will turn gray to brown and have a reddish purple margin. Individual leaf spots can coalesce to create irregular patterns of blighting on the leaf.

 

 

Resistance.  Soybean varieties vary in their resistance to frogeye leaf spot. Several genes are commonly used for resistance. You will want to know the susceptibility of your variety if you’re trying to decide whether a fungicide is warranted.

 

Cultural Practices. Frogeye leaf spot is more severe in continuously cropped soybean fields. Reduced tillage systems will tend to have more as the pathogen overwinters in residue.

 

Fungicide Application. In Nebraska a fungicide application typically is not warranted to manage frogeye leaf spot. Fields with a history of frogeye should be watched carefully and if disease develops, application of a strobilurin fungicide at the R3 (pod set) to early R4 growth stage is considered to be most effective.

 

In 2010 resistance of this pathogen to strobilurin fungicide was reported for the first time in Tennessee. Since then there has been significant spread of the resistance in the Mississippi valley, but none has been observed in Nebraska. If an application is made and control is not as expected, it is possible that resistance has spread; however, most likely, it will not be an issue for us in Nebraska for several years.  In addition, most fungicide products on the market today are combinations with different modes of action with activity against this fungus.