Frogeye leaf spot is caused by the fungus Cercospora sojina. This disease is becoming more common in Nebraska and is more common in the southeastern portion of the state. To date there have been several fields significantly affected and treated to protect soybean yield in Nebraska. In some years it may hasten maturity by causing premature defoliation. Yield loss estimates due to frogeye leaf spot have been reported as high as 30% nationally with extensive leaf blighting, but for Nebraska I would estimate less than 20% in highly susceptible varieties. 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 source for this disease is infested residue, infected seed and airborne spores.
Infection can occur at any stage of soybean development, but most often occurs after flowering and is typically in the upper canopy. Initial symptoms are small, dark spots on the leaves. Spots eventually enlarge to a diameter of about ¼ inch and the centers of the lesions become gray to brown and have a reddish purple margin. Individual leaf spots can coalesce to create irregular patterns of blighting on the leaf.
In addition, stems and pods can also be affected. Stem infections appear later in the season and will be long narrow dark lesions with flattened centers. Pod lesions will be circular to elongate, slightly sunken and reddish brown in color. As with the foliar lesions, the centers of these lesions will become gray to brown as they mature. Severe pod infection can result in infected and discolored seed. Seed symptoms will appear as gray and brown areas on the seed and which can be blotches to specks on the seed coat. Infected seed can have cracked and flaking seed coats.
Favorable Environmental Conditions
Infection occurs more readily in young leaves than older mature leaves. This is why the disease is observed mostly in the upper canopy when it occurs later in the season. This change in susceptibility of the leaves as they develop can result in layered occurrence of the disease in the plant canopy. Frogeye Leaf Spot development is favored by warm, moist weather, which promotes sporulation of the pathogen in the primary lesions. Conidia are dispersed by wind or splashing rain.
Management of Frogeye Leaf Spot
Soybean varieties vary in their resistance to Frogeye Leaf Spot and there are several genes commonly used for resistance.
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.
Application of fungicides to manage frogeye leaf spot in Nebraska is typically not warranted in most fields. Fields with a history of frogeye should be watched carefully and if disease develops application of a strobilurin fungicide at the R3 growth stage (pod set) are considered the most effective. In 2010, resistance to strobilurin fungicide was reported for the first time to this pathogen in Tennessee. If an application is made and control is not as expected, it is possible that resistance has spread. It will most likely not be an issue for us in Nebraska for several years.