Frogeye leaf spot is caused by the fungus Cercospora sojina. This disease is becoming more common 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. 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 the 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. Resistance gene Rcs3 has been reported to be resistant against all known races of the FLS pathogen in the United states. Growers should consult with seed company representatives and review disease ratings to identify resistant varieties.
Frogeye Leaf Spot is more severe in continuously cropped soybean fields. Reduced tillage systems will tend to have more as the pathogen overwinters in the residue.
Fungicide applications can be very effective controlling Frogeye Leaf Spot. Products applied to soybean in reproductive stages R3 (beginning pod) to R5 (beginning seed) are most effective. Group 11 Quinone Outside Inhibitor (QoI) fungicides (formerly referred to as “strobilurin”) were historically the most effective and widely used for management of Frogeye Leaf Spot. Since 2010, resistance to this class of fungicides has been documented in numerous other states. In 2019, soybean leaf samples with FLS were collected from fields in 10 Nebraska counties. QoI fungicide resistance was confirmed in samples from all 10 of the Nebraska counties sampled in this small survey (Fig. 1). It is unknown how widespread fungicide resistance is in the frogeye leaf spot pathogen in Nebraska yet. Soybean fields with a history of Frogeye Leaf Spot should be monitored carefully for disease development. If a fungicide application is necessary to control Frogeye Leaf Spot, avoid spraying products that only contain Group 11 QoI active ingredients. Products with two or more effective active ingredients with mixed modes of action (from multiple FRAC fungicide classes) will be more likely to provide effective control of the disease. Relative efficacy of fungicides on FLS can be found in the Crop Protection Network’s publication on Fungicide Efficacy for Control of Soybean Foliar Diseases.
- Fungicide Resistance and Management of Frogeye Leaf Spot of Soybean in Nebraska
- Fungicide Efficacy for Control of Soybean Foliar Diseases (Crop Protection Network)
- Frogeye Leaf Spot (publication CPH-1017), and Frogeye Leaf Spot of Soybean, published by the Crop Protection Network
- Frogeye Leaf Spot, including a short video on Frogeye Leaf Spot Identification and Management, in the Soybean Plant Disease section of CropWatch.
- Foliar Fungicide Use In Soybean, a Nebraska Extension NebGuide (G1862)