Frogeye leaf spot is a fungal disease of soybeans caused by Cercospora sojina that produces distinctive pale-centered lesions on leaves and can reduce yields significantly in warm, humid growing seasons. The disease has become increasingly difficult to manage in recent years due to widespread fungicide resistance, making an integrated approach essential.
What Frogeye Leaf Spot Looks Like
The hallmark symptom is small, round to irregular spots on soybean leaves, typically 1/8 to 1/4 inch across. Young lesions start as dark brown spots, but as they mature, the center fades to pale brown or white while the margin stays dark brown. That contrast between a light center and dark border is what gives the disease its name: the lesions resemble a frog’s eye.
Lesions can appear on leaves throughout the canopy, and when infections are severe, spots may merge together, killing large sections of leaf tissue. The fungus can also infect stems, pods, and seeds, though leaf symptoms are what most growers notice first.
Conditions That Drive Infection
Cercospora sojina thrives in warm, wet weather. Extended periods of rain, heavy dew, or relative humidity above 90% favor infection and disease development. The fungus needs moisture on the leaf surface to germinate and penetrate the tissue, so fields with dense canopies that hold humidity are especially vulnerable.
The pathogen overwinters in infested crop residue and on infected seed. Mild winter temperatures and conservation tillage both favor its survival between seasons, giving it a head start the following year. Continuous soybean planting and minimum tillage increase disease risk by keeping inoculum levels high in the field.
Diseases That Look Similar
Early frogeye leaf spot lesions, before the pale center develops, can be tricky to distinguish from other common soybean diseases. Target spot produces larger lesions, roughly 1/2 inch across, that are very dark brown (sometimes with a purple hue) and form concentric rings with varying shades of brown. Target spot lesions also tend to have a yellow halo and typically start on lower canopy leaves. Septoria brown spot causes small brown lesions that may also show concentric rings, making it easy to confuse with both target spot and frogeye in the early stages.
The key identifier for frogeye leaf spot is that mature lesion with a distinctly white or pale center ringed by a sharp dark brown border. If you’re seeing that combination, frogeye is the most likely diagnosis. For ambiguous cases, your state extension lab can confirm the pathogen from a leaf sample.
The Fungicide Resistance Problem
One of the most important things soybean growers need to know about frogeye leaf spot is that the pathogen has developed widespread resistance to strobilurin fungicides (FRAC group 11). This resistance was first documented in Tennessee in 2010 and has since spread across the soybean belt. In Nebraska, a 2020 survey found strobilurin resistance in 128 fields across 48 counties, with 100% of samples showing some level of resistance. Iowa reported similar results, with nearly all isolates from 51 counties testing resistant.
This means that if you’re relying on a strobilurin-only fungicide product for frogeye leaf spot control, you’re likely getting little to no benefit. The resistance is so prevalent in many states that strobilurins alone should no longer be considered a reliable tool against this disease.
Managing Frogeye Leaf Spot
Field trial results consistently show that an integrated approach provides the best control. No single practice is enough on its own, but combining several strategies can keep the disease at manageable levels.
Variety Selection
Planting resistant varieties is the most effective starting point. Soybean varieties vary considerably in their susceptibility. One resistance gene, Rcs3, has been reported to provide resistance against all known races of the frogeye leaf spot pathogen in the United States. Seed company disease ratings are the best way to compare options for your area.
Crop Rotation and Tillage
Rotating away from soybeans for two or more years with non-host crops like corn, sorghum, or small grains interrupts the disease cycle and reduces the amount of fungal inoculum in the field. The longer the rotation, the more effective it is, particularly in fields with a history of heavy frogeye pressure. Tillage buries infested residue and speeds its breakdown, further reducing the pathogen load. This isn’t practical for every operation, but where it fits, it helps.
Fungicide Strategy
Given the widespread strobilurin resistance, fungicide applications should use mixed-mode-of-action products rather than strobilurin-only formulations. Premixes that combine multiple chemical classes are the standard recommendation. Timing applications during early reproductive growth stages gives the best chance of protecting the upper canopy leaves that matter most for grain fill. Seed treatments can also reduce early-season infection from pathogen carried on the seed.
The combination of resistant varieties, rotation, residue management, and properly timed fungicides with effective modes of action gives growers the most reliable path to keeping frogeye leaf spot from cutting into yields.