By Ethan Stoetzer

Iowa State University Extension field agronomists have reported the appearance of frogeye leaf spot in soybeans as much of the crop across Iowa enters the R3 to R5 growth stage.

Frogeye leaf spot, caused by the fungus Cercospora sojina, can occur at any growth stage in soybeans, but most often occurs after flowering. Typically, symptoms can be observed from beginning flowering (R1) through beginning maturity (R7). Young, newly unfolding foliage is the most susceptible to fungal infection, which is why symptoms are mostly observed in the upper plant canopy.

Watch for these symptoms
Disease symptoms of frogeye typically start out as small, water-soaked spots or lesions in the upper plant canopy. As the disease progresses, lesions enlarge and become round to angular. Eventually, the lesion center changes color to gray or brown and is surrounded by a narrow, reddish-purple margin. In some soybean varieties, a light green halo around the lesion border can be observed.

If environmental conditions are favorable, fungal sporulation can occur, which gives the underside of lesions a fuzzy gray appearance. The lesions can then begin to coalesce to create blighted areas on leaves. When the disease is severe, plants could experience premature defoliation.

In addition to foliar symptoms, the pathogen can infect stems and pods late in the growing season, though these symptoms can be challenging to identify. When lesions appear on the stem, they are elongated. When lesions appear on pods, they tend to appear oblong and resemble the foliar symptoms. If pods are severely diseased, seeds can become infected and can experience discoloration, turning them a light purple-to-gray color. Infected seeds may also be symptomless.

 
IDENTIFY CORRECTLY: Difficult to diagnose, frogeye leaf spot is easily mistaken for other diseases and disorders — even herbicide injury.

Frogeye leaf spot can be difficult to diagnose correctly in the field, as it is easily mistaken for other diseases and disorders, including herbicide injury. It is recommended that plant samples with these symptoms be sent to a diagnostic laboratory to confirm the disease before implementing a management strategy, if diagnosis is unclear. Frogeye leaf spot can be confused with the following diseases and disorders: phyllosticta leaf spot, PPO herbicide injury and paraquat herbicide injury.

Conditions favoring this disease
Conditions that favor frogeye leaf spot include warm, humid weather, with frequent rains that persist over an extended period of time. Several days of overcast weather can also increase the spreading of the fungus.

Field conditions can also increase the susceptibility of plants to the disease. These conditions include continuous soybean production (the fungus that causes frogeye leaf spot can survive in infested soybean residue for at least two years), short rotations between soybean crops, practicing conservation tillage, and planting a susceptible soybean variety in a field with a history of frogeye leaf spot.

Management to control frogeye
There are soybean varieties that are resistant to frogeye leaf spot, but be aware that some varieties marketed as resistant to the disease might not entirely be so. The resistance gene, known as Rcs3, has been effective against all races of this fungus known to occur in North America. Crop rotation and tillage can also be effective in reducing the amount of fungal inoculum available to infect the next soybean crop.

Long crop rotations may be necessary if the disease has been severe in a particular field. Well-timed foliar fungicide applications can effectively control frogeye leaf spot. Research shows that applying a fungicide during pod development (R3 to R4 growth stage) is most effective for managing the disease. There no set threshold for foliar disease management of soybean, but growth stage, disease risk and varietal susceptibility should be considered when making treatment decisions.

Tips for resistance management
Resistance to quinone-outside inhibiting (QoI/strobilurin) fungicides has been reported occurring in the frogeye leaf spot pathogen in North America. It’s important to use fungicide products that contain active ingredients from different fungicide classes for resistance management purposes.

Never rely on only one class of fungicide to manage frogeye leaf spot. Always consider the risk factors of variety susceptibility, cropping history and environmental conditions listed on the label before you apply a fungicide. Do this to minimize the risk of further fungicide resistance developing.

If you decide to apply a foliar fungicide, scout fields two weeks after the application to determine if the fungicide is adequately managing disease. Although many factors influence fungicide efficacy (such as low-volume spraying, nozzle choice, carrier-water quality, etc.), inadequate control may indicate that the fungus is resistant to the fungicide you used. Also, remember that no fungicide will ever provide 100% disease control on a susceptible soybean variety. If you believe fungicide resistance may be an issue in your field, contact your local ISU Extension specialist.

Stoetzer is the communications specialist for ISU’s Integrated Pest Management program.