Frogeye Leaf Spot in Soybeans

Frogeye leaf spot, caused by the fungus Cerospora sojina, is a common soybean foliar disease in the South. It’s a more recent disease in Midwestern states and has spread into the upper Midwest as well. Warmer winter temperatures, susceptible soybean germplasm and reduced tillage lead to more frequent outbreaks of frogeye leaf spot. Plant pathologists recommend growers prepare to defend against this potentially yield-robbing disease every year.

“We have millions of acres of disease hosts across major soybean regions and bucket loads of inoculants because of minimum tillage and residue on soil surfaces,” says Dr. Alison Robertson, extension field crops plant pathologist, Iowa State University. “In areas such as low-lying river bottoms and with water sources such as humidity, you’re more at risk of frogeye leaf spot.”

Risk of disease is also highest when soybeans are grown continuously in the same field, especially in reduced-tillage situations. The disease primarily survives in infested crop residue, and spreads via infected seed and airborne spores.

Identification and Lifecycle

Although infection may begin at any stage of soybean development, frogeye leaf spot most often occurs after flowering. Typically, it’s seen in the upper canopy. Initial symptoms appear as small dark spots on the leaves. The spots are angular with light gray centers and distinct purple to red-brown margins. Individual leaf spots can merge to form larger patterns of blight on the leaf. When the infection occurs, frogeye leaf spot can cause premature leaf drop.

The lifecycle of frogeye leaf spot begins when the fungus survives in crop residue or on seed in storage. Seed infection is a common means of proliferation for this disease. Infected seed provides an avenue for long-distance spread of the fungus. When seeds are infected, they germinate poorly and the resulting seedlings are weak. Spores produced on the leaves of infected seedlings are the main source of the spread of frogeye leaf spot. Spores become airborne and are dispersed by wind or splashed on nearby plants by rainfall.

The first noticeable leaf symptoms occur after soybean plants begin to bloom. The likelihood of infection and spread are greatest during warm, humid weather with cloudy days and frequent rain. Under these conditions, the frogeye leaf spot fungus produces abundant spores, creating a cycle of leaf infection throughout the growing season. Extended wet weather exacerbates disease proliferation even more.

Stems and pods can also be affected, which poses another threat to yield. Stem infections appear later in the season as long, narrow, dark lesions with flat centers. If pod lesions occur, they will look circular to long in shape, reddish-brown in color and slightly sunken. As these lesions mature, centers will appear gray and brown. Infected seed symptoms are visible as gray and brown areas on the seed coat, which may crack and flake.

Crop Damage

Yield loss from frogeye leaf spot disease results primarily from lack of photosynthesis caused by the leaf infection and premature defoliation. When seeds are infected, they germinate poorly and the resulting seedlings are weak. The disease is reported to cause soybean yield losses of up to 30 percent in some fields during wet years.

Managing Frogeye Leaf Spot

Growers have several tools that together help manage frogeye leaf spot, including variety selection, scouting, cultural practices and fungicides. These best management practices also help minimize the potential for fungicide resistance. Following seasonal reports about disease occurrences from county extension agronomists is another helpful practice.

Variety selection

Avoid frogeye leaf spot in soybeans by planting resistant seed. If the disease was detected in your crop last year, planting a resistant variety is an effective component of a sound management strategy.

Plant high-quality seed within a maturity group to fit your climate, and don’t skimp on seeding rates. Good genetics can add 10 to 12 bushels of yield per acre. Choose more than one variety to manage risk and potential yield.

Scouting

When scouting soybean fields for weeds and insects, check susceptible varieties for the presence of frogeye leaf spot and other foliar diseases. Start looking for symptoms early in the season. Stunted seedlings with dark lesions of varying size is one of the first signs of frogeye leaf spot. Begin looking for lesions at the R2 soybean growth stage as they tend to begin to appear after floral initiation. Typically, the disease will be spread throughout a field. If you suspect frogeye leaf spot, take samples and send them to the diagnostic lab recommended by your county extension agent.

Cultural practices

Two-year rotations with other crops and cultivation of crop residues prior to planting can lower inoculum levels and reduce disease. Practices such as shredding or disking after harvest can also help.

Fungicides

Frogeye leaf spot is a serious disease that can cause up to 30 percent or greater yield loss if control measures are not taken. Research from multiple states shows that fungicide applications made between the R2 and R5 growth stage provide the best timing for economical control.

Fungicides belonging to Group 3 (triazoles) and Group 11 (strobilurins) chemistry classes are among the most frequently used to control frogeye leaf spot. Growers should be aware that resistance of C. sojina to Group 11 fungicides has been reported in Arkansas, Kentucky, Illinois, Tennessee and Missouri, as well as many other states across the South. To reduce the chance of fungicide resistance, triazoles and strobilurins should be tank mixed or rotated with fungicides belonging to a different fungicide group labeled for frogeye leaf spot, particularly if multiple fungicide applications are used. For example Delaro^® fungicide contains both triazole and strobilurin chemistries, and it’s labeled for frogeye leaf spot.

Bayer solutions to control diseases in soybeans

A fungicide seed treatment can provide a healthy start for seedlings. Multiple fungal pests can impact soybean seeds and seedlings immediately after planting, and EverGol^® Energy SB can protect against pests causing the most damage such as Rhizoctonia solani, Fusarium spp and Pythium spp.

Also, timely foliar fungicide applications help protect soybean plants from frogeye leaf spot and other diseases through the season. For growers, a fungicide decision is a matter of considering production needs, past history in the fields, commodity prices, and proper timing and risk management.

Bayer offers Stratego^® YLD, a fungicide featuring triazole technology combined with strobilurin chemistry for soybeans. Offering two modes of action, it provides both preventive and curative benefits and systemic movement to provide broad-spectrum, long-lasting disease control and higher yield potential. Trials show that when applied at early flowering, around growth stage R3, Stratego YLD can deliver an average yield increase of 3 to 4 bushels per acre over untreated soybean fields. For cost efficiency, fungicides can be tankmixed with insecticides and applied in the same trip across the field.

Before applying any fungicide, please read the entire label for the best possible results and to confirm that the product is effective on the disease you need to control. Not every product is suitable for every situation, and correct application technique will ensure the best results.

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