An Overview of Frogeye Leaf Spot

Frogeye Leaf Spot

Symptoms and Signs

Frogeye leaf spot initially appears on upper leaf surfaces as small, dark, water-soaked spots (lesions) (Figure 1). Eventually, these lesions enlarge and become round to angular.

The centers of frogeye leaf spot lesions progress from gray to brown to light tan, and are surrounded by a narrow reddish purple margin (Figure 2). On some soybean varieties, you may also see a light green halo around the lesion border (Figure 3).

When environmental conditions are favorable, fungal sporulation occurs, which gives the underside of lesions a gray and fuzzy appearance (Figure 4). Lesions can coalesce to create blighted areas on leaves. When frogeye leaf spot is severe, plants can prematurely defoliate.

In addition to leaf lesions, frogeye leaf spot symptoms can occur on stems and pods late in the season, but these symptoms can be difficult to identify. Stem lesions appear elongated (Figure 5). Pod lesions appear oblong and resemble foliar symptoms (Figure 6). Severely diseased pods can infect and discolor seeds. The fungus that causes frogeye leaf spot may infect seeds, which results in light purple to gray discoloration, but infected seed also may not show any symptoms.

Disease Cycle

The fungus that causes frogeye leaf spot survives in infested soybean residue for at least two years. Preliminary research suggests that other legumes and some weeds and cover crops may also be hosts of the fungus.

There are reports of the fungus being transmitted by seed, although this has rarely been observed in the field. Wind and splashing water may disperse spores. Spores produced on infected plants can move to new plants in the same field, and wind can also disperse the spores to nearby fields (Figure 7).

The fungus can infect leaves at any stage of soybean development, but is most common after flowering (Figure 8). Symptoms are most frequently observed from flowering (R1) through beginning maturity (R7). Young, expanding leaves are more susceptible than older, fully expanded leaves (Figure 9). Because of this, frogeye leaf spot symptoms are usually observed in the upper plant canopy.

Conditions that Favor Disease

Frogeye leaf spot is most severe when warm, humid weather with frequent rain persists for extended periods. Several days of overcast weather favor disease development and spread. Overhead irrigation may increase the risk of severe frogeye leaf spot compared to flood or furrow irrigation or dryland production systems.

Fields will have higher risk for frogeye leaf spot if:

• You plant a susceptible soybean variety in a field with a history of frogeye leaf spot

• Your fields have continuous soybean production

• Your fields have short rotations between soybean crops

• You practice conservation tillage

Yield Losses and Impact

Frogeye leaf spot’s effect on yield can vary greatly, depending on disease timing, varietal susceptibility to disease, and weather conditions during soybean reproductive stages.

If the disease begins late in reproductive stages (after growth stage R5.5) or disease severity is low, the yield impact will be minimal.

But if conditions are favorable and there are severe disease outbreaks early or just after flowering, yield losses can be up to 35 percent.

Diagnosis

Frogeye leaf spot can be difficult to diagnose correctly in the field, because it is easily mistaken for other diseases and disorders (such as herbicide injury). For this reason, we recommend you send symptomatic plants to a diagnostic laboratory to confirm the problem before you implement a disease management program. Obtaining an accurate diagnosis will allow you to determine the best management strategies for your soybean field.

Diseases and Disorders with Similar Symptoms

Frogeye leaf spot can be confused with similar diseases and disorders. We describe the most common of these below.

Diseases

Phyllosticta Leaf Spot (Phyllosticta sojicola)

Phyllosticta leaf spot lesions, if not formed on the edge of the leaf, appear similar to frogeye leaf spot lesions (Figure 10).

How to distinguish Phyllosticta leaf spot from frogeye leaf spot:

Phyllosticta leaf spot lesions can have dark specks (pycnidia) that develop in the center of lesions. Frogeye leaf spots do not contain dark specks but instead may have gray fuzzy growth (mycelium and conidia) in the center of the lesions.

Management

Resistant Varieties

There are soybean varieties with frogeye leaf spot resistance. However, varieties marketed as resistant may not be completely resistant to the disease (known as partial resistance). To date, the resistance gene known as Rcs3 has been effective against all races of this fungus known to occur in North America (resulting in a high level of resistance).

Information about specific resistance genes in a variety may not be readily available, so you should consult seed dealers to help locate varieties with the Rcs3 gene. Also, planting high-quality, certified seed will reduce the risk of introducing infected seed into a field.

Crop Rotation and Tillage

Because the fungus that causes frogeye leaf spot survives on residue, you should follow practices that encourage residue to decompose. This will reduce the amount of the fungus available to infect the next soybean crop.

Rotations to a nonhost crop such as corn, small grains, or grain sorghum will help reduce residue; however, long rotations may be necessary if the disease has been severe in a particular field. Tillage will help break up residue and reduce the amount of fungus for the subsequent crop.

Foliar Fungicides

Well-timed foliar fungicide applications can effectively control frogeye leaf spot. Researchers have reported that foliar fungicides applied during the pod development stages (R3-R4) are the most effective for managing frogeye leaf spot and protecting against yield reductions.

Foliar fungicide efficacy guides are updated annually by the NCERA-137 soybean disease working group. For a current fungicide list, see Fungicide Efficacy for Control of Soybean Foliar Diseases (CPN 1019).

Scout your fields and note other risk factors to determine if you need to apply a foliar fungicide. Factors that can affect risk of frogeye leaf spot are:

• Variety susceptibility — frogeye leaf spot is more likely to develop to economically damaging levels on susceptible varieties.

• Cropping history — fields in short rotations or con- tinuous soybean production will be at higher risk for developing frogeye leaf spot. These fields may be more likely to benefit from a fungicide.

• Environmental conditions — warm, humid weather with frequent moisture and heavy dews favor disease development. Irrigated fields (especially with overhead irrigation) will be at a greater risk for disease development.

While these factors can help you decide whether to apply foliar fungicides to manage frogeye leaf spot, the decision to apply depends on the farmer. There is not a set threshold for foliar diseases of soybean (including frogeye leaf spot), but you should consider growth stage, disease level, and variety susceptibility.

Yield response from fungicides applied to varieties with the Rcs3 gene is much lower than applications to susceptible varieties in trials with severe frogeye leaf spot pressure.

Fungicide Resistance Management

The genetic diversity of the frogeye leaf spot fungus is a key reason why fungicide-resistant isolates can be selected. Resistance to quinone-outside inhibiting (QoI/strobilurin) fungicides has been reported in the frogeye leaf spot pathogen in North America.

It is 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, and always consider the risk factors listed above before you apply a fungicide in order 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 percent control on a susceptible variety.

If you believe fungicide resistance may be an issue in your field, contact your local extension specialist.

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Acknowledgments

Authors

Carl Bradley, University of Kentucky; Martin Chilvers, Michigan State University; Anna Freije, Purdue University; Loren Giesler, University of Nebraska; Daren Mueller, Iowa State University; Ed Sikora, Auburn University; Adam Sisson, Iowa State University; Damon Smith, University of Wisconsin-Madison; Albert Tenuta, OMAFRA; and Kiersten Wise, Purdue University.

Reviewers

Tom Allen, Mississippi State University; Gary Bergstrom, Cornell University; Alyssa Collins, Penn State University; John Damicone, Oklahoma State University; Anne Dorrance, Ohio State University; Nick Dufault, University of Florida; Travis Faske, University of Arkansas; Heather Kelly, University of Tennessee; Nathan Kleczewski, University of Delaware; Dean Malvick, University of Minnesota; Hillary Mehl, Virginia Tech University; Lindsey Thiessen, North Carolina State University; and Trey Price, LSU Ag Center.

All photos were provided by and are the property of the authors and reviewers.

Sponsors

The Soybean Disease Management series is a multi-state collaboration sponsored by the North Central Soybean Research Program (NCSRP) through the Soybean Checkoff. This project was funded in part through Growing Forward 2 (GF2), a federal-provincial territorial initiative. The Agricultural Adaptation Council assists in the delivery of GF2 in Ontario. The authors thank the United States Department of Agriculture - National Institute of Food and Agriculture, the Grain Farmers of Ontario, and United Soybean Board for their support. Contributors to this series come from land-grant universities in the North Central states and Canada. 

This information in this publication is only a guide, and the authors assume no liability for practices implemented based on this information. Reference to products in this publication is not intended to be an endorsement to the exclusion of others that may be similar. Individuals using such products assume responsibility for their use in accordance with current directions of the manufacturer.