An Overview of Cercospora Leaf Blight of Soybean

Introduction

Cercospora leaf blight (CLB), caused by multiple species of Cercospora fungi, is an important soybean disease, particularly in the southern United States. This foliar disease can cause premature defoliation and yield loss. Historically, CLB was attributed to infections by C. kikuchii, but C. flagellaris has been reported to be the dominant species based on a recent sampling of symptomatic soybean plants from a large area stretching from Texas east to Alabama and north to Missouri and Kentucky. The main species causing CLB in other areas is not well understood. Additional species associated with CLB in the U.S. include C. alchemillicola, C. iranica, and C. sigesbeckiae, and a previously unidentified species of Cercospora.

Symptoms and Signs

Symptoms of CLB usually are not observed until beginning seed (R5) or later, and symptoms may not appear at the same time. Small, purplish specks (Figure 1) may be visible on leaflets in the upper canopy. As the disease progresses, the upper canopy of the soybean plant will develop a bronze to purple coloration, and the leaves develop a leathery feel (Figure 2). Purple lesions may develop on the petioles (Figure 3). As the disease progresses, brown necrosis (blighting) may occur on leaflets along with a grayish hue that is attributed to sporulation of the fungi that cause CLB (Figure 4). In severe situations, or on an extremely susceptible variety, petioles, the main stem, and pods can all develop a dark purple coloration. The fungi associated with CLB have also been associated with purple seed stain (Figure 5).

Conditions that Favor Disease

Hot (68 to 86 F) and humid (at least 92.5%) conditions with prolonged dew periods (8 to 24 hours) favor the development of CLB. Rainfall and wind can disperse spores and result in secondary infections (Figure 6). The fungi can also reproduce on additional crop and weed hosts (corn, cotton, pokeweed, ragweed, others) and survive on previous crop residue, which contributes to the inoculum for the subsequent cropping season. Late-planted soybean may have a greater risk of CLB due to the higher inoculum levels being present later in the season. Later planted and later maturing susceptible soybean varieties are more apt to develop CLB. Infected seeds may provide additional inoculum at planting.  

Yield Losses and Impact

Yield losses due to CLB can range from mild to severe. Fields can be completely defoliated under weather conditions favorable for the disease, although this is rare. The annual yield loss estimates compiled by plant pathologists throughout the U.S. soybean production system indicate that the estimated losses occurring between 2013 and 2023 ranged from $46 to $166 million.

Diagnosis

Since multiple symptoms are attributable to CLB and it can be confused with other problems, diagnosis can be challenging in the field. Symptoms can vary with location and variety, further confounding diagnosis. Extension pathologists can help with CLB identification and/or samples can be sent to diagnostic laboratories to confirm the presence of CLB pathogens. 

Diseases, Disorders, and Injury with Similar Symptoms

Cercospora leaf blight can be confused with other diseases and disorders. We describe the most common of these below and in Table 1.

Diseases

Target spot (Corynespora cassiicola) – Target spot infects various soybean plant parts. Initially considered minor, it now causes significant defoliation in susceptible varieties across the southern United States. Lesions are reddish-brown and round to irregular with yellow halos and may contain a concentric ring pattern (Figure 7). Severe infections lead to defoliation, beginning in the lower canopy. Stem and petiole lesions are dark brown and can be confused with CLB lesions (Figure 8).

How to distinguish Target spot from CLB: Petiole lesions caused by target spot generally are first observed in the lower canopy while CLB petiole lesions are initially observed in the upper canopy. Concentric ring patterns in lesions are present in target spot but absent in CLB. The presence of foliar symptoms in conjunction with petiole lesions in the lower canopy indicate target spot. Moreover, lesions associated with target spot tend to be smaller and not confined to the petiole.

Frogeye leaf spot (Cercospora sojina) – Frogeye leaf spot (FLS) is characterized by the small gray lesions with reddish-brown borders on upper leaf surfaces. Lesions start dark and water-soaked, enlarging over time. Severe cases lead to premature leaf drop and can spread to stems and pods. The fungus that causes frogeye leaf spot may also cause purple petiole lesions (Figure 9) and infect seed. 

How to distinguish frogeye leaf spot from CLB: The foliar phase of FLS is usually severe and easily distinguishable from CLB when FLS petiole lesions occur. The seed stain caused by C. sojina (the causal agent of FLS) is light purple to gray, while seed infection by CLB pathogens results in a royal purple color. 

Injury

Chloride toxicity – Chloride toxicity symptoms in soybeans include leaf tip scorching, premature yellowing, bronzing of leaves, and leaf loss that may resemble CLB.

How to distinguish chloride toxicity from CLB:  Chloride toxicity injury typically manifests along leaflet margins as chlorosis and necrosis, while CLB can occur anywhere on leaflets.   

Sunburn – Sunburn, also known as sunscald, appears as brick-red discoloration between veins of soybean leaves. This condition primarily affects the lower leaf surface, distinguishing it from CLB, which causes discoloration on the upper leaf surface. Injury occasionally can be seen on pods.

How to distinguish sunburn from CLB: Sunburn is typically on the lower leaf surface, while CLB is predominately on the upper leaf surface.

Table 1. Symptom distribution and expression for diseases and disorders with symptoms similar to Cercospora leaf blight.

Management

Soybean varieties with resistance to CLB may be available, and farmers should plant varieties that have been evaluated near their farm. Since the fungi that cause CLB can overwinter on soybean residue, rotation and/or tillage may reduce disease incidence.

Managing CLB mainly relies on using foliar fungicides from multiple fungicide classes, and applying them at appropriate reproductive growth stages. However, QoI (Group 11) and MBC (Group 3) fungicides are likely no longer effective in managing CLB since widespread QoI resistance within the CLB pathogen populations has been confirmed in eight states. However, some SDHI (Group 7) and DMI (Group 3) premixes applied from beginning pod to beginning seed (R3-R5) using appropriate rates and water volumes (10-15 gallons per acre) are still effective. Please refer to the most recent Fungicide Efficacy for Control of Soybean Foliar Diseases for more information.

Acknowledgements

Authors

Trey Price, LSU AgCenter; Tom Allen, Mississippi State University; Vinson Doyle, LSU AgCenter; Travis Faske, University of Arkansas; Daren Mueller, Iowa State University; Boyd Padgett, LSU AgCenter; Adam Sisson, Iowa State University; Terry Spurlock, University of Arkansas; Sara Thomas-Sharma, LSU AgCenter; Chris Ward, Louisiana Consultant; and Tessie Wilkerson, Mississippi State University.

Reviewers

Gary Bergstrom, Cornell University; Martin Chilvers, University of Michigan; Maira Duffeck, Oklahoma State University; David Langston, University of Virginia; Dean Malvick, University of Minnesota; Richard Webster, North Dakota State University; and Kiersten Wise, University of Kentucky.

Sponsors

Many Cercospora leaf blight management projects have been funded by the United Soybean Board, Mid-South Soybean Board, and respective state commodity boards. The authors thank soybean farmers across the nation for their support.