How to Treat Cercospora Leaf Blight in R2 Soybeans?

In soybeans, Cercospora leaf blight (CLB) is a common disease caused by the fungus Cercospora kikuchii, which also causes purple seed stain. Chemical treatments and optimized cultural management practices may help in certain circumstances.

Symptoms and Identification of Cercospora Leaf Blight in Soybeans

The OSU Extension has a Foliar Diseases of Soybeans Fact Sheet, which clarifies, “Foliar symptoms begin to appear at the beginning of pod fill and develop initially as small, reddish-purple, angular to irregular lesions on the upper leaf surface.”

The guide goes on to describe, “As the disease progresses, the infected leaves become leathery, and the upper surface of the leaf develops dark purplish-red to bronze discoloration. Heavily infected leaves rapidly turn yellow and drop from the plant, mimicking natural leaf fall. Lower leaves remain green and attached to the plant.”

Foliar Fungicides to Control Cercospora Leaf Blight

The University of Missouri explains, “A foliar fungicide application may be applied in the early pod development stage (R2-R4). However, this has only shown limited results.”

In the Crop Protection Network's guide Fungicide Efficacy for Control of Soybean Foliar Diseases, you’ll see that no fungicides have the rating “Very Good” when it comes to treating Cercospora leaf blight.

The guide points out, “Cercospora leaf blight efficacy relies on accurate application timing, and standard R3 application timings may not provide adequate disease control. Fungicide efficacy may improve with earlier or later applications; however, efficacy has been inconsistent with some products.”

Fungicides with the rating of “Poor to Good” include:

• flutriafol 11.8% (Topguard 1.04SC®)
• tetraconazole 20.5% (Domark 230ME®)
• azoxystrobin 18.2% and difenoconazole 11.4% (Quadris Top 2.72SC®)
• azoxystrobin 19.8% and difenoconazole 19.8% (Quadris Top SB)
• benzovindiflupyr 2.9%, azoxystrobin 10.5%, and propiconazole 11.9% (Trivapro®)
• cyproconazole 7.17% and picoxystrobin 17.94% (Aproach Prima 2.34SC®)
• flutriafol 19.3% and fluoxastrobin 14.84% (Fortix SC® and Preemptor SC®)
• pydiflumetofen 6.9% and difenoconazole 11.5% (Miravis Top 1.67SC®)
• pyraclostrobin 28.58% and fluxapyroxad 14.33% (Priaxor 4.17SC®)
• pyraclostrobin 28.58%, fluxapyroxad 14.33%, and tetraconazole 20.50% (Priaxor D 4.17SC® 1.9SC®)

Fungicides with the rating of “Fair” include:

• cyproconazole 8.9% (Alto 100SL®)
• thiophanate-methyl (Topsin-M®)
• azoxystrobin 7.0% and propiconazole 11.7% (Quilt 1.66SC®)
• azoxystrobin 13.5% and propiconazole 11.7% (Quilt Xcel 2.2SE®)
• trifloxystrobin 32.3% and prothioconazole 10.8% (Stratego YLD 4.18SC®)
• tetraconazole 7.48% and azoxystrobin 9.35% (Affiance 1.5SC®)

The guide also cautions, “Fungicides with a solo or mixed QoI or MBC mode of action may not be effective in areas where QoI or MBC resistance has been detected in the fungal population that causes Cercospora leaf blight.”

Cultural Practices to Manage Cercospora Leaf Blight in Soybeans

N.C. Cooperative Extension explains, “Crop rotation systems incorporating non-host plants such as corn or small grains can reduce inoculum in crop residue and disrupt the disease cycle. Tillage is also an option to reduce inoculum levels in the field.”