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Soybean Leaf Spot: Frogeye & Septoria

The warm, rainy conditions that help soybeans thrive are the same conditions favored by frogeye leaf spot and Septoria leaf spot. Be sure to monitor fields for signs of these diseases as the summer heat gets turned up. 

Controlling Frogeye Leaf Spot
Reports of frogeye leaf spot in soybeans emerge quickly as humidity and temperatures increase. Frogeye leaf spot is a foliar soybean disease caused by the fungus Cercospora sojina and can be identified by the dark, red-brown or purple rings around lesions. Severe defoliation can occur when lesions merge and cover large leaf areas.

To help control frogeye leaf spot, apply a fungicide such as Quadris Top® SBX between R1 to R3 stages for the most robust broad-spectrum preventive and curative disease control. 

If you have frogeye leaf spot this year, it will likely make another appearance next year. When planning for next season, select Golden Harvest varieties with a frogeye leaf spot rating of 3 or lower in continuous soybean fields, and 4 or lower in a soybean-corn rotation. Burying infested residue and rotating soybeans with a non-bean crop will also help reduce inoculum levels.

Managing Septoria Leaf Spot
The same warm, wet weather that favors frogeye leaf spot also creates a perfect storm for Septoria leaf spot. Sometimes referred to as “brown spot,” the fungus Septoria glycines is common in soybean residue and spreads from the soil to soybean plants by splashing rain. 

The disease severity at R6 is often predictive of yield at harvest.  Yield loss is most likely to occur in high-yield environments and is related to timing and defoliation rate. When significant defoliation occurs, the University of Illinois Extension has reported yield losses in the 8 to 34% range.

Infection can occur as early as the V2 growth stage on lower leaves and can be identified by irregular, dark brown spots on both upper and lower leaf surfaces. As pictured below, adjacent lesions frequently merge to form irregularly shaped blotches. As symptoms progress, leaves can become rusty brown and may also appear on stems and pods of plants approaching maturity. 

​​​​​​​Source: University of Wisconsin-Extension

Applying a fungicide like Quadris Top SBX between the R3 and R6 growth stages will effectively slow disease development, and promote beans that are larger and fuller with stronger roots. Septoria-resistant varieties aren’t available, and the only weather that can put infection on hold is dry heat. 

When thinking about how to prevent Septoria in the season ahead, select soybean varieties with later relative maturities to help reduce pressure, since symptoms typically appear sooner in the season on early-maturing varieties. Rotating next season’s high pressure fields to corn is another management strategy, as Septoria leaf spot is most severe in continuous soybean fields.

If you’d like to learn more, contact your Golden Harvest Seed Advisor.

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Syngenta hereby disclaims liability for third-party websites.

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