Managing Frogeye Leaf Spot in Soybeans

As summer temperatures and humidity increase, so do frogeye leaf spot (FLS) sightings in soybeans. The foliar disease, which is caused by the fungus Cercospora sojina, will likely show this year if you had it last year. If FLS appears, know which signs of disease development to watch and which management practices to use. 

Development Conditions
FLS typically develops between temperatures of 77 to 86° F with prolonged dew and humidity at 90% or above. These conditions produce spores that spread by rain and wind. Continuous soybean fields and areas with considerable residue favor disease development.  

Disease Identification
Mature FLS lesions show dark red-brown or purple rings, as pictured below. If lesions are still immature, there are no outside rings. 

Yield Impact and In-Season Risks
The United Soybean Board estimates U.S. soybean farmers lost an average of 7.2 million bushels annually from 1996 to 2014 as a result of FLS. Susceptible varieties have reported 20 to 30% yield loss. Resistant varieties typically yield better, but you should anticipate potential FLS pressure to make informed management decisions. Though FLS fungicide treatment thresholds haven’t been determined, the United Soybean Board developed a foliar disease risk chart to help forecast pressure. Once at-risk soybean fields are identified, monitor to see if a fungicide application is needed for extra control.
Management Control Practices
  • Cultural:
    • If your fields have a severe history of FLS, rotate to a crop other than soybeans. If planting soybeans is a must, bury infested residue with tillage to reduce inoculum. 
  • Resistant varieties:
    • Selecting resistant varieties is most important for reducing FLS. Choose soybean varieties with an FLS rating of 3 or lower in continuous soybean fields, and 4 or lower in a soybean-corn rotation. The Golden Harvest soybean product finder outlines FLS tolerance ratings for each variety.
  • Fungicides:
    • If an FLS foliar fungicide treatment is needed, avoid using a single mode of strobilurin since multiple states have confirmed resistance. Quadris Top® SBX and Miravis Top® contain multiple modes of action with preventative and curative disease control. Apply between the R1 to R3 soybean stages for optimal results. 
Contact your local Golden Harvest Seed Advisor with questions or for additional agronomic insights.

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