Managing Frogeye Leaf Spot in Soybeans

Categories: GROWING, SOYBEANS
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  • Frogeye leaf spot (FLS) in soybeans can be easily mistaken for other diseases or herbicide injury. 
  • There is currently no economic threshold in place for frogeye leaf spot management; it is largely discretionary based on a few key factors.   
  • Confirmed cases of resistance have grown rapidly in recent years, making it imperative to be conscientious about fungicide choice. 

Signs, Symptoms and Diagnosis
Frogeye leaf spot (FLS), caused by the fungus Cercospora sojina, produces lesions mostly found in the upper canopy of soybean plants. They begin as small, circular and dark water-soaked marks on the tissue, but will become larger, more angular, and lighten in color throughout the season, fading from gray, to brown, to tan, and surrounded by a thin, purple margin. In some late-season cases, lesions can also be found on the pods and stems, where they will sometimes appear more oblong (pods) and elongated (stems) than on the leaves. Severe cases on pods may cause infection in seeds and sometimes, but not always, cause a purple or gray discoloration. When conditions are right, fungal sporulation (spore formation) occurs, adding a gray and fuzzy appearance to the undersides of the lesions. In advanced cases, coalescence of the lesions may cause a blight of the leaves. Defoliation will result when the disease reaches its greatest severity. 

FLS can be easily mistaken for similar looking diseases and plant injuries, such as herbicide burn. In order to get the most complete and accurate diagnosis, it is recommended that a symptomatic sample be sent to a lab for official verification. The following conditions are often confused with FLS, but they can be distinguished: 

  • Phyllosticta leaf spot: Small black specks of fungi can form inside the older lesions but will not be present in FLS. 
  • Target spot: Most common in southern states, target spot secondary lesions lack a yellow halo. “Target zone” like rings can appear similar to FLS, but purple lesion margins will be absent.     
  • PPO herbicide injury: Herbicide injury is easily visible across larger, uniform areas following spray boom widths and new growth is typically unaffected. 
  • Paraquat herbicide injury: Differentiated from FLS by having healthy new growth on the plant, spray patterns are easily observed, and similar injury is found on weeds and other plants within the canopy. 

Disease Cycle and Conducive Conditions
Hosts of FLS fungus include infested soybean residue, with initial research suggesting some weeds, cover crops and other legumes as potential hosts as well. The disease is spread via wind and water splashing the spores onto nearby plants, and in very rare instances, through infected seed. Wind dispersal can carry the disease into fields beyond the ones initially infected. Any stage of soybean growth can be at risk, but infection is most prevalent from R1-R7 (flowering through early maturity), mostly impacting the upper canopy of the crop. The most likely conditions to contribute to infection include frequent bouts of precipitation and/or overhead irrigation, periods of overcast skies lasting a few days or more, and warm, humid weather occurring for extended periods of time. Fields that are continuously soybean, have short rotations between soybean crops, are conservation or no-tillage, are planted with a susceptible variety, or have a history of FLS are also at a greater risk for infection.          

Yield Potential Impact
​​​​​​​
Minimal or no yield potential impact: 

  • Low disease severity.
  • Disease occurs in reproductive stages (post R5.5).
Yield potential losses up to 35%: 
  • Early disease outbreak (before or just after flowering).
  • Favorable environmental conditions.

Management
There are several proactive and reactive approaches to managing FLS. Most notably, this includes planting a soybean variety containing the Rcs3 gene, which is the only gene currently available that is resistant to all strains of the FLS fungus. A variety labeled “resistant” without the presence of the gene is only partially resistant to the disease, not providing full coverage to all strains in existence. Residue management is another tool that can be used to help mitigate future impacts of a previously infected field. Tillage and crop rotation are viable options to consider that help break up and lessen overwintering host residues in the field, thereby reducing the risk of future infections.  

A foliar fungicide application, when applied at the proper stages of R3-R4, can be effective in controlling FLS. Golden Harvest® recommends Miravis® Top fungicide at 13.7 oz/acre rate for the most optimal control of FLS, providing additional coverage against Cercospora leaf blight and pod and stem blight. Trivapro® broad-spectrum fungicide is also rated for control of FLS and may be considered as a management option. It is important to keep in mind that there is no official economic threshold for treating this disease.   

Environmental conditions, field susceptibility and disease severity should all be considered when choosing a management plan for your field.  

Resistance
Since 2010, resistance among almost all strains of FLS toward the Qol (Group 11) class of fungicides has continued to increase substantially. Strobilurins are the most well-known type of fungicide to fall into this class. As of February 2020, the states with confirmed resistance include AL, AR, DE, IL, IN, IA, KY, LA, MS, MO, NE, NC, MI, MN, OH, TN and VA. When resistance is found in a field, it is assumed to be widely present across the growing area, and management plans should be adjusted accordingly to avoid Group 11 chemistries in the next application. Miravis Top fungicide is approved for use and effective against strobilurin-resistant FLS infections. 

For more information on managing FLS, contact your local Golden Harvest Seed Advisor.  

Photos are either the property of Syngenta or used under agreement.  
Syngenta hereby disclaims liability for third-party websites. 


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