Soybean Fungicides for Frogeye Leaf Spot at R3?

Frogeye leaf spot or FLS is a foliar disease caused by the fungus Cercospora sojina. The disease can become particularly problematic in systems where soybean is grown continuously. During the reproductive (R) stages of plant development is when symptoms of the disease are often most noticeable.

1. Identify the Symptoms of Frogeye Leaf Spot

The first step of proper treatment is to confirm that FLS is truly what you’re dealing with since FLS can easily be mistaken for other diseases including Phyllosticta leaf spot, PPO herbicide injury, and paraquat herbicide injury.

Consult scouting guides from a local Extension to see photographic examples of FLS lesions, which can range from being small and dark to larger and brown, gray or even purple.

2. Apply the Right Fungicides Prudently

Reference the most up-to-date fungicide efficacy ratings for control of soybean foliar diseases. To avoid resistance, opt for a mixed mode of action or rotate which products you spray if multiple applications are needed throughout the same growing season.

Some active ingredients that have been shown to be very good at killing frogeye leaf spot include:

• Flutriafol 11.8% (Topguard 1.04SC)
• Prothioconazole 41.0% (Proline 480SC)
• Thiophanate-methyl (Topsin-M) 

Effective mixed modes of action include:

• Azoxystrobin 25.3% and flutriafol 18.63% (Topguard EQ 4.29SC)
• Azoxystrobin 18.2% and difenoconazole 11.4% (Quadris Top 2.72SC)
• Azoxystrobin 19.8% and difenoconazole 19.8% (Quadris Top)
• Flutriafol 26.47% and bixafen 15.5% (Lucento 4.17SC)
• Pydiflumetofen 7.0% and azoxystrobin 9.3% and propiconazole 11.6% (Miravis Neo)
• Thiophanate-methyl 21.3% and tetraconazole 4.2% (Acropolis)
• Mefentrifluconazole 11.61% and Pyraclostrobin 15.49% and Fluxapyroxad 7.74% (Revytek)

In general, fungicides to treat FLS tend to be the most effective when applied during the growth stages R3, R4, or R5.

3. Adjust Management Practices

To help control frogeye leaf spot in the long-run, carefully consider seed selection and crop rotation.

When purchasing soybean seeds in the future, be on the lookout for varieties that are resistant to FLS, which will get you started off on the right foot.

You may consider introducing picks like wheat, corn, and sorghum, which can minimize issues with continued FLS since these crops don't host the fungal disease.