Soybean Fungicides for Septoria Brown Spot at R5?

Septoria brown spot--also known as brown spot, soybean brown spot, Septoria leaf spot, and brown spot of soybeans--is a common fungal foliar disease caused by Septoria glycines.

According to the University of Nebraska-Lincoln, “Yield loss estimates due to brown spot range from 8% to 15% nationally and occur when 25% to 50% of the canopy prematurely defoliates.”

Control options include chemical treatments like fungicides and adjusted management practices including crop rotation and some tillage since brown spot “is a residue-borne disease.”

Disease Progression and Symptoms

The Crop Protection Network explains that Septoria brown spot “is the most common foliar disease of soybean. Disease develops soon after planting and is usually present throughout the growing season.”

Although brown spot may be present during vegetative growth stages such as V2, symptoms of the disease are usually mild during this time. “Infected young plants have purple lesions on the unifoliate leaves. Lesions on later leaves are small, irregularly shaped, dark brown, and are found on both leaf surfaces.”

As the growing season progresses, Septoria brown spot extends upward from the lower canopy to the upper canopy. The University of Minnesota Extension explains, “Septoria brown spot is most common when conditions and leaves are wet and warm.”

Fungicide Treatments and Timing

When applied during R3, R4, or R5, fungicides can help protect yield by slowing the disease's progression into the upper canopy.

For treating brown spot in soybeans, these products have efficacy ratings of very good.

• Cyproconazole 8.9% (Alto 100SL®)
• Flutriafol 11.8% (Topguard 1.04SC®)
• Tetraconazole 20.5% (Domark 230ME®)
• Boscalid 70% (Endura 0.7DF®)
• Azoxystrobin 25.3% and flutriafol 18.63% (Topguard EQ 4.29SC)
• Benzovindiflupyr 2.9%, azoxystrobin 10.5%, and propiconazole 11.9% (Trivapro®)
• Flutriafol 26.47% and bixafen 15.5% (Lucento 4.17SC®)
• Prothioconazole 16.0% and trifloxystrobin 13.7% (Delaro 325SC®)
• Pydiflumetofen 6.9% and difenoconazole 11.5% (Miravis Top 1.67SC®)
• Pyraclostrobin 28.58%, fluxapyroxad 14.33%, and tetraconazole 20.50% (Priaxor D 4.17SC®)
• Tetraconazole 7.48% and azoxystrobin 9.35% (Affiance 1.5SC®)
• Mefentrifluconazole 11.61%, Pyraclostrobin 15.49%, and Fluxapyroxad 7.74% (Revytek12)

According to the Fungicide Resistance Action Committee (FRAC), the pathogen Septoria glycines shows “a medium risk of development of resistance to fungicides.” Hence, best practices should be used to minimize resistance: apply fungicides only when necessary and use a mixed mode of action.

Crop Rotation and Management Practices

The N.C. Cooperative Extension points out, “Since the disease overwinters in crop residue, destroying crop debris is an important control method for continuously cropped soybean fields. Tillage practices that favor rapid breakdown of crop residues are also beneficial to limiting losses from disease.”

Rotating your crops is another sound way to help manage continued problems. “Crop rotation to non-host crops may also help to reduce inoculum build up in the field. Common rotational crops like corn and cotton are non-hosts.”