Not all soybean diseases require a fungicide application, says University of Missouri Extension plant pathologist Kaitlyn Bissonnette.

Knowing if and when to spray fungicides saves money and reduces concerns about fungicide resistance, Bissonnette says.

She shares information about soybean diseases in a series of sessions on disease scouting throughout the state. Missouri Soybean Merchandising Council and MU Extension sponsor the sessions.

Timing, method and rate of application determine fungicide efficacy, says Bissonnette. Many common diseases don’t affect yield and don’t warrant treatment.

Always follow label instructions for best results. Fungicides applied too early often lack efficacy, Bissonnette says. Little fungicide may remain on leaves by the time disease shows. Most disease does not require a fungicide application before early pod development (R3 growth stage).

Bissonnette says farmers should check weather patterns and the disease-resistance level of their varieties when making fungicide application decisions. Regular scouting of the field, starting at the edges and walking to the interior areas, provides early detection of pests. Know your diseases. Some are easily identified while others are harder to distinguish from other types of crop damage, she says. An integrated pest management system that includes crop rotation and the use of resistant varieties is always the best approach.

Some require little treatment

Bissonnette gives a brief overview of diseases that seldom require foliar fungicide treatment:

“Know your number,” she says. “Testing tells how many eggs are in the soil and provides insight for future management needs.” In a single season, three to six generations of SCN can appear, with each female containing around 250 eggs. To test for SCN, submit samples to MU’s SCN Diagnostic Lab. Visit scndiagnostics.com for more information. Rotating resistant varieties as well as non-host crops such as wheat and corn is suggested to manage SCN.

Frogeye leaf spot

Bissonnette says frogeye leaf spot (FLS) has been verified in parts of Missouri recently. FLS, which can reduce yield by up to 35 percent, is one disease that can benefit from fungicide treatment. It generally appears in the middle of the growing season. FLS can be anywhere in the soybean canopy, but infection begins in newer leaves. The fungus that causes FLS survives in infected seed and soybean residue. It favors warm, humid weather. Rotate crops and select resistant varieties.

If applying a fungicide to control FLS, Bissonnette recommends using one with multiple modes of action. Before you spray, know if your soybean variety has resistance to FLS and scout for the disease. Ask agronomists in your area if FLS has been reported. Spray when soybean are at the R3 growth stage. “Preventative is always better than curative,” she says.

Agronomists at county MU Extension centers are available to help. You may also send plant samples to the MU Plant Diagnostic Clinic. Call (573) 882-3019 or visit plantclinic.missouri.edu for more information.

Bissonnette also recommends the publication “Determining Fungicide Efficacy” from the United Soybean Board. The publication gives research-based information from several university plant pathologists on the efficacy of many commercially available fungicides toward common soybean diseases. Download at iwilltakeaction.com/resources/fungicide-efficacy-fact-sheet.