MU Extension Plant Pathologist: Don't Treat All Soybean Diseases With Fungicides

Part 2 Of A 2 Part Series

   Sudden death syndrome (SDS) often looks like other diseases such as stem canker or brown stem rot. Unlike those diseases, however, the stems of soybean infected with the SDS fungus remain green. First symptoms of SDS are yellowing (chlorosis) and tissue death (necrosis) between leaf veins resulting from the movement of a toxin from the roots into the leaves. Because infection is in the roots, the disease will not respond to foliar fungicide. Choose resistant seed varieties and rotate crops between soybean and small grains or other non-host crops. Plant into warm soils and improve soil drainage.
   Soybean cyst nematode (SCN). This microscopic roundworm historically has been controlled with resistant varieties, most of which contain genes from the PI 88788 resistance source. However, the nematode has begun to develop resistance to these genes. In Missouri, two other resistant sources are available: Peking and Hartwig. Bissonnette suggests testing soil for SCN nematode numbers. Test in the fall after harvest or in the spring before planting.
   “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 for more information. Rotating resistant varieties as well as non-host crops such as wheat and corn is suggested to manage SCN.
   Charcoal rot. This late-season disease appears in dry years. It overwinters in the soil, so it is important to scout fields with a known history of charcoal rot. Light gray discoloration and premature yellowing of the leaves occurs. Leaves also are smaller than normal. Fungicides do not help.
   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 your county MU Extension center are available to help. You may also send plant samples to the MU Plant Diagnostic Clinic. Call 573-882-3019 or visit 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 ∆
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