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August, 13, 2004

White Mold and Sudden Death Syndrome Widespread in Soybeans

Soybean diseases are taking their toll across much of eastern Iowa. White mold (Sclerotinia stem rot) is especially widespread north of I80 and soybean sudden death syndrome is common south of I80. Both diseases showed up earlier than normal this year because of the cool weather. The wet weather earlier in the season resulted in the infection of the soybeans with the plant pathogens that cause these diseases.

White mold is caused by the fungus Sclerotinia sclerotiorum, which overwinters in the soil and plant residue as sclerotia. Sclerotia are dark, hard structures that look like mouse droppings. Infection of the soybeans occurs during flowering and is favored by cool temperatures and high soil moisture. Infected plants now have a mass of cottony white fungal mycelium and dark sclerotia on the stem. The herbicide Cobra and several fungicides are labeled for white mold, but these need to be applied during early flowering to be effective. There is nothing that can be done now to fields that have white mold. Tactics to reduce the problem in the future include; clean harvest equipment between fields or harvest white mold infected fields last, plant tolerant soybean varieties, and plant in wider rows.

Soybean sudden death syndrome is caused by the fungus Fusarium solani, which overwinters in soil and plant residue. Cool, wet conditions early in the growing season result in greater levels of plant infection. Symptoms usually do not show up until early August, when leaves show yellowing and death between the veins, with the veins remaining green. Often it occurs in fields where soybean cyst nematode is also present. It is sometimes confused with brown stem rot. To distinguish it from this disease, split the stem lengthwise. With brown stem rot the center (pith) of the stem is brown. With sudden death syndrome the pith is white but the rest of the stem is discolored. There is nothing that can be done now for fields with sudden death syndrome. It is common to see more of the disease in parts of the field that have poorer drainage or more soil compaction, such as near the edge of fields. Tactics to reduce the problem in the future include; plant more tolerant varieties, plant later, improve soil drainage, and use practices that reduce soybean cyst nematode. For more information about white mold and sudden death see“ Soybean Sclerotinia Stem Rot” (Pm-1519) at http://www.extension.iastate.edu/Publications/PM1731.pdf and the August 2 2004 ICM Newsletter at http://www.ipm.iastate.edu/ipm/icm/2004/8-2-2004/plantdis.html.

 

Late Summer Seeding of Forages

The target date to have forages seeded by for the best chance of establishment in central Iowa is August 20. Since most common weeds don't germinate well this late in the season, we do not recommend a companion crop or pre-plant herbicide. But, keep watch just in case a flush of weeds occurs. It may warrant a postemergence herbicide. The warm soil temperatures will get alfalfa off to a quick start, but still make sure you achieve good seed-to-soil contact when planting. For more information see the handout "Late Summer (Fall) Seeding of Forages in Iowa" or
http://www.exnet.iastate.edu/Pages/eccrops/latesummerforageseed.html.

 

If you have any questions, please feel free to contact the Iowa State University Extension Office.
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Last Update: August 13, 2004
Contact: Jim Fawcett fawcett@iastate.edu


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