White mold, caused by the fungus Sclerotinia sclerotiorum, is not common every year in this region, but farmers that have battled the disease in the past will want to assess the risk of white mold development as soybeans approach flowering (growth stage R1 – plants have at least one open flower at any node).
“White mold development is favored by cool, cloudy, wet, humid weather at flowering,” says Damon Smith, assistant professor of plant pathology at the University of Wisconsin-Madison. “The disease is more problematic in soybeans in high-yield environments where high plant populations, narrow row spacing, and an early-closing canopy are commonly used.”
In the temperate north central soybean production areas of the United States, white mold can be a significant yield limiting disease. Yield can be reduced two-to-five bushels per acre for every 10 percent increment increase in white mold incidence in soybeans at the R7 growth stage. These impacts on yield are significant and make white mold one of the most important diseases of soybean in the north central United States.
White mold can survive for many years in soils; therefore it is difficult to use crop rotation to control it. In soybeans, infection occurs via flowers during bloom. Incidence of white mold can be sporadic from one year to the next, and one field to another, because of specific environmental requirements necessary for infection. White mold incidence is often greater in fields with high yield potential resulting in a dense canopy and in situations where plants are in narrow rows and at high population. In these instances, canopy humidity and wetness can be high thereby promoting increased incidence of white mold.
“Management of white mold can include reduced tillage, crop rotation, canopy management, irrigation management, weed control, and chemical control,” says Smith. “Chemical control of white mold is variable.”
There are fungicides available for in-season management of white mold, however not all commonly used fungicides are labeled for use against white mold in soybean. For information on which fungicides are labeled for disease control and recommendations on fungicide efficacy, visit fyi.uwex.edu/fieldcroppathology/files/2013/07/2014-Soybean-Fungicide-efficacy-table.pdf
Fungicide recommendations are developed by the NCERA-137 national soybean disease committee, and recommendations are based on replicated research data collected from university trials.
“Last year, in Wisconsin numerous products were evaluated for white mold control in soybean,” Smith says.
Results of this trial can be viewed on the summary found at fyi.uwex.edu/fieldcroppathology/files/2013/11/2013-Fungicide-Test-Summary.pdf. Scroll down to pages 6 and 7.
“Consistent with results of the NCERA-137 research, our Wisconsin research identified several products having a rating of ‘good’ for white mold management, including Aproach, Endura, and Proline,” Smith says. “If using fungicides for white mold management, keep in mind that efficacy may be based on the ability of the fungicide to penetrate into the canopy, and the timing of the fungicide application.Fungicides will be most effective at reducing the impact of white mold when applied at, or close to, growth stage R1.”
Wisconsin research data indicates that fungicides applied up to growth stage R3 (early pod – pods are 3/16-inch long at one of the four uppermost nodes) may be effective, but later applications will likely not be effective at reducing disease. Once symptoms of white mold are evident, fungicides will have no effect on reducing the disease. Fungicide applications for white mold management may be most useful on fields where varieties rated as susceptible to white mold are planted in a field with a history of the disease.
“If a soybean field is diagnosed with high levels of white mold, this field should be harvested last,” Smith says. “This will help reduce the movement of the survival structures of the white mold fungus by harvesting equipment, to fields that are not infested. Also, be sure to clean all harvesting equipment thoroughly at the end of the season to avoid inadvertent infestation of fields.”
Read more on soybean white mold at fyi.uwex.edu/fieldcroppathology/2014/07/01/managing-white-mold-in-soybean. There is also a University of Wisconsin Cooperative Extension video that shows symptoms of white mold and discusses management options for the disease. The video can be found on YouTube at www.youtube.com/watch?v=X2qFTJyX_P0.