By XB Yang and S.S. Navi, Department of Plant Pathology
Soybean white mold has been a production problem for soybean producers since early 1990s. White mold outbreaks often occur in even years due to crop rotations, with rare severe occurrences in odd years. Iowa's cool, wet summer has increased the white mold risk for some growers in eastern Iowa — even though it is an odd year. Extension field agronomist Virgil Schmitt first reported the occurrence of white mold in east central Iowa and Jim Fawcett, northeast Iowa field agronomist, has received a report of severe white mold in a 60-acre field.
Although many Iowa soybean producers are experienced with this disease, there are questions when it shows up in mid-August. Some are asking if applying chemicals is a solution. Others want to know if an immediate application of a fungicide can stop the infestation of this disease.
The short answer is that it is too late to use any fungicides for white mold control. It wastes resources if fungicides are applied now. White mold fungus attacks soybean plants during flowering stage and treatments to protect soybean have to be made before or during the flowering period, depending on chemicals used. The dead plants visible now are the results of fungus infestation that happened before mid-July.
Management
If you have severe white mold in your soybean fields, consider no-tillage if corn is a rotation crop. Another consideration is to use Contan after soybean harvest. Contan is a biological control agent proven to be effective in white mold control in many crops. A tolerant variety of soybean should be planted in this location during the next rotation or a consideration given to using Cobra, a herbicide that can reduce white mold infection if applied correctly. There are other chemicals in the same family that are effective in reducing the risk of this disease.
Soybean field showing wilt symptoms due to white mold.
Mycelia (a) and sclerotia (b) formation on white mold infested soybean plant.
XB Yang is a professor of plant pathology with research and extension responsibilities in soybean diseases. Yang can be reached at (515) 294-8826 or by emailing xbyang@iastate.edu. SS Navi is an assistant scientist working on soybean diseases.
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