If the risk of soybean rust development is high and the crop is before growth stage R6, fungicides may be needed. For fungicides to be most effective, they need to be applied as close to the time soybean rust initially infects plants in a field. In most cases, this means that fungicide decisions will have to be made at a time when the risk is high, but before the time when rust is detectable in the field. Detecting low levels of soybean rust (<10 percent) typically requires incubating leaves and observing them under laboratory conditions by a trained diagnostician.
Iowa State University
Mueller, Daren S., "Foliar Fungicides Publication Available" (2008). Integrated Crop Management News. 958.
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