Based on foliage and what I have been observing, this may be stem canker. Stem canker symptoms have exploded in some of our plots since last friday. Look for a range of symptoms associated with plants with wilted and declining leaf health. Some plants may not show symptoms at all, probably will appear in a few days, to classical symptoms of 2-3 inch grayish to brown lesion at one lower node, to entire stem showing a brown discoloration and not traceable to a specific node.
Brown stem rot is in the mix, so both external stem lesions and brown pith tissues can be observed among plants.
We are observing strong differences among soybean varieties. See the following for more information on stem canker: http://www.plantpath.wisc.edu/soyhealth/stemcanker.pdf