1. Plant Genetics Research Unit, Agricultural Research Service, United States Department of Agriculture, Columbia, Missouri 65211, Plant Science Division, University of Missouri, Columbia, Missouri 65211, Soybean Genomics and Improvement Laboratory, USDA ARS, 10300 Baltimore Avenue, Beltsville, Maryland 20705, and USDA Agricultural Research Service, Soybean/Maize Germplasm, Pathology, and Genetics Research Unit, Department of Crop Sciences, University of Illinois, Urbana, Illinois 61801