1. United States Department of Agriculture, Agricultural Research Service, R. B. Russell Agricultural Research Center, Athens, GA 30605.
2. Corresponding author. Phone: (706) 546-3374. Fax: (706) 546-3607. E-mail: msohn@qaru.ars.usda.gov
3. Department of Crop and Soil Sciences, Penn State University, University Park, PA 16802.