1. International Institute of Tropical Agriculture (IITA), PMB 5320, Ibadan, Nigeria, and Department of Crop and Soil Sciences, Kwame Nkrumah University of Science and Technology (KNUST), Kumasi, Ghana
2. IITA, PMB 5320, Ibadan, Nigeria
3. Department of Crop and Soil Sciences, KNUST, Kumasi, Ghana
4. IITA, Chitedze Research Station, P.O. Box 30258, Lilongwe 3, Malawi
5. United States Department of Agriculture, Agricultural Research Service, School of Plant Sciences, University of Arizona, Tucson, AZ 85721