1Department of Plant Production and Soil Science, Faculty of Natural and Agricultural Sciences, University of Pretoria, P. Bag X20 Hatfield, Pretoria, 0028. South Africa.
2International Maize and Wheat Improvement Centre (CIMMYT), Southern Africa Regional Office, P. O. Box MP 16, Mount Pleasant, Harare, Zimbabwe.
3International Crops Research Institute for the Semi-Arid Tropics, Matopos Research Station, P. O. Box 776,
Email: [email protected]