[1] Department of Plant Pathology, University of the Orange Free State, Bloemfontein 9300, South Africa
[2] CIMMYT, Lisboa 27, Apdo. Postal 6-641, Mexico D.F, Mexico
[3] NAARI, P.O. Box 7084, Kampala, Uganda
[4] CIMMYT, P.O. Box 5689, Addis Ababa, Ethiopia