Historical overview of breeding for durable resistance to maize streak virus for tropical Africa

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Abstract
Maize streak virus (MSV) was first studied by Storey in Kenya in the 1920s and resistance transferred into East African maize in the 1960's. Yet the majority of farmers of East Africa are still growing streak-susceptible maize. At the International Institute of Tropical Agriculture (IITA) in Nigeria, an effort to incorporate streak resistance as an integral part of a holistic maize improvement program was begun in 1975. By 1979, usable resistance had been found and methods of large scale field challenge devised, and used routinely in the development of populations targeted at three major ecological zones in tropical Africa. Methods were continuously improved and resistant open pollinated varieties and hybrids produced that are now used on a large scale by farmers in West Africa. The strategy for success was based on: 1) Avoidance of virus strain specificity; 2) Large scale vector rearing and field infestation to ensure early infection; 3) Selection only for tolerance when infected; and 4) Not separating virus resistance selection from simultaneous selection for yield, adaptation, and resistance to other diseases important in each target ecology. The result was maize genotypes that exhibit low field incidence (tolremicity) and are tolerant (yield well even if diseased). The approach and methods have been extended to many interested national maize improvement programs and adopted by them. We believe that this form of tolerant resistance remains effective wherever deployed in Africa and thus that it has not ‘broken down’.