The most important diseases of Musa spp. in Cameroon, as in many other banana-producing areas, are those caused by Mycosphaerella musicola and M. fijiensis. Investigations during 1988 and 1989 in Cameroon in areas under high disease pressure revealed that Yangambi was highly resistant to M. fijiensis. During field observations between September 1992 and September 1994, necrotic symptoms were recorded on Yangambi from October to February in 1992-93 and 1993-94 and from June to September in 1994 at elevations of 700 to 1,100 m. In 1995, perithecia were recorded from necrotic tissues between May and November at 80 m, between September and October at 700 and 900 m, and between October and November at 1,100 m; mean number of perithecia per mm2 of samples was 8.6, 10.6, 17.6, and 17.9 at 80, 700, 900, and 1100 m, respectively. No viable ascospores was isolated from them. In 1997, this variation in the behavior of Yangambi was confirmed at 1,250 m elevation with necrosis between late August and early December, and ascospores of M. fijiensis were isolated from samples collected during October. Appearance of necrosis occurs when optimal climatic conditions for the development of M. fijiensis are obtained. This first and complete shift in pathogenicity of M. fijiensis to overcome the resistance of Yangambi is the end of a process that started 5 years ago after almost 8 years' exposure of the cultivar in the same locality.
