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DNA fingerprinting with synthetic simple repetitive ohgonucleotides such as (CA)s or (CAA), detected polymorphisms between various isolates of the ascomycete Mycosphaerella fijiensis. the causal agent of the black Sigatoka disease of Musa. These microsatellite motifs are present at multiple chromosomal locations and in high copy numbers in the Mvcosphaerclla genome, generating informative fingerprints with low background. Variability exists on a macro-as well as a microgeographical scale: it occurred within one lesion, between lesions of one plant, between plants, cultivars, and geographic locations. Mathematical analysis of the data produced dendrograms that demonstrated the presence of different genetically related groups of Mycosphaerella fijiensis in Nigeria.

Monitoring of banana streak badnavirus (BSV) antigens and symptoms in naturally BSV-infected plantain and banana (Musa spp.) plants showed a great variation in symptom expression, distribution and relative concentration of BSV between and within plants. Expression and distribution of symptoms was erratic within individual leaves as well as between different leaves of the same plant. The concentration of BSV antigens detected by triple antibody sandwich enzymelinked immunosorbent assay (TAS-ELISA) varied in different plant parts including leaf lamina, midrib and pseudostem, roots and young 'cigar' leaf. The concentration of BSV antigens was high in symptomatic tissues but was low or below the limits of detection in most asymptomatic tissues. During 'hot dry' seasons when symptoms were not fully expressed, the concentration of BSV antigens in leaf tissues declined drastically, often below the detection limit of TAS-ELISA. These results suggested that for more reliable detection of BSV antigens by TAS-ELISA, it is advisable to index plants using composite tissue samples comprising as many leaves as possible for each plant and collected during cool and/or rainy seasons when symptom expression is generally severe.

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Between 199 1 to 1996, more than 50 Musa hybrids and 10 landraces were evaluated under field and screenhouse conditions for virus symptoms resembling those caused by banana streak badnavirus (BSV). The symptoms included chlorotic streaks, leaf deformation, stunting, cigar leaf death, distortion of the peduncle, bunch or fruits, and internal pseudostem necrosis. Immunosorbent electron microscopy (ISEM) of randomly selected plants with one or more of these symptoms confirmed the presence of BSV particles in 15 tropical Musa plantain hybrids (TMPx) and five Musa landraces. Under both field and screenhouse conditions, the incidence of symptomatic plants in the hybrids was significantly higher than in the landraces. The hybrids also generally had a higher concentration of BSV antigens, as determined by enzyme-linked immunosorbent assay (ELISA). By contrast, most BSV-infected landraces were symptomless and had very low or undetectable amounts of BSV antigens. There was a significant variation in incidence of symptomatic plants between genotypes, experiments and year of observation. These results are discussed in relation to the higher natural BSV incidence observed on some Musa hybrids as compared with their parental genotypes.

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