Mycosphaerella leaf spot diseases of bananas - CBS

Mycosphaerella leaf spot diseases of bananas: present status and outlook
is maintained in all populations. Ecological conditions being favourable for disease
development and banana cultivation almost year round in most growing areas, low
genetic drift in large pathogen populations can maintain the high levels of genetic
diversity observed. Therefore, a certain level of variability in pathogenicity might also
be maintained in pathogen populations, allowing the pathogen to attack newly
introduced resistant genotypes, as has been observed with M. fijiensis on ‘Paka’ and
‘T8’ in Rarotonga, Cook Islands (Fullerton and Olsen, 1995). The existence of specific
interactions between the host and M. fijiensis isolates was suggested for highly resistant
genotypes (Fullerton and Olsen, 1995). Variability in aggressiveness was evaluated for
two M. fijiensis samples from Cameroon and the Philippines by inoculating five partially
resistant cultivars using a leaf piece assay (El Hadrami, 2000). Variability was similar
but low for both countries, however genetic diversity in the Philippines was much higher
(Carlier et al., 1996). Specific interactions between the isolates and the cultivar were
not detected. Only susceptible hosts are cultivated in these countries, and the lack of
a selection pressure being exerted by the host on the pathogens could explain the results.
The potential of pathogen populations to overcome partial resistance should be analysed
by following their evolution in fields of resistant hosts (see Abadie et al., this
proceedings).
Genetic recombination plays an important role in the genetic structure of
M. musicola and M. fijiensis. Genetic markers were statistically independent therefore
characteristics of pathogenicity could not be related to RFLP genotypes. With regards
to breeding programmes, introducing specific resistance genes in individual cultivars
(pyramiding) may not be a strategy for durable resistance in banana. Mixing varieties
or using partially resistant hosts might be more appropriate.
Table 1. Nei’s gene diversity estimates for populations of M. fijiensis (Carlier et al., 1996) and M. musicola (Hayden
et al., in prep.) from different geographical regions.
Species Asia Africa Latin America and Caribbean Australia and Pacific
M. fijiensis 0.57 0.25 0.40 0.28
M. musicola 0.41 0.27 0.21 0.33
A high level of genetic differentiation was observed between populations of
M. musicola and M. fijiensis from different regions (Figure 1). The Fst parameter (Wright,
1951) estimated for all loci between pairs of populations varied between 0.14 and 0.58
for M. fijiensis and between 0.025 and 0.55 for M. musicola. But whereas the African
populations of M. fijiensis were significantly different from the Latin
American/Caribbean ones (Fst = 0.49), no significant difference was observed between
the African and Latin American populations of M. musicola (Fst = 0.025, not significant).
This suggests a separate introduction of M. fijiensis in each region but a common one
for M. musicola.
On the other hand, the high levels of genetic differentiation observed between
Australian and African populations of M. musicola (Fst = 0.47) does not support the
hypothesis of Stover (1962) whereby spores of M. musicola were carried by air currents
from Australia to Africa. In general, the high level of genetic differentiation of both
pathogens at a global scale suggests occasional migration events between continents.
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