Mycosphaerella leaf spot diseases of bananas - CBS

Session 1
R.A. Romero
molecular methods are being used to prevent or delay the spread of these pathogens
to new areas. Little is known about the integration of these methods with strategies
to prevent the dissemination of the diseases caused by Mycosphaerella spp. A possible
limitation to the use of molecular methods is a lack of adequate infrastructure and
trained personnel in developing countries.
The arrival of black leaf streak disease to a new area has resulted in the replacement
of Sigatoka disease as the predominant disease of bananas and plantains. The
ability of M. fijiensis to displace M. musicola has not been adequately studied.
However, it is known that M. fijiensis has several biological characteristics that make
it more competitive than M. musicola, e.g. greater ascospore production, more sexual
cycles a year, and a higher rate of colonization of host tissue (Stover, 1980; Mouliom
Pefoura et al., 1996). Little information is available about differences in pathogenicity
between the two species.
The replacement of M. musicola by M. fijiensis, i.e. the elimination of one species
by another in a short time, is interesting from an ecological and evolutionary point
of view. Unfortunately, there are no studies that show whether M. musicola has in
fact disappeared. It is possible that M. musicola coexists with M. fijiensis at low
frequencies that are difficult to detect. If both pathogens survive in mixed
populations, with M. musicola at a low frequency and M. fijiensis as the predominant
species, it would be preferable to refer to a complex of Mycosphaerella leaf spot
diseases rather than to black leaf streak disease. Breeding strategies for resistance
would need to take this possibility into consideration because resistance to
M. fijiensis does not necessarily imply resistance to M. musicola. Similarly,
resistance genes incorporated by genetic engineering would need to be evaluated
against both pathogens. Current molecular techniques may be able to clarify whether
M. musicola survives at low frequencies with M. fijiensis.
Reports describe how M. musicola is better adapted than M. fijiensis to low
temperatures, and how M. musicola is prevalent at higher altitudes where
temperatures are cooler (Mouliom Pefoura et al., 1996; Romero and Gauhl, 1988;
Tapia, 1993). However, over the years, M. fijiensis has been replacing M. musicola
at higher altitudes in Costa Rica (Gauhl et al., 2000) suggesting that M. fijiensis may
be adapting to the environment. Plantains and other cooking bananas commonly
grown at higher altitudes, sometimes in combination with coffee, can be severely
affected due to the greater aggressiveness of black leaf streak disease in comparison
with Sigatoka disease.
The socio-economic impact of black leaf streak disease has continued to increase
as the pathogen reaches new areas. The impact has also increased as the disease
becomes more difficult to control because of increased resistance of the pathogen
to new fungicides. Thus, fungicide resistance is increasingly an important constraint
to control black leaf streak disease in plantations dedicated to the export market.
Recently M. eumusae, a new species pathogenic to bananas, has been described
(Anon., 1995; Carlier et al., 2000). Little is known about the biology and epidemiology
of this species, and hence it is not possible to evaluate whether or not it represents
a threat to small-scale and large–scale banana production. Research is urgently
needed to characterize the pathogenicity and aggressiveness of M. eumusa. In
particular, studies are needed on its competitive ability with respect to M. musicola
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