Mycosphaerella leaf spot diseases of bananas - CBS

Mycosphaerella leaf spot diseases of bananas: present status and outlook
Identification, taxonomy and diagnosis
In 1989, the discussions at the San José meeting concentrated on the availability of
diagnostic methods for the pathogens causing Sigatoka disease, black leaf streak disease
and black Sigatoka. The main observation was the risk of confusing the species when
diagnosis was based on the observation of symptoms alone.
In situ or in vitro observation of the anamorph was considered to be the most reliable
method to identify the diseases:
1) the anamorph Cercospora musae, now Pseudocercospora musae, for Sigatoka
disease caused by Mycosphaerella musicola,
2) the anamorph Cercospora fijiensis, now Paracercospora fijiensis, for black leaf
streak disease caused by Mycosphaerella fijiensis,
3) the two anamorphs for black Sigatoka which at the time were attributed
to Mycosphaerella fijiensis var. difformis were in reality the anamorphs of
M. fijiensis and M. musae. The species was described only in Latin America.
There has been much work on these aspects in recent years, using the methods
developed for molecular taxonomy on different populations of the pathogens
(Carlier et al., 1994, 2000; Johanson and Jeger, 1993; Johanson et al., 1994).
Molecular markers are highly sensitive at discriminating between fungal
species and have clarified the taxonomy of the banana pathogens.
Molecular markers have made it possible to distinguish clearly between
M. fijiensis and M. musicola and to confirm that M. fijiensis and M. fijiensis var.
difformis are synonymous (Carlier et al., 1994).
More recently, markers have been used to identify a new species pathogenic to
banana, Mycosphaerella eumusae. Initially, M. eumusae was thought to have a
Septoria anamorph (Carlier et al., 2000) therefore the new disease was called
Septoria leaf spot disease. Additional detailed work on the morphotaxonomy
attributed M. eumusae with a Pseudocercospora anamorph and the name was
changed to eumusae leaf spot disease, ELSD (Crous and Mourichon, 2002).
Other methods based on serology are still being developed. The methods are
intended above all to be sufficiently simple to diagnose the early stages of disease,
for example, within the framework of preventive control measures with rational
use of fungicides (Etienne et al., 1995). More specific methods to identify
pathogens as well as opportunist or non-pathogenic species of Mycosphaerella are
also being developed.
The use of molecular markers has provided a great deal of information in recent
years. In particular, molecular markers have made it possible to perform many
analyses of genetic diversity, mainly in M. fijiensis.
A high level of genetic diversity has been demonstrated in M. fijiensis
particularly in the genetic structure of populations at a macrogeographical
scale (Carlier et al., 1996). The diversity and geographical distribution of popu-
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