Mycosphaerella leaf spot diseases of bananas - CBS

Session 2
L.H. Jácome
At one level, a temporal analysis seeks gross comparisons between experimental
treatments, e.g. fungicide spray schedules in order to evaluate strategies for disease
management. At a second, more complex level, changes in specific environmental
factors, pathogens or host resistance lead to changes in the epidemic that are reflected
by changes in the disease progress curves. A third level of analysis corresponds to
comparative epidemiology, where the purpose is to identify similarities and
differences between epidemics based on the shape of the disease progress curves and
to look for the elements that serve as primary determinants. Geographical populations
of Mycosphaerella, with marked genetic differentiation, could be considered as
separate epidemiological units requiring independent disease management. This
suggests that studies and modelling of the epidemiology, distribution and population
structure of the three Mycosphaerella leaf spot pathogens should be undertaken at
the national, regional and global levels.
In general, there is little information on the biology, population structure and
epidemiology of the Mycosphaerella leaf spot pathogens. As a result, areas requiring
further investigation are pathogen variability, distribution of variants, sources of
resistance, epidemiology and population structure. The papers and posters in this
session have been selected to improve our understanding of the population biology
and epidemiology of the Mycosphaerella pathogens of Musa. Aspects of the
aerobiological pathway of M. fijiensis ascospores and conidia at small-scale and
mesoscale levels are discussed. Such studies are needed to clarify the temporal and
spatial patterns of the spread of leaf spot diseases within or across banana cropping
sequences, and in relation to environmental and host factors. Several forecasting
schemes for black leaf streak disease have been introduced and are based on those
developed for Sigatoka disease in the French Antilles. The forecasting schemes
combine local weather and ascospore trapping data. The value of ascospore trapping
for forecasting black leaf streak disease is discussed.
Aspects of the genetic structure and evolution of Mycosphaerella pathogens at
the global, regional and local levels are discussed. Genetic differentiation and
independency of introductions of the pathogens according to the region are also
described. Knowledge of the variability within Mycosphaerella is necessary for
breeding and management of disease resistance. The usefulness of microsatellite
markers for the study of fungal populations having high evolutionary rate is presented
by an analysis of the introduction and spread of black leaf streak disease in Mexico.
The presence of polymorphisms in chromosome length between molecular karyotypes
of M. fijiensis is discussed. Understanding the organization of the genome could
lead to the development of new strategies for disease control management.
Given that pathogens can evolve to break down host resistance, further research
on the evolution of the three Mycosphaerella pathogens on resistant hosts is needed.
As stated in the Musa disease fact sheet No. 8, published by INIBAP, population
studies of Mycosphaerella pathogens of banana are required to determine whether
the banana-producing regions correspond to one or several epidemiological units.
The studies should use molecular markers and determine pathogenicity. Techniques
are also needed to determine quickly and reliably host-pathogen interactions in
controlled conditions. The variability in pathogenicity in genetically differentiated
populations of the three pathogens could then be evaluated by means of a standard
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