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Background and
Objectives:
This project was initiated to
provide information on the presence of sexual state of the fungus and
its contribution to epidemics of wheat leaf blotch in Manitoba. The
fungus overwinters in stubble and infects wheat plants in late
spring/early summer. We proposed to provide direct as well as indirect
evidence on the presence of sexual spores of the fungus. Leaf blotch of
wheat is an important and destructive stubble-borne disease of wheat in
Manitoba. Spread of this disease in Manitoba is relatively recent
(early-mid 90's) and all common wheat varieties are currently
susceptible to the pathogen. We have conducted studies aimed at
developing a knowledge base to characterize the makeup of the virulence
of the pathogen, find sources of genetic resistance and understand the
mode of inheritance of resistance to this disease. By finding out if
ascospores (sexual spores) are formed (or not) in Manitoba, we will be
able to develop an appropriate control strategy for this disease. If
the pathogen undergoes sexual reproduction on farmers fields, we will
expect that resistant cultivars will become susceptible sooner than if
the sexual state was rare or non-existent. This is due to the fact that
the sexual process speeds up the emergence of new races, with new
virulence to newly released resistant cultivars.
Work
on this project was carried out by Charlotte Hoorne as partial
fulfillment toward a Master of Science degree.
Results
and Discussion:
Direct Methods of Detection
of the Sexual State of the Pathogen
Charlotte made
considerable progress in this field. After processing hundreds of
segments of wheat straw, she conclusively identified the sexual
structures of the pathogen. This positive evidence is what we were
after. Charlotte becomes the first scientist to report the presence of
ascospores of Mycosphaerella graminicola
(Septoria tritici)
in Canada. In order to confirm that the spores she observed
and isolated were indeed those of the pathogen, she fulfilled what is
known as the Koch postulates. These are steps to demonstrate that the
spores under investigation are the right ones, and if inoculated onto a
susceptible wheat plant, they would produce typical symptoms of septoria
leaf blotch. The pathogen is then re-isolated from artificially
inoculated plants and must be the same as the one inoculated. Charlotte
has now written a manuscript for submission to the Canadian Journal of
Plant Pathology. The presence of the sexual state of the pathogen in
Manitoba was anticipated, but never demonstrated by physical means
(direct observation) in Canada. We can now proceed to elaborate an
efficient strategy to control this pathogen, knowing that new races are
likely to arise, upon release of genetically resistant cultivars.
Charlotte's contribution will be regarded as significant by Canadian
plant pathologists. This work was published in the Canadian Journal of
Plant Pathology (Vol. 24, pages 445-449).
Indirect Method of Detection
of the Sexual State of the Pathogen
This
part of the project was included for two reasons:
-
As a back-up strategy to identify the sexual
state, using DNA techniques.
-
To train the student in the field of
molecular biology as part of her general training at the Master's level.
Until Charlotte's direct
determination of the presence of the sexual structures of the pathogen
on wheat stubble in Manitoba, the sexual state of the pathogen was
inferred from studies of DNA polymorphism (differences in DNA bands)
between isolates. This is based on the notion that high polymorphism
results (in part) from sexual recombination and that low polymorphism is
indicative of clonal populations (i.e. most isolates belong to the same
population and are produced by vegetative means, just like tree cuttings
or potato tubers). This technique has been widely used for this type of
study, both in Canada and elsewhere.
Charlotte completed the second part
of the study and confirmed that the spores that affect the crop are
likely to have originated from the sexual process (ascospores). This
finding confirms the result of the first study, which demonstrated that
the sexual process occurs in Manitoba fields.
Conclusions:
We
provided the proof that ascospores (sexual spores) are significantly
involved in the epidemics of speckled leaf blotch (septoria leaf blotch)
in Manitoba.
The
presence of the sexual form of the pathogen has very important
consequences to the management of the disease. It is expected that new
races of the pathogen are likely to arise through sexual recombination
as we introduce cultivars with genetic resistance. Sexual recombination
can also "make it easy" for the pathogen to generate new strains that
are resistant to commonly used fungicides. These aspects of the
epidemiology of the pathogen must be taken into consideration by plant
pathologists, agronomists and breeders when designing control measures.
It also means that we must continue to monitor the population of the
pathogen in western Canada for the appearance of new races and identify
new resistance genes in wheat.
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