Systematics of colletotrichum pathogens associated with strawberry plants.
Species of the genus Colletotrichum Corda are responsible for many diseases of
economically important crops world-wide. One particularly susceptible crop is
strawberry (Fragaria spp.) on which Colletotrichum spp. cause extremely destructive
anthracnose diseases, characterised by black necrotic lesions on any part of the plant.
Three "species" have been implicated in the disease: C. acutatum, C. jragariae, and
C. acutatum is considered the causal organism of strawberry blackspot in Europe and
is subject to quarantine regulations within the EU; c.jragariae has only been found in
the Americas and was formerly considered the sole cause of strawberry anthracnose in
North America; and C. gloeosporioides is a heterogeneous "species aggregate" which
has a morphological range which can encompass examples of other "species",
especially C. jragariae. The absence of a reliable test for distinguishing these
"species" - classical morphological approaches are unsatisfactory - led to an
international multi-disciplinary project to characterise Colletotrichum pathogens of
strawberry and develop diagnostic tests for individual taxa (\,,,;,,,\..c_,,\_ \:x~ t:k._ ~0).
The research reported in this thesis has demonstrated the utility of a multidisciplinary
approach to the systematics of Colletotrichum pathogens of strawberry. Data
obtained have supported taxon groupings at different levels. The use of mtDNA
provided data of two complementary types. Firstly, the NMS primers were utilised in
amplification of a fragment of "species aggregate" -specific size (with three notable
exceptions) from the mitochondrial small subunit rDNA. Secondly, the Hae III
restriction digests of A+T-rich DNA were able to highlight groupings below the level
of species aggregates. This latter method was able to distinguish the meioticallyreproducing
strains of C. jragariae from anamorphic isolates and elicited two distinct
subgroups in C. acutatum. PCR techniques previously described by Mills and co works and Freeman and Rodriguez were assessed for their utility and reliability.
Isozyme analyses were moderately useful for taxon differentiation at an infraspecific
level with the esterase systems.Study of extracellular enzyme activities using 4 MU substituted compounds was of limited diagnostic use. Additional
physiological and biochemical tests did not reveal many discriminatory tests. A few
tests, however, did reflect the greater vigour generally displayed by C. jragariae
isolates. Utilisation of glucuronamide and growth rate on casein were two such tests.
The physiological and biochemical tests were demonstrably more useful when put
together with the data from EU Project colleagues and subjected to cluster analyses
and ordination techniques, then when analysed as discrete tests.
Colleagues in collaborating laboratories undertook a battery of tests including
molecular, physiological and biochemical studies. It is appropriate to use the entire set
of information obtained from the overall EU-Project in order to fully define the
groups. The results reported in this thesis can stand alone, however, allowing
discrimination of the taxa at several levels.