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. gloeosporioides. 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.

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The yearly losses incurred by bacterial blackspot disease are high. Often trees are asymptomatic, with the pathogen either in the resident phase or latent stage of infection. Detection of the pathogen in these asymptomatic trees is one of the most important means of controlling the disease. Isolates which consistently differed in virulence were isolated from symptomatic mango plants. These isolates could be categorised into four groups based upon differences in virulence. Monoclonal antibodies (mAbs) were successfully raised using separate and pooled isolates for immunisation. MAbsraised were of the lgG class and reacted with a proteinaceous epitope. These monoclonal antibodies could distinguish between different virulence groups of Xanthomonas campestris pv. mangiferaeindicae by means of Western Blot analysis. These antibodies were used along with a selective medium, BVGA for detection of epiphytic populations as well as latent infections in mango. An enrichment step prior to the enzyme- linked immunosorbent assay (ELISA) is important, since bacterial counts on trees with latent infections are too low to result in a positive signal. These techniques in combination are thus useful for detection and monitoring of the pathogen, which may play an important role in controlling the spread of the disease. / Dissertation (MSc Agric)--University of Pretoria, 1993. / gm2014 / Microbiology and Plant Pathology / Unrestricted

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Le réchauffement climatique cause plusieurs modifications abiotiques et biotiques dans les milieux naturels. La hausse de la température de l’eau cause une diminution de l’oxygène dissous dans les lacs et augmente la quantité de zone hypoxique observée. Une autre conséquence de la hausse de la température est l’augmentation du métabolisme et de la consommation d’oxygène des espèces ectothermes dont les poissons et les parasites. Le parasitisme est omniprésent dans les réseaux trophiques et a un effet néfaste sur l’hôte affecté. Les parasites et l’hypoxie peuvent limiter la portée aérobie (AS) des poissons pour la réalisation d’activités journalières. Ainsi, cette étude analyse l’effet dans le temps d’une infection de trématodes causant la maladie du point sur le métabolisme et sur la tolérance hypoxique de l’hôte puisque le développement de ces parasites suggère un effet sur le poisson qui varie selon le temps de résidence des parasites. Nous avons utilisé des crapets-soleil (Lepomis gibbosus) infectés par ces trématodes comme système modèle. Nous avons émis l'hypothèse que l'infection parasitaire réduirait la portée aérobie et la tolérance à l'hypoxie des poissons en fonction du temps du développement de l’infection. Afin d’étudier cette relation hôte-parasite, des tests de respirométrie et d’hypoxie critique ont été performés à cinq moments lors des deux premiers mois suivant l’infection. Les traits métaboliques aérobies (taux métabolique standard et maximal, AS), des indices de la tolérance hypoxique et du métabolisme anaérobiques (tension critique d’oxygène, pression partielle d’oxygène entraînant la perte d’équilibre, la concentration de lactate) et le taux d’hématocrite sont les variables analysées à l’aide de la respirométrie et de prélèvements sanguins. Nous démontrons ici que l’infection expérimental de ces trématodes n’affecte ni la portée aérobie ni la tolérance hypoxique et ce indépendamment du temps de développement du parasite. Un faible effet temporel, mais significatif, est observé entre les premiers jours d’expérimentations et les derniers, des différences principalement dues aux faibles différences non significatives des taux métaboliques standards et maximaux. Le stress induit par captivité et l’effet des changements saisonniers sur les taux métaboliques sont possiblement en cause. Pour l’instant, selon les conditions environnementales actuelles, le crapet-soleil démontre une résilience à l’infection parasitaire ainsi qu’à l’hypoxie. / Global warming is causing several abiotic and biotic changes in natural environments. The rise in water temperature causes a reduction in dissolved oxygen in lakes and increases the amount of hypoxic zone observed. Another consequence of rising temperatures is the increased metabolism and oxygen consumption of ectothermic species, including fish and parasites. Parasitism is ubiquitous in food webs and has a detrimental effect on the affected host. Parasites and hypoxia can limit the aerobic range (AS) of fish for daily activities. Thus, this study analyzes the effect over time of a trematode infection causing the blackspot disease on the metabolism and hypoxic tolerance of the host since the development of these parasites suggests an effect on the fish that varies according to the residence time of the parasites. We used sunfish (Lepomis gibbosus) infected with these trematodes as a model system. We hypothesized that parasite infection would reduce the aerobic range and hypoxia tolerance of fish as a function of the time of infection development. Respirometry and critical hypoxia tests were performed at five time points during the first two months post-infection to investigate this host-parasite relationship and its impact over time. Aerobic metabolic traits (standard and maximum metabolic rate, aerobic range), indices of hypoxic tolerance and anaerobic metabolism (critical oxygen tension, partial pressure of oxygen leading to loss of equilibrium, lactate concentration) and hematocrit levels were analyzed using respirometry and blood sampling. We demonstrate here that experimental infection with trematodes affects neither aerobic range nor hypoxic tolerance independently of parasite development time. A small but significant temporal effect is observed between the first and last days of experimentation, differences mainly due to small non-significant differences in standard and maximum metabolic rates. This may be due to stress induced by captivity and seasonal changes affecting metabolic rates. For now, considering actual environmental conditions, sunfish show high resiliency to parasitic infection and to hypoxia.

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