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1977 - 197911980 - 19861

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    Plant pathogenic species of Stemphylium wallr. in New Zealand : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy at Massey University
    (Massey University, 1977) Singh, Gurmit
    A taxonomic study was conducted of 48 isolates of plant pathogenic Stemphylium Wallroth species, all of which produced a Pleospora Rabenhorst perfect state in culture. Specific identification was attempted using gross colony characters and morphological features of the imperfect and perfect states. Only conidial and ascospore features proved to be of taxonomic value. The diagnostic conidial features were shape, dimensions, length/width ratio, number of longitudinal and lateral septa, number of lateral septal constrictions, ornamentations and pigmentation. Ascospore features of taxonomic significance were dimensions, and shape of both juvenile and mature spores. On the basis of these criteria it was concluded that three Stemphylium species were present, namely Stemphylium botryosum Wallroth, Stemphylium globuliferum (Vestergen) Simmons, and Stemphylium vesicarium (Wallroth) Simmons, each with a corresponding Pleospora state. This cross-pairing had value since identification of form-species allowed reliable prediction of the Pleospora species, and vice-versa. The cultural conditions providing the best expression of conidial features (and thus facilitating separation of Stemphylium species) were 5% V-8 agar, pH 7.5, 20C and an 8h photoperiod. Synchronous production of conidia was induced with an injury technique and ascostromata were matured most rapidly at either 12C or a diurnal temperature regime of 8h at 16C/16h at 8C. A taxonomic survey of 166 Stemphylium isolates from 12 host species revealed the three previously mentioned species and a further undescribed species. The latter was isolated from annual phlox and was characterized by exceptionally large ascospores. S. vesicarium was by far the most common species, occurring on asparagus, chrysanthemum, blue lupin, Russell lupin, tree lupin, onion, pepper and tomato. The legumes were hosts of more than one species; lucerne for instance was a host for S. botryosum, S. globuliferum and S. vesicarium, while Russell lupin and tree lupin were only infected by S. botryosum. and S. vesicarium. By contrast, lettuce and carnation were only infected by S. botryosum. Cross-pathogenicity tests indicated that host specialization was relatively uncommon. S. botryosum from lettuce and the Stemphylium sp. from annual phlox were the only isolates exhibiting host specificity. The results of ultrastructural studies of conidiogenesis, the phenomenon of vegetative reversion of conidiophores and juvenile conidia, and the mechanism of ascospore release in Pleospora are discussed. A previously undescribed, saprophytic, chain-forming species of Stemphylium is also described.
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    Studies on the host-parasite relationships of Phytophthora cactorum (Leb. and Cohn) Schroet. and apple trees : a thesis presented in partial fulfilment of the requirements for the degree of Doctorate of Philosophy in Plant Pathology at Massey University
    (Massey University, 1986) Kelliher, Kevin John
    The present work was undertaken to gain a greater understanding of the nature and location of resistance mechanisms present in apple cultivars to Phytophthora cactorum . Artifical inoculation methods of evaluating resistance to this pathogen were also studied. The amount and type of sporangial germination in the presence of root exudates was not related to the susceptibility of the apple cultivar involved. The greatest accumulation of zoospores occurred at root tip regions and wound sites (the sites of maximal root exudation). The numbers of zoospores which encysted at root tip regions was not related to cultivar resistance and the large variation suggested that the quantity and/or quality of exudation varies considerably from root to root. The germination of zoospore cysts was markedly enhanced by root exudates of all cultivars tested. Ninety-five percent of the cysts germinated on the root surface while only 80% germinated on cellophane membranes laid over roots. Very few germ tubes formed appressoria upon membranes. More than 95% of the germ tubes grew towards the root and over 98% of them formed appressoria on the root surface. Appressorial formation appeared to be induced by specific surface configurations such as breaks in the outer wall and junctions between cells, on the root surface. The severity of P. cactorum infection of the seedling unsuberised apple roots (cv. 'COP' and 'GS') was dependent upon the initial inoculum loading, up to a saturation point i.e. extensive infection; a larger inoculum loading was required to reach this point with the more resistant 'GS' cultivar. The resistant cv. 'M 793' tended to have a lower amount of infection than the susceptible 'MM 106' at corresponding levels of inoculum. However, in general, mechanisms of resistance did not appear to be operating external to apple tissue. Electron microscopy studies showed that the process of infection was similar in susceptible (MM 106) and resistant (M 793) cultivars. Hyphae grew both intracellularly and intercellularly within root cortical tissue. Penetration through cells appeared to be both mechanical and enzymatic, although penetration by mechanical pressure alone may sometimes occur. Cellular deterioration was frequently obvious 2-4 cells in advance of the mycelium. The histochemical studies showed that in vivo, wall degrading enzymes produced by P. cactorum appeared to have a significant role in assisting fungal growth through host tissue but were relatively insignificant in its destruction . The lack of apparent structural wound associated responses (e.g. papilla formation) formed in cells of either tested cultivar in response to P. cactorum infection indicates that the mechanisms of resistance of the more resistant cultivars are primarily physiological. Investigations of internal resistance were hampered by the lack of a fully reliable method of assessing resistance. The most convenient system, inoculation of excised twigs, was further characterised revealing that the type of tissue tested (cortex, phloem-cambium or unlignified xylem) and the basal-distal location of the sampled piece of the shoot needs to he specified for valid comparisons with other workers and correlations with other criteria. A technique of assessing levels of infection by oospore numbers in unsuberised roots is described and would possibly be a better method for determining a cultivar's resistance to the root rot and perhaps the crown rot forms of the disease. Endogenous levels of nutritional compounds: total nitrogen, soluble sugars and starch and of phenolic compounds did not appear to determine the rate of apple tissue colonisation by P. cactorum. Phloridzin, the major phenolic of apple tissue, was toxic to P. cactorum in vitro yet mean pathogen growth rates of over 10 mm per day were measured within host tissue indicating that the hyphae are not physically encountering inhibitory amounts of phloridzin in vivo.