Please read the abstract in the section 00front of this document / Dissertation (MSc)--University of Pretoria, 2005. / Microbiology and Plant Pathology / Unrestricted

Typescript, etc.

Studies on the seasonal changes in the moisture content of western hemlock Tsuga heterophylla (Raf.) Sargent, demonstrated two moisture minima, one in the spring and one in the fall, and two maxima, one in the spring and the second in the winter. Significant differences were noted for both bark and wood moistures between good and poor sites, between seasons of the year and for the interaction between site and seasons of the year. The relative turgidity of bark from the good site was below the 80 per cent level for 95 days through an entire year. On the poor site it remained under this critical value for 123 days of the 8 months observation period. Dwarf mistletoe did not appear to affect the water economy of the portion of branches that had not been invaded by the endophytic system. However the parasite produced moisture stress in the bark at the central area of infections. An investigation of the fungi that were associated with cankered areas of dwarf mistletoe infections revealed the occurrence of twelve different species of fungi. Nine of these were Ascomycetes and three were Fungi Imperfecti. There were two undescribed species, and one species, Mytilidion decipiens Karst. had not been reported previously for North America. Preliminary tests of the parasitism of seven species indicated that one, a member of the Fungi Imperfecti, was mildly parasitic. / Science, Faculty of / Botany, Department of / Zoology, Department of / Graduate

The behaviour of the orchard mites Bryobia pretiosa (Koch) and Metatetranychus ulmi (Koch) and four species of predators was investigated both on unsprayed and on sprayed apple trees. On unsprayed trees the host mites were found to be most abundant on the east sides; no consistent directional preference was found for the predacious species. Small apple leaves supported the same number of M. ulmi per unit area as large leaves, but more mites of the genus Tetranychus. On unsprayed trees a negative correlation between the total number of predators and the number of host mites was recorded. In abandoned orchards the predators were capable of controlling the host mites and maintaining them at a level below that of economic significance. Each of the insecticides DDT, Parathion, and lead arsenate, was found to be very toxic to all species of predators recorded, and thus hindered or entirely prevented natural control of the host mites. In every instance lead arsenate and DDT produced an increase in the population of B. pretiosa, this effect being partially chemical and partially due to a lack of predators on the sprayed trees. Lead arsenate caused an increase in the population of M. ulmi whereas DDT reduced it to a slight extent. Under arid conditions Parathion caused an increase in the population of B.pretiosa; under less arid conditions the mite was eliminated. Parathion had no effect on M. ulmi in a commercial orchard while, in the same orchard, it caused a great increase in Tetranychus spp. The latter species were reduced by DDT in the commercial orchard. / Science, Faculty of / Zoology, Department of / Graduate

Rhabdogloeum pseudotsugae Sydow has been suspected of being the imperfect stage of Rhabdocline pseudotsugae Sydow since their original description by Sydow in 1922. Because Rhabdogloeum has been reported so seldom in North America, and never in Europe, this possibility has been considered slight by most investigators. Investigation of the problem of Rhabdocline pseudotsugae in British Columbia revealed several new aspects of the association between the two leaf-cast diseases. Inspection of Rhabdocline and Rhabdogloeum collections from the interior of British Columbia indicated that the association of Rhabdogloeum pseudotsugae and Rhabdocline pseudotsugae is more prevalent than was formerly believed. Collections from the coast, and on Vancouver Island in particular, indicated that the association is probably less prevalent there than in the interior. In relation to the frequent and widespread occurrence of Rhabdocline throughout the Douglas fir range however, Rhabdogloeum is seldom found. If Rhabdogloeum pseudotsugae is the imperfect stage of Rhabdocline pseudotsugae, then it appears that frequently the Rhabdogloeum stage and occasionally the Rhabdocline stage is completely suppressed, as both stages have been found dissociated from each other. Frequent observation of a group of tagged Rhabdocline-infected trees on Vancouver Island over a period of two years did not reveal the presence of a Rhabdogloeum stage. Tissue cultures of lesions from these tagged trees, however, resulted in the growth of a fungus quite similar in appearance to that produced in tissue cultures by lesions typical of those produced by Rhabdogloeum pseudotsugae. This supports the view that Rhabdogloeum pseudotsugae is the imperfect stage of Rhabdocline pseudotsugae. Observation of these trees indicated that Rhabdocline pseudotsugae is capable of vegetating two years before producing apothecia. Apparently it is also capable of vegetating more than two years or of infecting leaves other than those of the current year. / Science, Faculty of / Botany, Department of / Graduate

Poria weirii Murr. isolates cultured from various hosts may be divided into two groups on the basis of their ability to parasitize Thuja plicata Donn and to grow on media containing water-soluble T. plicata heartwood extractives. The role of these extractives in determining whether a Poria weirii isolate can parasitize Thuja plicata was examined. Measurement of linear growth along wood micro-sections confirmed that substances inhibitory to one isolate group are hot-water soluble T. plicata heartwood extractives. The tolerance of isolates from T. plicata (Thuja) and Pseudotsuga menziesii (Mirb.) Franco (non-Thuja) to a hot-water extract of Thuja plicata heartwood and its hexane soluble (principally thujaplicins) and insoluble (thujaplicin-free) fractions was studied in detail. Dry weight of mycelium produced in liquid glucose-asparagine medium was measured. Thuja isolates were more tolerant than non-Thuja of the hot-water extract and its fractions; thujaplicins caused the greatest response differences. Thuja isolates showed greater stimulation than non-Thuja at subinhibitory thujaplicin concentrations. Higher thujaplicin concentrations were required to produce fungistatic and -toxic conditions in Thuja isolates than in non-Thuja. There were highly significant differences between isolate groups in response to 7.5 ppm thujaplicin. Tolerance was not related to date of isolation or geographical location. With sufficient inoculum potential, a non-Thuja isolate may overcome T. plicata heartwood toxicity. The resulting infection is usually localized. Unlike Thuja isolates, non-Thuja were unable to increase thujaplicin tolerance during three serial generations on liquid medium containing thujaplicins. Long exposure of non-Thuja isolates to T. plicata heartwood under field conditions did not increase their thujaplicin tolerance. Thuja isolates may be able to adapt the enzyme systems blocked by thujaplicin or to detoxify thujaplicins. These results have significance for control of Poria weirii. Planting Thuja plicata in and around infection centers caused by non-Thuja, isolates could limit their size by reducing the number of root contacts between susceptible species. / Science, Faculty of / Botany, Department of / Graduate

Nutritional mutants of Ustilago hordel were used to demonstrate that parasexual recombination occurs within the host plant (Hordeum vulgare) prior to the production of teliospores. The nutritional mutants were also used to show that resistance of the newly-germinated seedling of H. vulgare to U. hordei and of subsequently formed tillers to infection was not correlated, and was probably not controlled by the same gene or genes. The application of gibberellic acid to H. vulgare was found to cause an increase in the overall tiller height of healthy plants by increasing the elongation of Internodal regions 0-1, 1-2, 2-3, and 3-4. There was no increased elongation of internodal regions 4-5 and 5-6. The healthy tillers of diseased plants showed no Internodal elongation when gibberellic acid was applied. Diseased tillers, which are usually shorter than healthy tillers, were also unaffected by the presence of gibberellic acid. The injection of both mating types of U. hordei into the young developing spike of a normally resistant strain of H. vulgare resulted in the production of diseased spikes. This would indicate that blockage to normal infection occurs at the time of seedling penetration, at the level of tiller primordia development, or at the time of spike primordia development. / Science, Faculty of / Botany, Department of / Graduate

Soybean cyst nematode (SCN), Heterodera glycines, is a threat to soybean production in North Dakota. Studies on the biology of SCN were conducted to improve my understanding and management of this plant parasitic nematode. The objectives of the research were to; (1) determine if SCN reproduces on crops commercially grown or being tested for production in North Dakota, (2) evaluate the effects of SCN on growth of dry bean, (3) determine if there could be a shift in the SCN population toward greater ability to reproduce on dry bean, and (4) characterize the spatial distribution of SCN in research size field experiments. Canola, clover, lentil, and sunflower were nonhosts while borage, camelina, chickpea, crambe, cuphea, field pea, nyjer, and safflower were poor hosts for SCN with female indices (FI) less than 8. Lupines were susceptible hosts with FI’s of 42 to 57. FI’s of dry bean cultivars varied from 5 to 117. Kidney beans averaged the highest FI at 110 followed by navy, pinto and black at FI’s 41, 39, and 16, respectively. Pod number (PN), pod weight (PW), seed number (SN), and seed weight (SW) of GTS-900 (pinto bean) were significantly less at 5,000 and 10,000 eggs/100 cm3 soil compared with the control by 44 to 56% averaged over the two years. Significant reduction in growth of Montcalm (kidney bean) and Mayflower (navy bean) was observed at 2,500 and 5,000 eggs/100 cm3 soils in 2009, but not in 2008. There was no evidence that SCN was increasing reproduction during two 11 month periods of continual reproduction on roots of dry bean cultivars Premiere and Cirrus (navy), Buster and Othello (pinto), and Eclipse and Jaguar (black). The spatial distribution of SCN in field plots was aggregated in nine of ten field sites with large differences in egg numbers between plots. Lloyd’s index of patchiness ranged from 1.09 to 3.34. Spatial distribution of SCN can be an important factor affecting the results of field experiments.

No description available.

No description available.