In the genus Fusarium are many important plant pathogens. The diversity of hosts the genus attacks, the number of pathogenic taxa and the range of habitats in which they cause disease are the greatest in plant pathology. One important species complex within the genus Fusarium is Fusarium oxysporum Schlecht. This species complex consists more than 80 pathogenic forma specialis and is particularly difficult to control. The fungi can survive in soil for decades as specialized spores, known as chlamydospores. Interestingly, however, this species complex also contains beneficial non-pathogenic forms that can be exploited to manage Fusarium wilt diseases. In this study, the ability of non-pathogenic F. oxysporum strains, Trichoderma harzianum Rifai Eco-T®, soluble silicon, and their combination was evaluated on two important crops, banana (Musa sp. L.) and beans (Phaseolus vulgaris L.), for their potential to suppress pathogenic strains of F. oxysporum. The ability of these crops to take up and accumulate silicon in their organs, and its effect on low temperature stress was also investigated. Several endophytic fungi, mainly Fusarium spp. and bacteria, were isolated from healthy mature banana plants. After preliminary and secondary in vivo screening tests against F. oxysporum f.sp. phaseoli on beans (Phaseolus vulgaris L.) cv. Outeniqua, two non-pathogenic F. oxysporum strains were selected for further testing. These two non-pathogenic F. oxysporum strains were found to colonize banana (Musa sp.) cv. Cavendish Williams and bean plants, and to suppress Fusarium wilt of these crops. In order to improve the efficacy of these biocontrol fungi, soluble silicon was introduced. The relationship between plant mineral nutrition and plant diseases have been reported by several authors. Plants take up silicon equivalent to some macronutrients, although it is not widely recognized as an essential element. In this study, we established that roots, the target plant organ for soilborne plant pathogens, accumulated the greatest quantity of silicon of any plant organs when fertilized with high concentrations of silicon. On the other hand, the corm and stem accumulated the least silicon. Such observations contradict the concept of passive uptake of silicon via the transpiration stream in these plants as the only uptake mechanism. The prophylactic properties of silicon have been documented for many crops against a variety of diseases. In vitro bioassay tests of silicon against these wilt pathogens showed that silicon can be toxic to Fusarium wilt fungi at high concentrations (>7840 mg .-1), resulting in complete inhibition of hyphal growth, spore germination and sporulation. However, low concentrations of silicon (<490 mg .-1) encouraged hyphal growth. Silicon fertilization of banana and beans significantly reduced disease severity of these crops by reducing the impact of the Fusarium wilt pathogens on these crops. However, it could not prevent infection of plants from the wilt pathogens on its own. Synergistic responses were obtained from combined applications of silicon and non-pathogenic F. oxysporum strains against Fusarium wilt of banana. Combinations of silicon with the non-pathogenic F. oxysporum strains significantly suppressed disease severity of Fusarium wilt of banana, caused by F. oxysporum f.sp. cubense (E.F. Smith) Snyder & Hansen, better than applications of either control measure on their own. Banana production in the subtropical regions frequently suffer from chilling injury, and from extreme variations between night and day temperatures. Such stress predisposes banana plants to Fusarium wilt disease. Silicon, on the other hand, is emerging as important mineral in the physiology of many plants, ameliorating a variety of biotic and abiotic stress factors. We established that silicon fertilization of banana plants significantly reduced chilling injury of banana plants. Membrane permeability, lipid peroxidation (MDA level) and proline levels were higher in silicon-untreated plants than the treated ones, all of which demonstrated the stress alleviating effect of silicon. Low temperatures damage the cell membrane of susceptible plants and cause desiccation or dehydration of these cells. Levels of sucrose and raffinose, recognized as cryoprotectants, were significantly higher in silicon-amended banana plants than unamended plants. / Thesis (Ph.D.)-University of KwaZulu-Natal, Pietermaritzburg, 2008.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:ukzn/oai:http://researchspace.ukzn.ac.za:10413/1225 |
| Date | January 2008 |
| Creators | Kidane, Eyob Gebrezgiabher. |
| Contributors | Laing, Mark D. |
| Source Sets | South African National ETD Portal |
| Language | English |
| Detected Language | English |
| Type | Thesis, Thesis |
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