Browsing by Author "Kidane, Eyob Gebrezgiabher."

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Biocontrol of three fusarial diseases.
(2004) Kidane, Eyob Gebrezgiabher.; Laing, Mark Delmege.
Over the past one hundred years, research has repeatedly demonstrated that phylogenetically diverse microorganisms can act as natural antagonists of various plant pathogens. Interest in biological control research continues reflecting the desire of multiple constituencies to develop sustainable methods for controlling plant diseases. The review of the literature comprises information on the epidemiology, economic importance and the different control options available against Fusarial diseases of cabbage, maize and pine, and the safety of microorganisms intended for use as biocontrol agents, their management and strategy of control. Trichoderma and Bacillus isolates used as biocontrol agents were obtained mainly from the rhizosphere of cabbage, maize and pine with a view that they would be adapted to those habitats where they would eventually to be used as innundative biocontrol agents. Preliminary selection was made based on in vitro antagonism of those isolates towards Fusarium oxysporum f. sp. conglutinans (Wollenweb.) W.C. Snyder & H.N. Hans. Ultrastructural studies of mycoparsitism of Trichoderma Isolates ET23, ET13 and Trichoderma harzianum Eco-T® which caused significant reduction in disease incidence and severity on later study under greenhouse conditions, were investigated on the vascular pathogen, F. oxysporum f. sp. conglutinans. Although the mode of action of the three isolates wall not fully elucidated, certain mechanisms such as mycoparasitism and antibiosis or production of antimicrobial substances, which cause cell wall degradation and lysis, have been identified. Twenty Trichoderma and 18 Bacillus isolates which showed antagonism towards F. oxysporum f. sp. conglutinans were tested against the same pathogen on cabbage under greenhouse conditions. Trichoderma isolates were delivered to the soil in two different ways, i.e., seed treatment and drenching, while Bacillus isolates were only drenched as spore suspensions. More than two-third of the biocontrol isolates caused significant reductions in disease incidence and severity of the vascular wilt disease. Application of Trichoderma isolates by drenching resulted in better control of the disease than when applied as a seed treatment. Of the 38 Bacillus and Trichoderma isolates tested against the cabbage yellows fungus, three Trichoderma and four Bacillus isolates were selected for further testing against Fusarium sp. and Rhizoctonia solani Kuhn on maize and Fusarium circinatum on Pinus patula seedlings. Since none of the Fusarium isolates obtained from diseased kernels and cobs of maize were pathogenic to the two maize cultivars, yellow maize and PAN 6479, provided by Pannar® seed company, biocontrol experiments on Fusarium diseases of maize could not be conducted. Only Trichoderma Isolate ET23 and T. harzianum Eco-T® were found to significantly control Rhizoctonia preemergency damping-off on maize while none of the Bacillus isolates caused significant increase in seedling emergence. In the test against F. circinatum on pine, in most cases, significant reduction in seedling mortality was observed in the first 4 to 8wk, however, after 12wk they were no longer effective. Improvement in the survival of pine seedlings were observed when T. harzianum Eco-T® was applied prior to the introduction of F. circinatum. There was almost a direct relationship between the inoculation time and percentage of survival of seedlings. Prior inoculation gives the biocontrol agent time to colonize the potential infection courts for the pathogen in the root area and to be established in the rhizosphere of the pine seedlings. It has been reported that the inconsistent and poor performance of biocontrol agents in the field can be improved with the use of mixtures of biocontrol agents to mimic the naturally suppressive soils which comprise numerous saprophytic microorganisms. However, these organisms have co-evolved for many years that they are adapted to live together in the same soil ecosystem. Therefore, when combinations of biocontrol organisms are used, the compatibility between these isolates is important. Compatibility tests between and among Bacillus and Trichoderma isolates were carried out in vitro. The tests revealed that the Bacillus and Trichoderma isolates are not all compatible. Trichoderma Isolate ET13 showed antagonism towards Isolates ET23 and T. harzianum Eco-T®; Bacillus Isolates B81 and BF011 were slightly antagonistic to Isolates EXR and JR01, and Isolate JR01 was slightly antagonistic to Isolate EXR. Comparisons of single versus mixtures of Bacillus or Trichoderma isolates showed that mixtures of Bacillus or Trichoderma isolates did not result in significantly greater reduction in disease incidence and severity of cabbage yellows.
Management of fusarium wilt diseases using non-pathogenic Fusarium oxysporum, and silicon and Trichoderma harzianum (ECO-T®)
(2008) Kidane, Eyob Gebrezgiabher.; Laing, Mark Delmege.
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.