Biocontrol of three fusarial diseases.
Kidane, Eyob Gebrezgiabher.
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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.
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