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M (Plant- en Bodemwetenskappe), North-West University, Potchefstroom Campus Biological control is a natural form of control. Man tries to manipulate one part of the environment in such a way as to control another. The aim of this study was to find a biological control agent for Fusarium o-xysporum f. sp. dianthi. This pathogen is a soil inhabitant that causes vascular wilts in carnations (Dianthus caryophyllus). It infects the roots, which it penetrates directly or through wounds. Once inside the root, the mycelium reaches the xylem vessels and spreads internally to the rest of the plant. The pathogen and 91 potential antagonists were isolated from soil and plant samples. An in vitro trail run eliminated 17 organisms that didn't inhibit the pathogen. The inhibition of the growth rate was used to test for antagonism. The growth rate of the organisms was determined by culturing the potential antagonist opposite the pathogen in a special glass tube. Aspergillus sp., Fusarium moniliforme, F. prolif eratum (3 isolates), Trichoderma harzianum (7 isolates), T. fasciculatum, Pseudomonas sp. ( 4 isolates) and P. aeruginosa either inhibited the growth of F. oxysporum f. sp. dianthi or had a higher growth rate than the pathogen. Rooted cuttings of four carnation cultivars were planted in sterile soil. The pathogen and potential antagonist were added to the soil. The resistance of cultivars 1, 2 and 3 were determined by planting rooted cuttings of the cultivars in sterile soil. The pathogen was added to the soil. Cultivar 1 is the most susceptible for isolate 1 of the pathogen and cultivar 3 the least. There is no difference between the susceptibility of the three cultivars for isolate 2 and 3 of the pathogen. The following organisms were tested in nursery conditions: Pseudomonas sp., Fusarium moniliforme and Trichoderma harzianum and a mixture of the seven T. harzianum isolates. Isolate 2 was used because that isolate was the most virulent of the three pathogen isolates. Cultivar 1 was used because that cultivar was the most susceptible. Rooted cuttings of that cultivar were planted in a mixture of the potential antagonist, pathogen and sterile soil. The best results were obtained from T. harzianum. After 100 days 76% of the plants treated with an isolate of T. harzianum showed no disease symptoms. Forty seven percent of the plants in the control plot showed symptoms. Trichoderma harzianum inhibits the growth of Fusarium oxysporum f. sp. dianthi. More research is necessary to find a method for implementation in practise. It must also be determined if the biological control of plant pathogens is a viable undertaking. Masters |
