Biological control of Verticillium dahliae by Talaromyces flavus

<em><p>Verticillium dahliae</em> causes vascular wilt in a wide range of host plants. Control of Verticillium wilt is by soil disinfestation and to a lesser extent by crop rotation or, for a few host plants, by growing resistant varieties. For environmental reasons, the development of alternatives to chemical soil disinfestation is being sought. Biocontrol by microbial agents is one of the options. The potential of <em>Talaromyces flavus</em> as a biocontrol agent in management of the disease is the subject of the thesis.</p><p>The effect of the pathogen on plants was studied to adequately interpret results of biocontrol experiments. Under controlled conditions, a linear relationship was demonstrated between pathogen inoculum density in soil and its population density on roots or in sap extracted from stems. In field experiments, incidence of stem infection by <em>V. dahliae</em> and yield of tubers showed a clear dose-response relation to the amount of pathogen inoculum applied to soil. Incidence of stem infection and density of stem colonisation by the pathogen gradually increased during the season. For studying the effects of antagonists on dynamics of <em>V. dahliae</em> a quantitative bioassay is recommended using eggplant as a test plant.</p><p>Recovery of viable propagules from old samples of seeds coated with ascospores of <em>T. flavus</em> in a clay formulation indicated that products containing the antagonist may have a long shelf life. On the pelleted seeds, a significant proportion of the ascospores had survived a storage period of 17 years.</p><p>In pot experiments with field soils, the fungus moved from seed tubers of potato and seeds of eggplant coated with ascospores to the developing roots, including the root tips, the site where <em>V. dahliae</em> infects the root, albeit at a low density. The population of <em>T. flavus</em> decreased log-linearly with distance from the seed. Results suggest that passive movement along the growing root is one of the main factors involved in colonisation by the fungus.</p><p>The potential of <em>T. flavus</em> to control <em>V. dahliae</em> was evaluated in several experiments. Its application to senescent stems collected from a field with a diseased potato crop reduced viability of microsclerotia. Incorporating an alginate wheat-bran preparation of <em>T. flavus</em> in soil (0.5% w/w) was followed by a decrease of >90% of the population of <em>V. dahliae</em> in soil. The microbial antagonist also reduced colonisation by <em>V. dahliae</em> of roots and infection of eggplants. Although to a lesser extent than with the antagonist, alginate wheat-bran without <em>T. flavus</em> also reduced pathogen colonisation. <em>T. flavus</em> was tested for efficacy to control wilt in two independent field experiments with potato. After application of a <em>T. flavus</em> preparation, stems were less densely colonised by <em>V. dahliae</em> in the treated plots than in the control plots in the first growing season of experiment 1 and in the second growing season of experiment 2. A reduction in plant colonisation by the pathogen was not followed by higher yields of potato tubers. Although population density of <em>T. flavus</em> in soil had not increased during the growing season, it remained at a higher level in treated plots than in non-treated plots, also in the second year after introduction of the antagonist.</p><p>Treatment with combinations of <em>T. flavus</em> with <em>Bacillus subtilis</em> , <em>Fusarium oxysporum</em> or <em>Gliocadium roseum</em> gave similar control of root colonisation and stem infection by <em>V. dahliae</em> as application of the single antagonists. The results suggest that <em>T. flavus</em> is compatible with these antagonists.</p><p>The significance of propagule distribution in soil for the performance of mycoparasites was demonstrated using a simple model. Progress of infection of the host fungus was slower with a random distribution than a uniform distribution of mycoparasite propagules. With a random distribution, the average distance between propagules was 40-50% smaller than with a uniform distribution.</p><p>The results of the experiments presented in this thesis and in the literature, demonstrate that <em>T. flavus</em> has the potential for biocontrol of <em>V. dahliae</em> . However, thus far the results with this antagonist tend to be inconsistent. Control is often partial or even fails completely. This means that application of <em>T. flavus</em> as a single control method has little commercial potential. For the time being, biocontrol is considered as a method to be applied in combination with cultural methods, biological disinfestation, the use of tolerant cultivars or the selective application of chemical control. The next step in further research should focus on the nature of the inconsistency of biocontrol using this fungus.</p&gt