Biological control of Verticillium dahliae by Talaromyces flavus

Verticillium dahliae 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 Talaromyces flavus as a biocontrol agent in management of the disease is the subject of the thesis.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 V. dahliae 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 V. dahliae a quantitative bioassay is recommended using eggplant as a test plant.Recovery of viable propagules from old samples of seeds coated with ascospores of T. flavus 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.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 V. dahliae infects the root, albeit at a low density. The population of T. flavus 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.The potential of T. flavus to control V. dahliae 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 T. flavus in soil (0.5% w/w) was followed by a decrease of &gt;90% of the population of V. dahliae in soil. The microbial antagonist also reduced colonisation by V. dahliae of roots and infection of eggplants. Although to a lesser extent than with the antagonist, alginate wheat-bran without T. flavus also reduced pathogen colonisation. T. flavus was tested for efficacy to control wilt in two independent field experiments with potato. After application of a T. flavus preparation, stems were less densely colonised by V. dahliae 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 T. flavus 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.Treatment with combinations of T. flavus with Bacillus subtilis , Fusarium oxysporum or Gliocadium roseum gave similar control of root colonisation and stem infection by V. dahliae as application of the single antagonists. The results suggest that T. flavus is compatible with these antagonists.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.The results of the experiments presented in this thesis and in the literature, demonstrate that T. flavus has the potential for biocontrol of V. dahliae . 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 T. flavus 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

The effect of spatial distribution of mycoparasites on biocontrol efficacy: a modelling approach

The spatial distribution of propagules in soil is an important factor in determining the ability of mycoparasites to control soilborne plant pathogens. The assumptions of uniform, random and aggregated propagule distribution were used to evaluate the importance of spatial distribution patterns of propagules of a mycoparasite. For the random and uniform cases explicit expressions were obtained for the average distance between propagules. Average distances among propagules are 40-50% smaller for the random compared to the uniform distribution. For the aggregated case no explicit expression is possible and numerical simulations were used to generate spatial distributions. The consequences for host inactivation by the mycoparasite were evaluated using a simple model of omnidirectional and constant growth of the mycoparasite. A random distribution of propagules gave a considerably slower rate of inactivation than the uniform distribution. Numerical simulations were made to generate comparable patterns of host inactivation for aggregated distributions in which propagule clusters were located at random in three-dimensional space and the distances between propagules with centres followed a normal distribution. The number of propagule centres and propagules/centre varied for a given inoculum density. Parameters were estimated from published data for sclerotia of Sclerotium minor inactivation at different densities of macroconidia of Sporidesmium sclerotivorum. Differences in host inactivation between the uniform and random distributions were small but both gave poor predictions of the field data at low and high densities. The aggregated distribution gave an improved fit for the higher propagule densities but no improvement at the lower. In studying the dynamics of mycoparasites it may be more significant epidemiologically to design treatments based on differences in mean distances between propagules rather than population densities. Density-dependent effects on growth rate need more attention in models and studies on mycoparasite ecology