The spatial ecology of phytopathogenic zoospores in the rhizosphere
This thesis is concerned with an analysis of swimming of oomycete zoospores, particularly in relation to the hypothesis that electrotaxis mediated host root colonisation. A correlation was found between the electrotactic behaviour of zoospores of Phytophthora palmivora and Pythium aphanidermatum and their localisation relative to anodic or cathodic regions of the majority of non-host roots. Cathodotropic P. aphanidermatum zoospores were found to be attracted to the cathode generated at the wound site on monocotyledonous and dicotyledonous plant roots studied. Zoospores of P. aphanidermatum were also found to become gradually less attracted to these wound sites as they dissipated over time. Anodotropic P. palmivora zoospores were found to be repelled by the cathodic wound sites on roots of all plants investigated, with the exception of Petunia hybrida. In addition to this zoospores of P. aphanidermatum were found not exhibit chemotaxis or encystment in gradients of the wound-specific metabolite acetosyringone. This suggests that electrotaxis and chemotaxis both operate in directing zoospore accumulation around roots. This reduction of the conductivity of the bathing medium by the addition of different concentrations of sodium chloride salts did not appear to affect the accumulation of both zoospore species around roots of rye grass. The results of this study supports the hypothesis that zoospores use electrotaxis as one means to locate new plant hosts in the rhizosphere. However, chemotaxis may still augment the regulation of zoospore colonisation and encystment.