Zoospores of the plant pathogen use a number of tactic responses to target specific infectible regions of host roots. Although the dominant one is believed to be chemotaxis, it has been shown that zoospores of oomycetes may also use the exogenous proton/electrical currents generated by plant roots for guidance. Since these proton currents also generate significant pH gradients in the rhizosphere, the tactic response of zoospores to changes in pH was examined. Using ‘swim-in’ capillary tests, zoospores of were found to be repelled by solutions of high pH and attracted to solutions of low pH, relative to a control at neutrality. This tactic response was generally consistent with the measured pH at sites of zoospore accumulation around intact and wounded roots. However, the endogenous pH gradient around host roots could be abolished with buffers and this treatment did not affect the extent or pattern of zoospore accumulation. Therefore, detection of root-generated pH gradients is unlikely to have a major role in the homing response of zoospores towards plant roots.


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