Negative Chemotaxis of Zoospores of the Fungus Phytophthora palmivora

347 Phytophthora palmivora zoospores are repelled by many low molecular weight cations. This negative chemotactic response occurs at a different threshold concentration for each cation, that for the most effective ion, H+, being 150 ,!AM. The effectiveness of different cations parallels their ionic conductivities, the most active cations having the highest conductivities. It is suggested that the close approach of cations to the cell surface reduces the negative charge at the surface and hence changes the transmembrane potential, altering flagellum activity in such a way as to cause turning and negative chemotaxis. Zoospores of Phytophthora palmivora (Butler) Butler, an important plant pathogenic fungus, tend to swim upwards, i.e. to show negative geotaxis (Cameron & Carlile, 1977). They are also attracted by root exudates and pod extracts of a major host, cocoa, and by various pure substances produced or likely to be produced by the host (Cameron & Carlile, 1978). Phytophthora cinnamomi zoospores also show positive chemotaxis but in addition are repelled by various cations (Allen & Harvey, 1974). We here show that this negative chemo-taxis occurs in P. palmivora, confirm Allen & Harvey's conclusion that the repulsion is a threshold response taking place at a critical concentration characteristic of each cation, and suggest a possible mechanism for the response. METHODS Production of zoospores. The strain of Phytophthora palmivora, routine culture, media and production of zoospores were as described previously (Cameron & Carlile, 1978). The pH 5.8 buffered medium in which the zoospores (1 x lo5 to 2 x lo5 ml-l) were suspended had the following composition (mg 1-The organism was used under licence from the Ministry of Agriculture, Fisheries and Food. Chemotaxis assay chambers. A block of clear acrylic plastic (Perspex; 48 mm x 48 mm x 5 mm deep) was machined to produce six assay chambers on its upper surface. The chambers were 20 mm x 6 mm x 1 mm deep and from the end of each chamber a groove extended to the edge of the block to facilitate the insertion of a capillary tube into the chamber. After filling with test or control solutions the chambers were covered with a large coverslip. Assessment of negative chemotaxis. The method used was based on that of Allen & Harvey (1974). A zoo-spore suspension was drawn into a capillary tube (5 p l ; Microcaps, Drummond Scientific Co., U.S.A.) and one end of the tube was sealed with petroleum jelly …