Swimming zoospores of the fungus Phytophthora cinnamomi exposed to a range of ions were much more sensitive to cations than anions. One cation, Ca2+, induced zoospores to encyst and subsequently germinate, but most cations induced encystment only and were toxic at higher concentrations. In general, the sensitivity of zoospores to a cation increased with its charge density. At 0.3 FM, La3+ reduced the viability of a zoospore population to 50%, while Fe3+ (20 p~) and Mn2+ (50 p~) induced encystment with only a slight decrease in viability. The other divalent and monovalent cations tested (Mg2+, Ca2+, Ba2+, Lit, Na+, K+, Cs+, NH,+) were effective in inducing encystment or reducing viability at higher concentrations. Each ion showed a distinctive concentration-response curve. Only F-and CH,COO-among the anions tested (Cl-, NO,-, F-, H,P0,2-, Sod2-, CH,COO-) had any effect on zoospores, and at 20 mM (pH 6-0) they reduced viability.The cysts (cystospores) of the fungus were generally less sensitive to cations than were swimming zoospores, and only Cs+ and K+ (50 mM) reduced viability to the same extent in each population. Both zoospores and cysts of this fungus had a broad tolerance to pH and temperature, but cysts were more resistant to low temperatures than were motile zoospores.

A method for the production of zoospores from the plant pathogen Phytophthora cinnamomi is described. The use of siliconized cellulose pads as a hyphal support, which in turn allows shake- grown mycelia to be used, results in the production of zoospores in concentrations of 107-108/litre. It allows one person to produce the equivalent of 1-10 mg of zoospore protein per batch, which is sufficient for a variety of biochemical studies.

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A number of organic compounds were tested to determine their ability to induce encystment or germination of zoospores of the fungus, Phytophthora cinnamomi. The basic amino acids, lysine and arginine, induced encystment at concentrations in the millimolar range. Pectin (500 pg ml-l) also induced encystment and in addition stimulated the encysted cells to germinate. A wide range of carbohydrates and the amino acids, glutamate and aspartate, stimulated cysts to germinate although they did not themselves induce encystment.The polypeptide, polyc-lysine and the basic proteins, histone 1, histone 4 and lysozyme together with the lectin, concanavalin A, immobilized swimming zoospores at concentrations between 0.3 and 10 pg ml-l. At concentrations below 1 pg ml-1 cysts were formed but at higher concentrations cells lysed and the viability of the population was reduced. These compounds did not induce germination of the encysted zoospores. Ethanol, isovaleraldehyde and hexanal also induced encystment but only at concentrations in the millimolar range.These data, together with previous work on cations show that the triggering of motile cells to encyst does not in this species invariably result in the cells being committed to germination.

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