Comparative studies on the mode of action of aluminium ethyl phosphite in four Phytophthora species

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Aluminium ethyl phosphite (fosetyl-aluminium) at concentrations of 250, 500, 750 and 1000 ppm caused severe morphological changes in mycelia of Phytophthora cactorum and P. capsici; P. megasperma was susceptible only at the two highest concentrations, whereas P. infestans was not affected at the concentrations tested. Thickening of the mycelial tips and knot-like swellings along the hyphae were particularly characteristic changes seen in the sensitive Phytophthora species. The fungicide caused a reduction in ion efflux from mycelia of P. cactorum (reduced membrane permeability), whereas the ion efflux from mycelia of P. megasperma and P. infestans was increased (enhanced membrane permeability). The compound did not affect the metabolism of neutral lipids in the four fungi tested. It had marked effects on the metabolism of phospholipids in P. cactorum and P. capsici but only slightly altered these components in P. megasperma and P. infestans. The relatively insensitive P. megasperma and the insensitive P. infestans contain two additional phospholipid components in their membranes compared with those in the sensitive P. cactorum and P. capsici. The fungicide induced a significant increase in the incorporation of glucose into the cellulose/cell-wall fraction of P. cactorum and P. capsici, whereas the synthesis of cellulose in P. megasperma and P. infestans was not affected. The results suggest that toxicity of aluminium ethyl phosphite may be related to the disturbance of physiological processes involved directly or indirectly with membrane function and cell-wall synthesis.