A comparison of the biochemical and physiological effects of the systemic fungicide cyprofuram with those of the related compounds metalaxyl and metolachlor

Abstract

Strains of Phytophthora megasperma insensitive to the fungicide metalaxyl showed limited cross-resistance to the structurally related fungicide cyprofuram and to metolachlor, a herbicide with antifungal properties. The three compounds each inhibited nucleic acid synthesis, but the wide variation in resistance factors and in the degree of nucleic acid inhibition at the concentrations inhibiting mycelial growth by 50% indicated that cyprofuram and metolachlor possess alternative mechanisms of toxicity. Studies with protoplasts suggested that cyprofuram has a direct effect on the plasma membrane which may be important against those resistant strains where nucleic acid synthesis is least affected. Metolachlor also was extremely effective in causing protoplast lysis. Sporangia production by P. palmivora was stimulated by low concentrations of cyprofuram, and germination was delayed. As with other ‘acylalanine-type’ fungicides, inhibition of nucleic acid synthesis is considered to be the main mode of action. Some degree of cross-resistance between cyprofuram and other acylalanines is likely to occur in the field but the dual mode of action of cyprofuram might delay its onset. The effect of increased sporogenesis on the development of resistance is more difficult to assess.