RELATIONSHIP BETWEEN CHEMICAL-STRUCTURE AND BIOLOGICAL-ACTIVITY OF TRIAZOLE FUNGICIDES AGAINST BOTRYTIS-CINEREA

The inhibitory activity of commercial and experimental triazole fungicides on the target enzyme, sterol 14 alpha-demethylase (P450(14DM)), was studied in a cell-free sterol synthesis assay of Botrytis cinerea Pers. ex Fr. In order to assess structure-activity relationships, the inhibitory activities of the compounds on radial growth of the fungus were tested as well. The EC(50) values (concentrations of fungicide inhibiting radial growth of B. cinerea on PDA by 50%) of all triazoles tested ranged between 10(-8) and 10(-5) M. IC50 values (concentrations of fungicides inhibiting incorporation of [2-C-14]mevalonate into C4-desmethyl sterols by 50%) generally ranged between 10(-9) and 10(-7) M and correlated with inhibition of radial mycelial growth. However, differences in IC50 values did not reflect quantitatively the observed differences in EC(50) values, since the ratio between EC(50) and IC50 increased with decreasing fungitoxicity. For a limited number of compounds the correlation between intrinsic inhibitory activity and fungitoxicity was low. Both in-vitro tests were used to investigate structure-activity relationships for stereoisomers of cyproconazole, SSF-109 and tebuconazole. Fungitoxicity and the potency to inhibit cell-free C4-desmethyl sterol synthesis correlated for all stereoisomers tested. Mixtures of isomers of tebuconazole or cyproconazole were slightly less active than the most potent isomer. The high activity of several commercial triazoles in both experiments implies that poor field performance of triazole fungicides against B. cinerea is due neither to insensitivity of the P450(14DM) nor to low in-vitro sensitivity of the fungus.