Action of fungicidal triazoles of the diclobutrazol series on ustilago maydis

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Abstract
Diclobutrazol [(2RS, 3RS)‐1‐(2, 4‐dichlorophenyl)‐4, 4‐dimethyl‐2‐(1,2, 4‐triazol‐1‐yl)pentan‐3‐ol] decreased the rate of growth of Ustilago maydis during the log phase. Marked changes in sterol composition were observed with a decrease in ergosterol and an increase in methyl‐sterols, indicating a block in the removal of the 14‐methyl group. The inhibition was of rapid onset (<4 h). Changes in other lipid constituents were minor and there was no build up of unsaturated fatty acids. The fungicidal activity resides in the (2R, 3R)‐isomer, which is known to be more potent in blocking a yeast 14‐demethylase enzyme than the (2S, 3S)‐isomer. This relationship held in the 4‐chlorophenyl homologues, which as a group were less fungicidal and less potent inhibitors than the 2, 4‐dichlorophenyl compounds. The evidence presented indicates that the primary lesion caused by the fungicide was a build up of membrane sterols containing extra methyl groups; this, in combination with a loss of ergosterol, is believed to prevent proper membrane ordering and thus to cause a loss of membrane function.