Investigation of the sterol composition and azole resistance in field isolates of Septoria tritici

We report here a biochemical study of resistance to azole antifungal agents in a field isolate (S-27) of a fungal phytopathogen. Isolates of Septoria tritici were compared in vitro, and their responses reflected that observed in the field, with S-27 exhibiting resistance relative to RL2, In untreated cultures, both RL2 and S-27 contained isomers of ergosterol and ergosta-5,7-dienol although in differing concentrations, Under azole treatment, this phytopathogen exhibited a response similar to that of other pathogenic fungi, with a reduction in desmethyl sterols and an accumulation of 14 alpha-methyl sterols, indicative of inhibition of the P450-mediating sterol 14 alpha-demethylase. Growth arrest was attributed to the reduction of ergosterol combined with an accumulation of nonutilizable sterols. Strain S-27 exhibited an azole-resistant phenotype which was correlated with decreased cellular content of azole.