Sterol Composition of DMI-Resistant and -Sensitive Field Isolates of Cercospora beticola

The effect of the sterol 14a-demethylase-inhibitor (DMI) fungicide flutriafol, on the sterol composition of DMI-sensitive and -resistant strains of Cercospora beticola, was studied. In the absence of flutriafol, these strains had the same sterol composition regardless of their sensitivity to DMIs, with ergosterol the major sterol in the fungal membranes. Following treatment with flutriafol, the desmethyl sterols decreased and the eburicol, obtusifoliol and 14a-methylergosta-8,14, 24(28)-trienol content increased, indicating that sterol 14a-demethylase activity was inhibited in all C. beticola strains tested. In the resistant strains, C14-methyl-sterols accumulated at a higher flutriafol concentration than they did in the sensitive strains. The results demonstrate that DMI-resistance in C. beticola is not conferred a sterol 14a-demethylase deficiency, suggesting an alternative mechanism in the observed resistance phenotypes.