Variable resistance factors of fungicides acting as sterol demethylation inhibitors

Abstract

AbstractThe ED50 values and resistance factors of 20 fungicides that all act as inhibitors of the C‐14 demethylation of 24‐methylenedihydrolanosterol were determined for one wild‐type and four resistant strains of Ustilago avenae. All fungicides were cross‐resistant to each other; however, the resistance factors varied considerably, ranging from 50 (triadimenol) to 2·2 (miconazole). A tentative structural requirement for low resistant factors was the presence of two phenyl rings separated from each other by at least three atoms. Labeling of lipids with [14C]acetate in the absence and presence of the inhibitors and subsequent sterol analysis revealed that the variable resistance factors were not related to the presence of a second target site. In spite of reported second modes of action of fenarimol, tebuconazole or miconazole, accumulation of C‐14 sterol precursors in both sensitive and resistant isolates was necessary to accomplish growth inhibition.