Mode of action of the fungicide flusilazole in ustilago maydis

Flusilazole is a potent inhibitor of Ustilago maydis sporidial growth (I 50 = 20 μg liter −1 ). Incorporation of [ 14 C] acetate into ergosterol of growing sporidia is inhibited 50% by 0.5 μg liter −1 of the fungicide. Inhibition of ergosterol biosynthesis is concomitant with the accumulation of the precursors eburicol, obtusifoliol and 14α‐methylfecosterol. A novel cell‐free assay has been developed to measure the 14α‐demethylation of [ 3 H ] dihydrolanosterol. Flusilazole inhibits the cell‐free demethylation with an I 50 of 15 μg liter −1 . These data provide strong evidence that the mode of action of flusilazole is by inhibiting ergosterol biosynthesis through direct inhibition of the 14α‐demethylation of ergosterol precursors .