Fungicides inhibiting sterol biosynthesis belong to chemically distinct classes such as imidazole, morpholine, pyridine, pyrimidine and triazole derivatives. Incubation of mycelium of for 90 min with representatives of these fungicides induced an efflux activity which prevented accumulation of fenarimol, a pyrimidine derivative, into the mycelium. Induction of this efflux activity reduced the fungitoxicity of fenarimol. Addition of oligomycin to mycelium in which fenarimol-efflux activity was induced immediately increased the uptake of fenarimol, indicating that the efflux activity is energy-dependent. Subsequent disruption of membrane permeability with sodium lauryl sulphate instantaneously caused leakage of fenarimol from the mycelium into the medium. The ability to induce fenarimol efflux is rather specific for inhibitors of sterol biosynthesis: except for pimaricin, fungicides with unrelated mechanisms of action did not have this ability.


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