Effects of imazalil on a wild-type and fungicide-resistant strain of Aspergillus nidulans

The effects of different incubation conditions on toxicity and uptake of imazalil by mycelium of a wild-type (003) and fungicide-resistant (R264) strain of Aspergillus nidulans were determined. In agar plate and liquid shake culture, imazalil was 200 and 40 times less toxic to the R264 strain, respectively, than to the wild-type strain. Inhibition of C-4 desmethyl sterol (ergosterol) biosynthesis occurred rapidly in mycelium of both strains at minimum growth inhibitory concentrations. Imazalil was neither detoxified nor converted into a toxic compound by mycelial suspension. Increased uptake of imazalil by the two strains occurred as concentrations of the fungicide were increased. However, the percentage uptake of imazalil by the wild-type strain was highest at the lowest concentration. These results suggest that binding in the wild-type strain involved a small number of high-affinity sites which became saturated as fungicide concentrations increased; and that at higher concentrations considerable nonspecific binding occurred in both strains. Uptake of imazalil during the initial 10 min of incubation was considerably lower in resistant than in the wild-type strain. However, upon prolonged incubation, both strains took up near equal amounts of fungicide. Uptake of fungicide by both strains was not inhibited by incubation at low temperature but was stimulated by respiratory inhibitors. These data support, in part, the hypothesis that resistance to imazalil, as reported previously for fenarimol (M. A. de Waard and J. G. M. van Nistelrooy, Pestic. Biochem. Physiol. 13, 255 (1980)), is based on reduced uptake of fungicide by mycelium of the resistant mutant.