CHARACTERIZATION OF ENERGY-DEPENDENT EFFLUX OF IMAZALIL AND FENARIMOL IN ISOLATES OF PENICILLIUM-ITALICUM WITH A LOW, MEDIUM AND HIGH DEGREE OF RESISTANCE TO DMI-FUNGICIDES

Differential accumulation of [C-14]imazalil and [C-14]fenarimol by germlings of wild-type and DMI-resistant isolates of Penicillium italicum was studied at various pH values. At pH 7 and 8 the low-resistant isolate E300-3 accumulated 22% and 35%, respectively, less imazalil than the wild-type isolate W5. Imazalil accumulation at pH 5 and 6 was similar. Isolate E300-3 also accumulated less fenarimol as compared with the wild-type isolate. This difference was much more obvious than for imazalil and was observed at all pH values tested. Differences in accumulation of both imazalil and fenarimol between low (E300-3), medium (H17) and high resistant (I33) isolates were not observed. These results suggest that decreased accumulation of DMIs is responsible for a low level of resistance only and that additional mechanisms of resistance might operate in isolates with a medium and high degree of resistance. With all isolates fenarimol accumulation was energy-dependent. This was not obvious for imazalil. The wild-type and DMI-resistant isolates had a similar plasma membrane potential as determined with the probe [C-14]tetraphenylphosphonium bromide ([C-14]TPP+). Various test compounds, among which ATPase inhibitors, ionophoric antibiotics and calmodulin antagonists, affected the accumulation of [C-14]TPP+, [C-14]imazalil and [C-14]fenarimol. No obvious correlation between the effects of the test compounds on accumulation levels of the fungicides and [C-14]TPP+ could be observed. These results indicate that the plasma membrane potential does not mediate the efflux of DMI fungicides by P. italicum.