Stepwise development of laboratory resistance to DMI-fungicides in Penicillium italicum

Isolates ofPenicillium italicum with differential levels of resistance to imazalil were obtained via step-wise mass selection of conidia of the fenarimol-resistant isolate E300-3 on imazalilamended PDA. Three out of five selection steps were successful. The resistance level to imazalil of isolates acquired after the two last selection steps was on average 122 and 197. The differential level of resistance was also apparent in decay control on oranges by imazalil inoculated with the various isolates. The isolates showed a similar cross-resistance pattern to other fungicides which inhibit C-14 demethylation of sterols (DMIs), although the level of resistance to these fungicides was significantly higher. All isolates displayed negatively-correlated cross-resistance to tridemorph and dodine. Most isolates had a normal virulence on oranges. In competition experiments with mixed-inocula of the wild-type and a resistant isolate, the proportion of the wild-type increased in successive generations on untreated oranges and the proportion of the resistant isolate increased on imazalil-treated oranges. The lower competitive ability of the resistant isolate on untreated oranges may be due to a decrease in spore production as compared with the wild-type.Since isolate E300-3 was obtained in two selection steps on fenarimol-amended PDA, the isolates obtained in the last selection steps on imazalil-amended PDA may have at least five different genes for resistance to DMIs. This is consistent with resistance to DMIs being under polygenic control, with the genes involved having an additive interaction, although this is not the only possible explanation of the results.