Phenotypic Instability of Cercospora beticola Sacc. Strains Expressing Resistance to the Sterol Demethylation-Inhibiting (DMI) Fungicide Flutriafol after Cold Exposure

A study was conducted in order to elucidate whether decreased sensitivities of Cercospora beticola Sacc. strains to sterol demethylation-inhibitors (DMIs) is a temporary adaptation or a stable trait. Phenotypic stability of resistance to the DMI fungicide flutriafol was tested with 30 single-spore strains collected from diseased sugar beet plants. Strains used in the study represented a wide range of sensitivity to flutriafol. Two different sets of experiments were conducted. The first set of experiments studied the effect of successive transfers on the pathogen’s sensitivity to flutriafol, both in vitro and in vivo. In the second set of experiments, the effect of cold storage on the sensitivity to flutriafol was tested for mycelial isolates and for conidia of the pathogen. Results showed that after successive transfers of isolates either in vitro or in vivo in the absence of flutriafol, sensitivity was not changed significantly in comparison with the initial sensitivity, suggesting a stable genetically controlled trait. Conversely, when fungal strains were exposed to cold conditions, either as mycelial isolates or as conidia, an increase in sensitivity was observed. This increase was most pronounced for strains with moderate and high levels of resistance to DMIs