Changes in sensitivity of Cercospora beticola populations to sterol-demethylation-inhibiting fungicides during a 4-year period in northern Greece

From 1996 to 1999, single-lesion isolates of Cercospora beticola, the cause of sugar beet leaf-spot disease, were obtained from sugar beet fields with different histories of use of the sterol-demethylation-inhibiting (DMI) fungicides, flutriafol and difenoconazole in Amyndeon, Imathia and Serres in northern Greece. Fields were sampled twice per year, before and after the within-season exposure of fungal populations to applications of DMI fungicides. The sensitivity to flutriafol and difenoconazole of 150 isolates of C. beticola per sampling site and sampling period was determined. In areas with a history of prolonged DMI use, sensitivity to both fungicides decreased gradually over the study period, suggesting directional selection on a polygenically controlled character. The size of the shift in a particular year seemed to depend on which fungicide was applied during that year in the fields, suggesting only partial cross-resistance between flutriafol and difenoconazole. Within the Serres population there was a significant correlation (r=0.57) between sensitivities to flutriafol and difenoconazole. Decreased sensitivity was the result of the selection of less sensitive strains during the spraying periods. The frequency of such strains significantly increased through the years and finally reached values of 32% for both flutriafol and difenoconazole in the Serres population and 20 and 16%, respectively, in the Imathia population, at the end of 1999. The high frequency of less sensitive strains in the Serres population was related to a slight decline in DMI field performance in this area during that year. However, the frequency of the less sensitive strains was not stable. The sensitivity assays carried out early in each season, before the DMI spray applications, showed that the mean sensitivities of the populations were significantly higher (P<0.05) than those of the previous autumn. These changes were mainly the result of a significant decline in frequency of the less sensitive strains, indicating that such strains have either reduced saprophytic survival ability during the overwintering period or lower competitive ability in the absence of DMI treatments.