Reduced sensitivity of Cercospora beticola isolates to sterol-demethylation-inhibiting fungicides

In a survey conducted during October 1995, single-lesion isolates of the su gar beet leaf-spot fungus, Cercospora beticola, were tested for sensitivity to the sterol demethylation inhibiting fungicides (DMIs) flutriafol and bi tertanol. The isolates were collected from fields in three different areas of northern Greece. Fields at Serres and Imathia had been sprayed with DMIs for about 15 years to control sugar beet leaf-spot. At the third site, Amy ndeon, DMI fungicides had not been used. From each area 150 isolates were t ested. ED50 values were calculated for individual isolates by regressing th e relative inhibition of colony growth against the natural logarithm of the fungicide concentration. The mean ED50 values for flutriafol for the Serre s, Imathia and Amyndeon populations were 1.07, 0.73 and 0.5 mu g mL(-1), re spectively (significantly different at P = 0.05). For bitertanol the mean E D50 values for the Serres and Imathia populations were 0.72 and 0.81 mu g m L(-1), respectively, which were not significantly different at P = 0.05. Th e mean ED50 value of the Amyndeon population was 0.48 mu g mL(-1), which wa s significantly lower than those of the other two populations (P < 0.05). A cross-resistance relationship was found to exist between the two triazole fungicides tested when log transformed ED50 values of 60 isolates were subj ected to a linear regression analysis (r = 0.81).