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

In a survey conducted during October 1995, single-lesion isolates of the sugar beet leaf-spot fungus, Cercospora beticola, were tested for sensitivity to the sterol demethylation inhibiting fungicides (DMIs) flutriafol and bitertanol. 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, Amyndeon, DMI fungicides had not been used. From each area 150 isolates were tested. ED50 values were calculated for individual isolates by regressing the relative inhibition of colony growth against the natural logarithm of the fungicide concentration. The mean ED50 values for flutriafol for the Serres, Imathia and Amyndeon populations were 1.07, 0.73 and 0.5 mu g mL(-1), respectively (significantly different at P = 0.05). For bitertanol the mean ED50 values for the Serres and Imathia populations were 0.72 and 0.81 mu g mL(-1), respectively, which were not significantly different at P = 0.05. The mean ED50 value of the Amyndeon population was 0.48 mu g mL(-1), which was 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 subjected to a linear regression analysis (r = 0.81).