Sensitivity of Cercospora beticola Isolates to Azoxystrobin

Sugar beet leaf spot, caused by the air-borne fungus Cercospora beticola Sacc., leads to a decrease in sugar beet leaf mass and the consequent regrowth of leaves based on exploiting the sugar reserves stored in the plant’s roots, thus ultimately resulting in lower yields and sugar contents of sugar beets. Azoxystrobin belongs to the group of QoI fungicides, which inhibit mitochondrial respiration by blocking cytochrome c reductase. The QoI fungicides are characterized by a very high risk of resistance interfering with their biological activity. For the purpose of testing the azoxystrobin sensitivity of the Cercospora beticola population found at the site of Rimski Šančevi, a collection of 84 isolates was assembled and tested for sensitivity to azoxystrobin by measuring the mycelial growth on fungicide-amended media with the addition of SHAM. The results obtained indicate that none of the isolates tested exhibited complete sensitivity to azoxystrobin, 4% were found to have reduced sensitivity, 26% were moderately resistant and 70% were highly resistant. A higher proportion of resistant isolates recorded is associated with the loss of azoxystrobin biological efficacy at the experimental site.