Temporal population monitoring of fungicide sensitivity in Cercospora beticola from sugarbeet (Beta vulgaris) in the Upper Great Lakes

In recent years, there has been an increase in the frequency of Cercospora beticola (Sacc.), the cause of Cercospora leaf spot (CLS) in sugarbeet (Beta vulgaris L.), isolates with demethylase inhibitor (DMI) and quinone outside inhibitor (QoI) fungicide resistance collected from commercial fields in Michigan, USA and Ontario, Canada. This study evaluated sensitivity distributions of C. beticola populations in the Upper Great Lakes sugarbeet production region to DMIs and organotin fungicides and determined shifts in the temporal frequency of fungicide sensitivity phenotypes from 2014 through 2017. The effective concentration (mg L⁻¹) for inhibiting growth by 50% (EC₅₀) was determined for six chemistries from the above fungicide classes. Isolates of C. beticola were categorized phenotypically into five sensitivity categories based on EC₅₀ values: Sensitive (<1); reduced sensitive (1–10); moderately insensitive (10–50); insensitive (50–100) and resistant (>100). Significant temporal shifts towards insensitivity were detected from 2014 to 2017 in the population median and mean EC₅₀ values, and frequencies of sensitivity phenotypes to DMI and organotin fungicides. Individual isolates of C. beticola were recovered with cross resistance to DMI and organotin fungicides. There was a population shift towards insensitivity, and thus an increase in the frequency of recovered isolates of C. beticola with lower sensitivity to five DMI fungicides (difenoconazole, fenbuconazole, flutriafol, prothioconazole and tetraconazole) and fentin hydroxide. Results from sensitivity monitoring agree with other studies that an integrated approach, combining knowledge of fungicide efficacy and pathogen biology is essential in developing fungicide resistance management recommendations and thus effective disease management strategies.