LAUSR

Abstract Cercospora leaf spot (CLS), caused by Cercospora beticola Sacc., is the dominant foliar disease affecting the profitability of table beet production in New York, USA. The disease is controlled primarily with fungicides, but sustainability is critically threatened by strobilurin resistance within the C. beticola population and a limited suite of alternatives. This study evaluated the temporal dynamics of strobilurin resistance within the C. beticola population over three years, and quantified sensitivity to selected demethylation inhibitors (DMIs; propiconazole and difenoconazole) and succinate dehydrogenase inhibitors (SDHIs; fluopyram, penthiopyrad, boscalid and benzovindiflupyr). Resistance to azoxystrobin (EC50 > 0.2 µg mL−1) was prevalent and detected in all fields sampled in 2015 and 2016, and six of 10 fields in 2017. Within fields, the incidence of azoxystrobin-resistant isolates ranged between 4.8% and 70%. No significant differences were detected in the frequency of azoxystrobin-resistant isolates within populations from fields with conventional and organic practices in 2016 and 2017. Reduced sensitivity to propiconazole and difenoconazole was detected in less than 1% of isolates. The EC50 values for the SDHIs were higher than the maximum concentrations (≥ 20 µg mL−1) evaluated. Two small-plot replicated field trials were also conducted over two years to evaluate alternatives to the commercial standards, propiconazole and azoxystrobin. Two applications of benzovindiflupyr + difenoconazole and pydiflumetofen + difenoconazole significantly reduced CLS severity and epidemic progress compared with plots that received propiconazole and non-treated controls.