Copper-based Chemicals Provided Effective Control of Fungicide-resistant Cercospora beticola Populations

Cercospora leaf spot (CLS) caused by Cercospora beticola poses a significant threat to the profitability of the sugar beet industry by negatively impacting recoverable sucrose, yield tonnage, and pile storability. In North Dakota and Minnesota, growers rely heavily on quinone outside inhibitors (QoIs) and demethylation inhibitors (DMIs) to provide effective control of CLS resulting in several CLS epidemics because of widespread resistance to QoIs and DMIs. Copper-based chemicals in various formulations with different active ingredients have a different mode of action compared to QoIs and DMIs and therefore could be used to manage these resistant populations of C. beticola. One in vitro study evaluated the sensitivity of QoI- and/or DMI-resistant C. beticola isolates to nine copper-based products. An in vivo study assessed the efficacy of copper-based chemicals alone and in mixtures with a sulfur-based product in controlling CLS. Results showed that copper-based products inhibited spore germination of DMI and QoI resistant C. beticola isolates, with the EC50 values ranging from 1.73 to 7.52 μg/ml. In the in vivo study, Fertileader® Copper caused phytotoxic burning on leaves. However, other copper-based products significantly reduced CLS severity and their efficacy was enhanced by a sulfur-based product in mixtures. These findings suggest that copper-based products that inhibit spore germination could provide an effective means for controlling C. beticola. These compounds represent a valuable management tool, that could be used as a mixing partner or alternating with single-site resistant risk fungicides to sustain CLS control and to prevent fungicide resistance buildup.