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 yield stability and profitability of the sugar beet industry by negatively impacting recoverable sucrose, yield tonnage, and sugar extraction. 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 with 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 eight copper-based products and one sulfur-based product. An in vivo study was conducted in a glasshouse on sugar beet plants to assess the efficacy of copper-based fungicides alone and in mixtures with a sulfur-based product in controlling CLS. The results showed that copper-based fungicides inhibited spore germination of DMI- and QoI-resistant C. beticola isolates, with the EC 50 values ranging from 1.73 to 7.52 μg/ml. The copper-based fungicides significantly reduced CLS severity, and their efficacy was enhanced by a sulfur-based product in a mixture. These findings suggest that copper-based fungicides 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 resistance risk fungicides to sustain CLS control and to prevent fungicide resistance buildup.