Effects of pyraclostrobin rotations and tank-mixtures on Cercospora beticola sensitivity and Cercospora leaf spot control in sugarbeet

Abstract

Sugarbeet production supplies approximately 50–60% of domestic sugar in the United States. One of the most economically important foliar diseases of sugarbeet is Cercospora leaf spot (CLS), caused by Cercospora beticola. Fungicide resistance is a major concern due to the frequent applications needed to control multiple infection cycles each season and the reliance on single-site fungicides. Reduced C. beticola sensitivity to multiple fungicide groups, including the quinone outside inhibitors (QoIs) has been observed. The QoI fungicide Headline (23.6% pyraclostrobin) was exceptionally effective in controlling CLS until high levels of reduced sensitivity stabilized in C. beticola populations. This study aimed to test application programs to minimize C. beticola pyraclostrobin resistance development and maximize CLS management. Rotation (high- and low-risk) and tank-mixture programs integrating pyraclostrobin (Headline™) with mancozeb (a multi-site fungicide) treatments were evaluated in 2019 and 2020 field studies. For all Headline programs, distributions of C. beticola pyraclostrobin sensitivities were not significantly different from each other but differed from the non-treated control (P < 0.05). In 2020, a significant increase in the percentage of isolates with half-maximal effective pyraclostrobin concentrations ≥5 μg/ml was noted in all Headline treatments (P < 0.05), even where a single application of Headline alone was used. Furthermore, no additional CLS control, yield, or sugar benefits from Headline were observed when compared to grower standard programs without Headline (P > 0.05). In conclusion, pyraclostrobin-based products used in chemical rotations or tank-mixtures were not more beneficial than lower-risk programs despite their historical CLS control efficacy.