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

Authors: HERNANDEZ, ALEXANDRA - Michigan State University; RUTH, SARAH - Michigan State University; BLOOMINGDALE, CHRISTOPHER - Michigan State University; Hanson, Linda; WILLBUR, JAIME - Michigan State University

Submitted to: Crop Protection
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/23/2024
Publication Date: 4/30/2024
Citation: Hernandez, A.P., Ruth, S., Bloomingdale, C., Hanson, L.E., Willbur, J.F. 2024. Effects of pyraclostrobin rotations and tank-mixtures on Cercospora beticola sensitivity and Cercospora leaf spot control in sugarbeet. Crop Protection. 182. Article 106710. https://doi.org/10.1016/j.cropro.2024.106710.
DOI: https://doi.org/10.1016/j.cropro.2024.106710

Interpretive Summary: Sugar beet, which supplies approximately half the domestic sugar in the United States, is subject to losses due to disease. One of the economically most important foliar diseases in the US is Cercospora leaf spot, caused by the fungus Cercospora beticola. Fungicides are an important tool in managing this disease, but resistance to several fungicides has developed. Among these, the strobilurin fungicides have been highly effective in the past, but resistance has been reported. This study looked at different spray programs to determine whether they could affect the resistance development to a major strobilurin fungicide, pyraclostrobin, while still managing the disease. Rotating chemicals, either starting with the target fungicide or another fungicide, and mixing the target fungicide with a different fungicide were tested in field studies in 2019 and 2020. For all programs, distributions of C. beticola strobilurin responses were not significantly different from each other but differed from a non-treated control (P < 0.05). In 2020, a significant increase in the percentage of isolates with effective concentration for 50% reduction in growth (EC50) values over 5 µg/ml was noted in all strobilurin treatments (P < 0.05). This is above the baseline sensitivity of less than 1 µg/ml, but lower than field rates. No additional disease control, yield, or sugar benefits were found compared to grower standard programs without the strobilurin (P > 0.01). Based on these results, strobilurins used in chemical rotations or in a tank-mixture were not more beneficial than lower-risk programs despite historical efficacy.

Technical Abstract: Sugarbeet production is responsible for supplying approximately 50-60% of domestic sugar in the United States. However, this production can be affected by biotic factors such as weeds, insects and diseases. One of the most economically important foliar diseases of sugarbeet is Cercospora leaf spot (CLS), caused by the fungal pathogen Cercospora beticola. Fungicide resistance is a major concern due to the frequent applications necessary to control multiple infection cycles of CLS each season and the importance of fungicides with single modes of action. Reduced sensitivity has been observed for C. beticola to multiple fungicide groups, including quinone outside inhibitors (QoIs). 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 programs to minimize C. beticola pyraclostrobin resistance development and maximize management of CLS. Rotation (high- and low-risk) and tank-mixture programs integrating pyraclostrobin with mancozeb (a multi-site mode of action fungicide) treatments were evaluated in field studies conducted in 2019 and 2020. 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 exhibiting effective concentration for 50% reduction in growth (EC50) values over 5 µg/ml was noted in all Headline treatments (P < 0.05), even where only a single application of Headline alone was applied. Furthermore, no additional CLS control, yield, or sugar benefits were noted when compared to grower standard programs without Headline (P > 0.01). Based on these results, pyraclostrobin-based products used in chemical rotations or in a tank-mixture were not more beneficial than lower-risk programs despite historical CLS efficacy.