Mancozeb, copper, sulfur, phosphite, and Bacillus subtilis enhance the efficacy of DMI fungicides for managing Cercospora leaf spot of sugar beet.

Cercospora leaf spot (CLS), caused by Cercospora beticola Sacc., remains a major foliar disease of sugar beet (Beta vulgaris subsp. vulgaris L.) worldwide. Increasing prevalence of resistance to demethylation inhibitor (DMI) fungicides also constrain effective management practices. The efficacy of commercially available DMI fungicides (prothioconazole, tetraconazole, difenoconazole plus propiconazole, and mefentrifluconazole) was evaluated when tank-mixed with broad-spectrum partners including mancozeb, copper, sulfur, potassium phosphite, sodium bicarbonate, and Bacillus subtilis. Field trials were conducted in 2020 and 2021 in Northwest Minnesota. All DMI fungicides suppressed CLS relative to the nontreated control, and mixtures with select partners significantly enhanced disease control and sucrose yield. Notably, mancozeb and copper consistently improved outcomes when combined across multiple DMIs, while sulfur, phosphite, and B. subtilis exhibited improved outcomes in specific pairings. Fungicide efficacy varied by combination, emphasizing the importance of tank mix partner selection. These findings demonstrate that strategic tank mixes can enhance CLS disease suppression and improve sucrose yield outcomes in sugar beet.