Characterization of the sensitivity of Cercospora beticola to fungicides commonly used to manage cercospora leaf spot of sugar beet in the Red River Valley

Abstract

Cercospora leaf spot (CLS), caused by Cercospora beticola, is a major foliar disease of sugar beet (Beta vulgaris L.) in the Red River Valley, often reducing yield and sucrose content by up to 50%. Current management relies on resistant cultivars and fungicides, though resistance has developed to some chemistries, including triphenyltin hydroxide (TPTH). This study assessed fungicide sensitivity in 40 C. beticola isolates collected from a commercial sugar beet field in Foxhome, Minnesota, during 2022. Isolates were cultured on CV-8 media amended with Headline (QoI), Proline (DMI), Champ® WG, Manzate Max, and Supertin (TPTH) at 0.01-800 ppm. After 14 days at 23 °C, EC₅₀ values revealed low sensitivity to Manzate Max (8 ppm) and Supertin (2 ppm), but reduced sensitivity to Champ® WG (70 ppm), Headline (101 ppm), and especially Proline (496 ppm). Greenhouse assays confirmed poor performance of Proline in disease suppression. Together, the results highlight emerging fungicide resistance and stress integrated management strategies