Abstract
Cercospora leaf spot (CLS), caused by the airborne fungus Cercospora beticola, is the most destructive and economically significant foliar disease affecting sugar beet. Yield losses can exceed 40% when environmental conditions favor disease development. While CLS is typically managed with fungicides on a timely basis, excessive use not only adds to production costs but also increases the selection pressure for fungicide resistance in the pathogen. Since 2021, improved sugar beet varieties with the BvCR4 gene that confers higher CLS tolerance (CR+) have become available to producers. Since there is a history of C. beticola populations overcoming the effectiveness of fungicides, we conducted field research to observe the ability of C. beticola to infect CR+ varieties in the presence and absence of fungicides. Field research was conducted from 2022 to 2024 in Moorhead, Minnesota, using two CR+ and one standard susceptible variety, to evaluate the effectiveness of these improved varieties against C. beticola and revealed that their tolerance is beginning to break down. Disease infection and severity was very low and limited to a few plants in 2022. The frequency of C. beticola infection and disease severity increased significantly in 2023 and 2024. Initially, the disease progressed slowly, with the first spots appearing 2-3 weeks later than in a standard susceptible variety; however, disease progression eventually matched that of these susceptible varieties. In untreated CR+ controls, the area under the disease progress curve (AUDPC) steadily increased from 0.7 and 1.3 to 15 and 24 (P < 0.05), indicating a decline in CLS tolerance over time. In contrast, fungicide applications initiated on a calendar basis, at disease onset, and as needed significantly reduced the disease severity, with AUDPC values of 2.9, 5.9 and 6.1 respectively, (P < 0.05), compared to the untreated control (AUDPC = 19, P < 0.05). Although the calendar-based application yielded the lowest AUDPC, recoverable sugar (RSA) did not differ significantly compared to early-start or as-needed applications (P < 0.05), with as needed approach requiring fewer fungicide applications. These results indicates that the fungus, over a period of 3 years, was able to overcome the natural durability of the improved CR+ varieties, underscoring the importance of timely and judicious fungicide application to prevent significant economic losses and help sustain fungicide effectiveness over the long term.