Abstract
Cercospora beticola causes Cercospora leaf spot (CLS), the most economically destructive disease of sugar beet in the North Central USA. From 1999 through 2015, most growers in Minnesota and North Dakota used quinone outside inhibitor (QoI), demethylation inhibitor (DMI), triphenyltin hydroxide (TPTH), and to a lesser extent thiophanate methyl in an alternation program to successfully manage sensitive populations of C. beticola. Over time, the pathogen developed resistance to most of the fungicides used for its control. In 2016, growers in the USA lost over $200 million because of QoI resistance, reduced sensitivity to DMI fungicides and a CLS epidemic. There is no individual fungicide that currently provides effective control of C. beticola and no new chemistry identified for CLS control in the near future. Sugar beet seed companies have been working at developing varieties with improved tolerance to C. beticola. Current field trials in the USA indicate that new cultivars (CR+) have better resistance to C. beticola compared to the currently approved non-CR+ cultivars. Field trials using currently approved moderately tolerant varieties (non-CR) and new improved CR+ varieties were conducted in 2019 through 2022. Fungicide applications were done on a calendar basis, and on an only when needed based on the presence of symptoms and favorable environmental conditions. In 2019, under environmental conditions favorable for fungicide applications, the newer improved CR+ varieties provided higher recoverable sucrose than one non-CR+ variety and similar yields to a second non-CR+ variety. In 2020, under conditions favorable for CLS, the new CR varieties produced significantly higher recoverable sucrose compared to the non-CR varieties. One CR-variety was highly resistant to C. beticola and its yield of recoverable sucrose in the absence of fungicides was not significantly different from treatments that received multiple fungicide applications. In a dry and warm 2021 season, fungicides provided protection for the non-CR varieties, but were not necessary the CR+ varieties. In 2022, conditions were not favorable for development of C. beticola until late August. One or two timely fungicide applications based on the presence of symptoms and thresholds resulted in low disease severity and recoverable sucrose similar to treatments with five applications that started before or at row closure. The availability and use of the newer CR+ varieties will improve the economic viability of the sugar beet industry. However, fungicides will have to be used judiciously to prolong the usefulness of the CR+ varieties.