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TECHNICAL SESSION: Chemical Control of Fungal Diseases

The role of multi-site fungicides for controlling Cercospora beticola and managing fungicide resistance
Mohammad F R Khan - North Dakota State Univ & Univ of MN.

Cercospora beticola causes Cercospora leaf spot (CLS), the most destructive foliar disease of sugar beet worldwide. Growers combine improved genetics, cultural practices, and fungicides to manage CLS. C. beticola has a history of developing resistance to several classes of fungicides. In 2016, widespread QoI resistance resulted in a CLS epidemic and over $200 million reduction in revenue for growers in North Dakota, Minnesota and Michigan. Since 2016, field studies were conducted in the USA to evaluate the efficacy of fungicides for controlling a C. beticola population resistant to QoI fungicides and with reduced sensitivity to demethylation inhibitors (DMI). Results indicated that QoI fungicide treatments resulted in disease severity that was not significantly different from the non-treated check, and DMI fungicides were becoming less efficacious. The efficacy of DMI fungicides improved when used in mixtures with multi-site fungicides, such as triphenyltin hydroxide, copper products, and mancozeb. Results also showed that the use of only multi-site fungicides in a rotation program provided season-long control of CLS. Deploying only multi-site fungicides for CLS control may be a strategy to reduce C. beticola populations that are resistant to site specific fungicides.