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Managing QoI resistant Cercospora beticola on sugar beet (Beta vulgaris L.) in the USA

Mohamed Khan: North Dakota State University & University of Minnesota

<div>Cercospora leaf spot (CLS), caused by the fungus<em> Cercospora beticola,</em> is one of the most damaging foliar diseases of sugar beet in warm and humid regions. Fungicide application is an important component of an integrated pest management program for controlling CLS. In the last two decades, growers in Europe and USA have been using mainly single site mode of action fungicides in mixtures or individually with rotation to control <em>C. beticola</em>. Recently, resistance of <em>C. beticola</em> to QoI fungicides were reported in Austria, Serbia, and the USA. In 2016, USA growers suffered over $200 million economic losses due to QoI fungicide resistance. Therefore, we conducted field studies in 2016 and 2017 to evaluate the efficacy of fungicides individually, in mixtures, and in a rotation program for controlling a <em>C. beticola</em> population that was resistant to QoI fungicides and had reduced sensitivity to demethylation inhibitors (DMI). In both years, QoI fungicide treatments showed no significant difference in disease control from the non-treated check. However, the DMI fungicides had an improved efficacy when used in mixtures with broad-spectrum fungicides, such as triphenyltin hydroxide and mancozeb. The use of multiple applications of fungicide mixtures at 10 day intervals were the most effective at controlling CLS resulting in the highest recoverable sucrose content. This work provides a practical way for better controlling CLS in areas with known QoI resistant <em>C. beticola</em> population.</div>