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Multiple fungicide resistances in Cercospora beticola of sugar beet
Gary Secor: North Dakota State University; Viviana Rivera-Varas: North Dakota State University; Melvin Bolton: USDA ARS NCSL
<div>Cercospora leaf spot (CLS), caused by the fungus <i>Cercospora beticola</i>, is the most damaging disease of sugar beet worldwide. It is endemic in the midwestern region of the US causing yield losses and reduced sugar quality. Disease is managed by a combination of crop rotation, resistant varieties and timely fungicide applications. This integrated management strategy has minimized disease losses with no major disease outbreaks for many years. Six fungicides from four classes of fungicides are applied one to six times during the growing season. The fungicides used are triphenyl tin hydroxide, thiophanate methyl, tetraconazole, difenoconazole, prothioconazole and pyraclostrobin. Since 1998, sensitivity to these fungicides has been monitored in over 1000 samples/year from commercial fields collected by sugar company agronomists throughout Minnesota and North Dakota. Changes in sensitivity to all fungicide classes have been documented in all factory districts, and isolates with resistance to multiple fungicide have steadily increased since 2013. In 2016, 14.4% of the isolates tested (n=1326) were resistant to all fungicide classes used for CLS management. The combination of decreased fungicide sensitivity, resistance to multiple fungicide classes and increased disease pressure in recent years may have an impact on future disease management and an accelerated need for resistant varieties.</div>

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