Alignment of the Stars and Fungicide Resistance Leads to Disease Epidemic in Sugar Beet.

Minnesota and North Dakota produce 57% of the United States sugar beet, Beta vulgaris.  Cercospora leaf spot (CLS), caused by the fungus Cercospora beticola, is one of the most damaging foliar diseases of sugar beet resulting in reduced recoverable sucrose and higher processing costs.  In 1998, growers lost over $75 million because of a CLS epidemic. Since 1999, growers have used an integrated system including better CLS resistant varieties, crop rotation, and judicious use of fungicides to manage CLS. Growers reduced fungicide use by 48% from 1998 to 2014 resulting in an average saving of $16 million annually. In 2016, early planting accompanied by adequate rainfall and warm temperatures resulted in relatively early row closure starting in mid-June. Cercospora inoculum present from the 2015 crop resulted in symptoms in late June with growers starting fungicide applications in late June and early July. Demethylation inhibitors (DMIs) and triphenyltin hydroxide (TPTH) were used early in the season and quinone outside inhibitors (QoI) later in the season, typically in August. Reports on the inability of fungicides to control CLS were made in August. More than 90% of fields in southern and central Minnesota had severe CLS in 2016. Sampling and laboratory testing of C. beticola from fields sprayed with fungicides but with severe CLS indicated widespread resistance to pyraclostrobin, a widely used QoI fungicide. The pathogen also had reduced sensitivity to DMI, TPTH, and thiophanate methyl fungicides. Growers lost over $300 million in gross revenue because of CLS in 2016. Growers will need more tolerant varieties and fungicides with different modes of action in a rotation program that will control CLS and manage resistance to produce sugar beet economically.