Cercospora leaf spot (CLS) (Figure 1 A, B) is the most damaging leaf disease of sugarbeet in North Dakota and Minnesota. CLS is caused by the fungus Cercospora beticola which does most damage in warm weather (80 to 90 degree F during the day and over 60 degree F in the night) and in the presence of moisture from rain or dew on the leaves. The fungus destroys the leaves (Figure 2) and thus adversely impacts photosynthesis resulting in reduced tonnage and lower extractable sucrose.
Figure 1A. Typical early symptoms of Cercospora leaf spot - circular spots or lesions about 1/8 inch in diameter with ash gray centers.
Growers have done a great job of managing CLS by using crop rotation, CLS resistant varieties, and timely fungicide applications. This has resulted in low inoculum pressure in growers’ fields.
Growers should not allow the disease to become severe before they start fungicide application since that will be a recipe for disaster. As such, after row closure fields should be scouted every 5 days so that the first application can be made at first symptoms. Currently, rows have already closed in many fields so scouting should commence in those fields, especially fields close to waterways, shelterbelts, last year’s sugarbeet fields, and next to corn fields. I have personally confirmed the presence of CLS in a grower’s field in the Moorhead factory district.
The best way to control CLS during the growing season is to apply fungicides in a timely manner. For ground application, apply fungicides in 15 to 20 gallons of water per acre at 100 psi pressure; aerial applicators should use 5 gallons of water per acre for best results. Use full rates of fungicides when they are used alone, and a minimum of ¾ times the labeled rates of each fungicide when using mixtures.