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Extension > Minnesota Crop News > Managing Cercospora leaf spot of sugarbeet

Monday, July 20, 2015

Managing Cercospora leaf spot of sugarbeet

by Mohamed Kahn, Extension Sugarbeet Specialist

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.

Figure 1B. Cercospora lesion with "black pepper–like" spots and conidiospores in the center.



Figure 2. Later season symptoms of Cercospora leaf spot on sugarbeet.

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.

Figure 3. Sugarbeet treated with fungicide for CLS.

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.


Figure 4. CLS infection on sugarbeet in non-treated check.
Research shows that application of effective fungicides at first symptoms, and subsequent applications at least at 14 day intervals based on the presence of leaf spots and favorable environmental conditions (Daily Infection Values for two consecutive days of 7 or higher) consistently provided the most effective and economical control. In principle, the timely use of different chemistries of fungicides in a rotation program will effectively control the disease and manage fungicide resistance. Mixtures of effective fungicides should be used especially in the first application to prevent the pathogen from increasing its population. Over the past decade, a mixture of Triphenyltin hydroxide (TPTH) (4L) and Topsin (F) fungicides as the first application has shown to be most effective (Figure 3) compared to a non-treated check (Figure 4). This mixture should help to control any overwintering CLS isolates which are resistant to the triazoles. The triazoles can then be used in rotation with the strobilurins (or QoI), with a fourth application, if necessary in a heavy disease year, in the form of TPTH.

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