July 3, 2003

 

Sugarbeet Diseases Appearing in Area Fields

Environmental conditions favor the development of cercospora leaf spot, according to Mohamed Khan, North Dakota State University Extension soil science specialist. "Cercospora leaf spot is the most damaging foliar disease of sugarbeet. The disease causes lower tonnage, reduced sucrose content, poor storage ability and increased impurities that lead to higher processing costs."

The most common source of the cercospora fungus is infected sugarbeet debris in the field. The fungus is spread mainly by wind and water and develops rapidly in warm, humid and rainy weather. Day temperatures of 80 to 90 degrees and night temperatures above 60 degrees with humidity above 87 percent favor disease development. Symptoms of the disease may appear five to seven days after infection.

"Cercospora infection produces circular spots about one-eighth inch in diameter with ash gray centers and dark brown or reddish-purple borders," Kahn says. "In humid conditions, the spots may become gray and velvety with the production of spores. The spores further the spread of the disease especially within fields, resulting in many infection cycles during the growing season."

Early control of cercospera leaf spot is important, Kahn notes. He recommends using an integrated approach.

• Use cultural practices such as burying infected tops by tillage.
• Plant approved varieties in fields with a minimum rotation interval of three years.
• Select fields as far away as possible from the previous year’s infected field.
• The first fungicide application should be made when conditions first favor disease development or at disease onset. If the first application is late, control will be difficult all season.
• Use the recommended rates of fungicides. Do not cut rates.
• Only one application of a Topsin in combination with a protectant fungicide such as Penncozeb or Triphenyltin hydroxide should be used in Hillsboro, East Grand Forks, Crookston and Drayton factory districts.
• Never use the same fungicide or fungicides from the same class back-to-back.
• Avoid using fungicides from a particular class of chemistry as a stand-alone in areas where there is known resistance or tolerance to that chemistry.
• Growers have two strobilurins – Headline and Gem, triphenyltin hydroxide-Tin, a triazole- Eminent 125, a benzimidazole –Topsin, and an EBDC – mancozeb for cercospora control. Always alternate fungicides from different classes. If possible, use only one application of a strobilurin during the season.

"Sugarbeet fields with more susceptible varieties with closed rows that are close to shelter belts, waterways or close to previously infected fields should be sprayed first because they would be the first to become infected," Kahn says.

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Source: Mohamed Khan, (701) 231-8596, mkhan@ndsuext.nodak.edu
Editor: Rich Mattern, (701) 231-6136, richard.mattern@ndsu.nodak.edu