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Controlling Cercospora Leaf Spot with Fungicides (08/06/15)

Cercospora leaf spot is the most damaging leaf disease of sugarbeet in our area.

Controlling Cercospora Leaf Spot with Fungicides

Cercospora leaf spot is the most damaging leaf disease of sugarbeet in our area.  It is caused by the fungus Cercospora beticola when day temperatures range from 77 to 90°F and night temperatures over 60°F in the presence of adequate moisture in the form of rain or dew for at least 8 continuous hours within the crop canopy. Typical foliar symptoms are circular spots about 1/8 inch in diameter with ash gray centers and dark brown or reddish-purple borders.  Under favorable conditions, the fungus may have 4 to 5 disease cycles during the season, and with each cycle there is a substantial increase in the amount of inoculum.  As such, timely fungicide application starting at first symptoms is necessary to effectively manage the fungus.  If the fungus is not controlled, the leaves of the plants - starting with the oldest – are killed resulting in lower tonnage and significantly lower extractable sucrose.

Research shows that the most effective and economical control were obtained when fungicides were applied at first symptoms, with subsequent applications on at least 14 day intervals based on the presence of leaf spots and favorable environmental conditions when Daily Infection Values for two consecutive days were 7 or higher.

Guidelines that will help in effective disease control include the following:

  1. The first fungicide application should be made when conditions first favor disease development or at first symptoms.  If the first application is late, control will be difficult all season.
  2. Use the recommended rates of fungicides to control Cercospora leaf spot - do not use lower than recommended rates when using individual fungicides.
  3. Only one application of Topsin in combination with a protectant fungicide such as Supertin should be used during the season. 
  4. Never use the same fungicide or fungicides from the same class ‘back-to-back’.
  5. Do not use fungicides of a particular class of chemistry as a stand-alone where there is known resistance to that chemistry.
  6. If using one application per year, do not use the same product year after year, otherwise the fungus will develop resistant isolates. 
  7. Use of high spray pressure (100 psi) and high water volume of 15 to 20 gal/ac will result in better disease control.

Over the years, growers in North Dakota and Minnesota have done an excellent job of controlling Cercospora leaf spot resulting in low levels of Cercospora inoculum.  Current conditions are very favorable for disease development.  Fungicides should be applied as soon as first symptoms are observed.  Fungicides typically provide 14 days of protection under heavy disease pressure.  Scouting of fields will contribute to better disease control since it helps in better timing of fungicide applications. 

Mohamed Khan

Extension Sugarbeet Specialist

NDSU & U of MN


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