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Controlling Cercospora Leaf Spot with Fungicides (8/11/11)

Cercospora leaf spot is the most devastating foliar disease of sugarbeet in Minnesota and North Dakota. The disease is caused by the fungal pathogen Cercospora beticola. The fungus overwinters in infected sugarbeet debris in the field. Cercospora leaf spot develops rapidly in warm, humid and wet conditions, typically after canopy closure.

Day temperatures of 80-90 F and night temperatures above 60 F favor disease development.  Day temperature above 93 F is unfavorable for disease development.  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, early control (at first symptoms) is necessary to effectively manage the fungus.  Since the fungus damages the leaves, it adversely impacts the photosynthetic capacity of plants and reduces yield; the disease also results in higher impurities in the juice which reduces sucrose extraction.  

Research shows that application of effective fungicides at first symptoms with subsequent applications 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.

Sugarbeet fields with more susceptible varieties that closed rows the earliest and are close to shelter-belts, waterways, and those close to previously infected fields should be the first to be scouted since they would be the first to become infected. 

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 cut rates.
  3. Only one application of Topsin in combination with a protectant fungicide, such as Supertin, should be used during the season.  When mixing fungicides, use ¾ of the labeled rate of each fungicide.
  4. The fungicides that were most effective individually and in rotations at Foxhome, MN in 2010, were Headline 2.09 EC, Proline 4SC with a non-ionic surfactant, Inspire XT, Supertin 4 L, and a mixture of Supertin 4L and Topsin 4.5 F.
  5. Never use the same fungicide or fungicides from the same class ‘back-to-back’.
  6. Avoid using fungicides of a particular class of chemistry as a stand-alone where there is known resistance to that chemistry.
  7. If using one application per year, do not use the same product year after year – resistance will develop. 
  8. Use of high spray pressure (100 psi) and high water volume of 15 to 20 gal/ac will result in better disease control.

Sugar price is favorable for growers.  As such, growers should make an extra effort to get the highest recoverable sugar per acre possible to reap the benefit of a good sugar price. Most growers 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

mohamed.khan@ndsu.edu

This site is supported in part by the Crop Protection and Pest Management Program [grant no. 2017-70006-27144/accession 1013592] from the USDA National Institute of Food and Agriculture. Any opinions, findings, conclusions, or recommendations expressed are those of the website author(s) and do not necessarily reflect the view of the U.S. Department of Agriculture.

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