Maintaining healthy sugar beet plants is always a top priority for farmers. Cercospora leaf spot (Cercospora beticola) is the most serious foliar disease of sugar beets in our growing area. Uncontrolled Cercospora can cause yield losses of five or more tons per acre, reducing sugar levels by up to three percentage points. Yield and quality losses can be significant when leaves have as few as one spot per square inch.

Disease development

Cercospora leaf spot (CLS) overwinters in the soil on decomposing beet leaves from previous crops and on weed residue. High temperatures and humidity are ideal conditions for spore development. These spores are then blown by winds or splashed by rainfall onto the growing sugar beet crop and the spores germinate and infect the leaves. The natural defense mechanism of sugar beets limits the damaged tissue to spots of 1/8th inch in diameter. 

Cercospora leaf spots have a dark brown to purple ring around the spot and the grey colored center will have tiny black dots on them. Spores from these “dots” are released which re-infect the plants when daytime temperatures are between 75 and 90 degrees Fahrenheit ( 21 to 32 degrees Celsius) with night time temperatures above 60 degrees Fahrenheit (15 degrees Celsius), coupled with leaf wetness for 10 to 12 hours or more. Extreme temperatures of 95 degrees Fahrenheit (35 degrees Celsius) or above slows Cercospora development. Infection will begin to form on leaves within 7 to 10 days. Without control measures in place entire leaves can become covered with spots which merge together killing large sections of leaves or entire leaves.

Management strategies

Cercospora leaf spot control requires multiple and integrated and intensive approaches.

1. Crop rotation: a longer rotation period will lower the Cercospora leaf spot disease pressure.
2. Weed control: pigweed and lambsquarter hosts CLS so they need to be controlled in all rotational crops as well.
3. Variety selection: Grow approved varieties with good Cercospora tolerance.
4. Fungicides: Triazole fungicides (group 3) are the most effective products against Cercospora and new to this group for the 2020 Ontario sugar beet growers will be a new BASF Triazole called Cevya. Strobilurin fungicides (group 11) is another option but due to resistance to CLS it is not a viable option. EBDC an ethylene bisdithiocarbamate (group M3) and Copper (group M1) should be used early in a fungicide program and tank mixed with Triazole fungicides.
5. Application timing: Spray early before you see any leaf spot because once you have seen infection you have lost yield. June 15 is the targeted date on a normal year. Those following BEETcast 50 DSV’s is a good starting point as well.

Resistance management

To maintain the effectiveness of the fungicides do not spray the same fungicide class back-to-back. Always add either copper or an EBDC with your Triazole fungicide tank mix.