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Using Fungicides to Control Cercospora Leaf Spot on Sugarbeet (07/09/20)

Cercospora leaf spot (CLS) (Figure 1) is the most damaging and economically important foliar disease of sugarbeet in North Dakota and Minnesota.

Cercospora leaf spot (CLS) (Figure 1) is the most damaging and economically important foliar disease of sugarbeet in North Dakota and Minnesota. The causal agent of CLS is the fungus Cercospora beticola which is most severe in warm weather (day temperature of 77 to 90° F and night temperature above 60° F) and in the presence of moisture from rain or dew on the leaves for  8 or more hours. The fungus destroys the leaves which are the photosynthetic factories responsible for producing the sugar in the tap roots of the plants. The longer and more severe the infestation, the greater the reduction in tonnage, and lower sugar concentration and recoverable sucrose. Roots of CLS infected plants have higher impurities which increases processing costs and delays the sugar extraction process. 

Growers typically mange CLS by integrating rotation with non-hosts crops including corn, soybean, wheat, and barley, planting CLS tolerant varieties, planting away from a previously infected crop, and applying fungicides in a timely manner. 

What are the fungicide options for 2020?

Current warm and wet conditions in most areas are favorable for C. beticola infection and rapid disease development. Because of the high overwintering population of C. beticola, the first fungicide application should be made soon after row closure or when the first lesion is observed in the factory district. Symptoms typically appear first in fields close to waterways, shelterbelts, last year’s sugarbeet fields, and near corn fields. The lesions are first present on the oldest and lowermost sugarbeet leaves close to the ground.  

The best way to control CLS during the growing season is to apply effective fungicides in a timely manner. For ground application, apply fungicides in 15 to 20 gallons of water per acre at 75-100 psi pressure; aerial applicators should use 3 to 5 gallons of water per acre for best results.

Since 2016, C. beticola have developed widespread resistance to fungicides which are quinone outside inhibitors (QoI) including Headline, Priaxor, Gem and Quadris and reduced sensitivity to triazole fungicides including Eminent, Inspire XT, Proline, Topguard, and Enable. As such, QoI and triazole fungicides, when used alone do not provide effective control of CLS (Figure 2).  Over the past three years, triphenyltin hydroxide (TPTH) fungicides (Supertin and Agritin) have consistently provided effective control of CLS (Figure 3). Tin and triazole fungicides are the mainstay of our program and these should always be mixed with other chemistries including ethylenebisdithiocarbamates (EBDCs) such as mancozeb and penncozeb, copper fungicides, and Topsin can be used in areas where CLS is not very severe. In the northern Red River Valley where CLS is not severe, the mixture of TPTH and Topsin (Figure 4) can be used early in the season for effective and economical control of CLS. Other mixtures which will work well in the northern Red River Valley includes mixtures of TPTH + Manzate / Badge SC + Manzate / TPTH + Badge SC (Figure 5) which are mainly multisite fungicides that can help to reduce the population of C. beticola that are resistant to QoIs and triazoles. In areas where CLS is severe, growers should budget for six applications – three where TPTH is mixed with either a copper product or an EBDC, and three where triazoles are mixed with either a copper product or an EBDC. The mixture of Proline (triazole) and Manzate (EBDC) (Figure 6), and the mixture of Inspire XT (triazole) and Badge (Copper) (Figure 7), each provided effective control of CLS. Growers also have the option of using a mixture of copper and an EBDC for effective CLS control (Figure 8). The use of fungicide mixtures in a rotation program, starting at first symptoms and continuing at 12 to 14 day intervals until mid-September will provide effective control of CLS. Please should consult your agriculturists for the best recommendations for your specific growing districts based on the fungal population known sensitivity to different fungicides.

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Mohamed Khan

Extension Sugarbeet Specialist

NDSU & U of MN

218-790-8596

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