Cercospora Leaf Spot in Sugarbeet
This page was adapted from the article, "Cercospora Leaf Spot in Sugarbeet," which appeared in Crop & Pest Report on July 21, 2022
The fungus Cercospora beticola causes Cercospora leaf spot that is the most damaging foliar disease of sugarbeet in Minnesota and North Dakota. This disease results in lower tonnage, lower sucrose concentration, reduced extractable sucrose 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 spreads from field to field mainly by wind. Cercospora leaf spot develops rapidly in warm and wet conditions. Day temperatures of 80-90° F and night temperatures above 60° F favor disease development. Leaf spot symptoms may occur about 5-7 days after infection under favorable conditions. Cercospora infection produces circular spots about 1/8 inch in diameter with ash gray centers and dark brown or reddish-purple borders. Severe infections result in death of leaves (Figure 1).
In humid conditions, the spots may become gray and velvety with the production of spores. These spores further spread the disease, especially within fields, resulting in many infection cycles during the growing season
Because of the multi-cyclic nature of the pathogen, it is important to have early control of Cercospora leaf spot using an integrated approach. This includes cultural practices such as burying infected tops by tillage, planting improved tolerant varieties especially CR+ varieties, using a crop rotation interval of a minimum of three years, selecting fields as far away as possible from the previous year’s infected field, and the timely and proper use of recommended fungicide mixtures in high water volume (15 to 20 GPA) (Figure 2).
Infected leaf samples are collected annually from all factory districts in Minnesota and North Dakota. These samples are tested to determine sensitivity of the fungus to the different fungicides used in their control. C. beticola has developed resistance and/or reduced sensitivity to most of the fungicides used for its control. Currently, no individual fungicide provides season long control of CLS. Mixtures that contain a multisite fungicide and used in a rotation program are most effective at controlling CLS. Consult your agriculturists for fungicide mixtures recommended for your factory district.
Research done at NDSU and University of Minnesota showed that application of effective fungicide mixtures after rows are closed and at disease onset, or at first symptoms in a field or in the factory district, with subsequent applications based on the presence of leaf spots and favorable environmental conditions, consistently provided the most effective and economical CLS control (https://apsjournals.apsnet.org/doi/epdf/10.1094/PDIS-91-9-1105). Frequent rainfall, especially soon after fungicide application, reduces the efficacy of fungicides. Information on the potential for disease in your area based on weather conditions is available at http://ndawn.ndsu.nodak.edu
Sugarbeet fields with more susceptible varieties with closed rows that 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. The most effective strategy to manage CLS is to use improved tolerant varieties (where available) with judicious timely fungicide applications. The development and availability of CR+ varieties with improved resistance to C. beticola will contribute significantly in managing leaf spot (Figure 3).