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Managing Cercospora Leaf Spot of SugarbeetManaging Cercospora Leaf Spot of Sugarbeet (06/22/17)

Cercospora leaf spot (CLS) is the most damaging leaf disease of sugarbeet in North Dakota and Minnesota.

Managing Cercospora Leaf Spot of Sugarbeet

Cercospora leaf spot (CLS) (Figure 1 and 2) is the most damaging leaf disease of sugarbeet in North Dakota and Minnesota. The causal agent of CLS is the fungus Cercospora beticola which is most damaging 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 hours or more. The fungus destroys the leaves and thus adversely affects photosynthesis. The longer and more severe the infestation, the greater the reduction in tonnage, sugar concentration and recoverable sucrose. Higher impurities in roots of infected plants results in higher processing costs.

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Growers typically manage CLS by integrating crop rotation with non-hosts, planting CLS resistant varieties, planting away from a previously infected crop, and timely fungicide applications.

From 1999 to 2015, growers effectively controlled CLS mainly by using effective fungicides with different modes of action in a rotation program starting after first symptoms were observed. The fungicides that were most effective at controlling CLS during that period were the triazoles (Eminent/Minerva, Minerva Duo, Proline, and Inspire XT), the QoIs (Headline, Gem, Priaxor – a mixture of a QoI and SDHI), TPTH, and Topsin used in a mixture with TPTH. During 1999 to 2015, growers reported they used Eminent on 100% (Please note that multiple applications of fungicides to the same acreage were counted as multiple treated acres; thus treated acres may exceed 100% of acres planted) of reported acres; Headline on 86% of reported acres; and TPTH on 108% of reported acres (from 1983 to 2015). Other triazoles used included Proline and Inspire on 38% and 32% of reported acreage, respectively, from 2008 through 2015. Priaxor was used on 26% of reported acres in 2015.

In 2016, favorable environmental conditions for CLS infection and disease development in most production areas resulted in a C. beticola population that became resistant to QoI fungicides (91%), triazoles (24%), TPTH (46%), Topsin (86%), and 5% of isolates evaluated were resistant to QoIs, triazoles, TPTH and Topsin (Secor et al. 2017. Sensitivity of Cercospora beticola to foliar fungicides in 2016. Sugarbeet Res. Ext. Rep. 47:154-162). As a result, CLS control with fungicides in 2016 was not very effective, especially in the southern production areas although 5 to 6 fungicide applications were made during the season in those areas (Figure 3).

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What are the options for 2017?

The sugarbeet crop was planted during 3 to 4 different planting dates with most planting completed by mid-May. Emergence and plant stand was good with most fields having 175 to 225 plants per 100 foot of row. Adequate soil moisture and rainfall coupled with relatively warm temperature has resulted in good crop growth with the expectation that the earliest planted crop will close rows by the end of June and the remainder of the crop in early July. There is great potential for a high yielding sugarbeet crop.

However, conditions favorable for crop growth are also conducive for severe CLS. Sugarbeet fields have the highest amount of overwintering inoculum since 1999. Growers should start spraying for CLS after rows are closed and conditions become favorable for infection (warm and wet conditions) or as soon as first symptoms are observed in fields or the factory district. It is recommended that after row closure, fields should be scouted every 3 to 5 days so that the first application can be made at first symptoms. Symptoms typically appear first in fields close to waterways, shelterbelts, last year’s sugarbeet fields, and corn fields.

Growers should check with their respective cooperatives for recommendations made for specific areas.

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 the pathogen has developed resistance to multiple fungicides and no fungicide provided effective control when used alone in research trials in 2016 (Figure 4), all applications should be a mixture with different modes of action in 2017. The best treatment in 2017 was a mixture of a triazole and TPTH.

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Growers who will start with a TPTH + Topsin (or a non-triazole) treatment as their first treatment should start application after row closure and at disease onset (77 to 90°F, wet leaves with moisture from rainfall or dew). Growers starting with a triazole in the mixture in the first application can start at first symptoms or at disease onset. Growers should shorten spray intervals during periods with heavy rainfall. Try to avoid using site specific fungicides with the same mode of action (triazoles, QoI, Topsin) back to back. Please note that 24 fl oz (liquid) or 15 oz (dry) of TPTH may be used per season and that one application of Minerva Duo (16 fl oz) has 4 fl oz of TPTH. In areas where the C. beticola population is still sensitive to QoI fungicides and growers plan to use a QoI fungicide, it should be mixed with another chemistry such as TPTH or an EBDC (such as mancozeb). It is recommended to do a compatibility test with each batch of mixtures to ensure that the mixture will not result in nozzle plugging. Research is ongoing to determine the utility of using different mixtures for controlling CLS and managing fungicide resistance. Feel free to call me for current research updates or visit our research site to observe real time CLS control using individual and fungicide mixtures.

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Mohamed Khan

Extension Sugarbeet Specialist

NDSU & U of MN

218-790-8596

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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