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Cercospora Leaf Spot Control (08/06/20)

Is the weather helping or hurting the sugarbeet crop? The warm weather coupled with rainfall provided favorable conditions for Cercospora leaf spot. How widespread is leaf spot at this time? Will rainfall impact the efficacy of fungicides? How should growers control Cercospora leaf spot this season? What precautions should growers use with fungicides? How long should the sugarbeet crop be protected from Cercospora leaf spot?

Is the weather helping or hurting the sugarbeet crop?

Prolonged wet conditions along with hail damage in some areas will adversely impact average sugarbeet yield in affected areas. However, environmental conditions are generally favorable for crop growth in most areas which should result in a high yielding sugar beet crop – hopefully a 30 ton per acre crop!

The warm weather coupled with rainfall provided favorable conditions for Cercospora leaf spot. How widespread is leaf spot at this time?

Warm day temperature above 77° F and night temperatures above 60° F along with free moisture from dew and rainfall provided favorable conditions for Cercospora leaf spot infection and development in most areas. As such, growers in all of our sugarbeet production areas have started their fungicide applications to control this disease. At this time, growers in southern production areas already have applied about four applications and growers in northern areas, where the disease is less severe, have applied an average of two fungicide applications.

Will rainfall impact the efficacy of fungicides?

Yes - Rainfall will impact the efficacy of fungicides, especially contact fungicides which have to be on the leaf surface to provide protection. Fungicides which are applied 2 to 6 hours before a rainfall and those which are translaminar or are systemic are expected to provide protection for 10 to 14 days. Rainfall will wash off contact fungicides such as Tin, Coppers, and the EBDC’s that have mancozeb as the active ingredient. It will be ideal to have plants protected with a systemic fungicide - such as thiophanate methyl (Topsin, Incognito) and the triazoles that include fungicides such as Proline, Inspire XT, Provysol, Eminent, Minerva, Enable and Topguard – during rainy periods.

How should growers control Cercospora leaf spot this season?

Growers have started their fungicide applications and most are doing an excellent job of controlling the disease (Figure 1). Fungicide mixtures need to be applied in a timely manner starting just after row closure or at first symptoms, with subsequent applications at 12 to 14 day intervals. In areas with lots of rainfall, the application interval may have to be shortened to 10 days especially where contact fungicides are used. Mixtures should comprise of two fungicides with different modes of action. Tin and triazoles are the foundation of our fungicide program and should be mixed with fungicides containing mancozeb, copper, and topsin. In the Red River Valley, growers have the option of using a QoI fungicide such as Priaxor or Gem in their mixtures. It is best to apply fungicides by ground rig complemented by aerial application when fields are too wet and impractical for tractors.

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What precautions should growers use with fungicides?

Since the sugar cooperatives in North Dakota and Minnesota are expecting a high yielding crop, pre-pile harvesting - where the roots are processed soon after harvest - will start around August 10 for Minn-Dak Farmers Cooperative and Southern Minnesota Beet Sugar Cooperative producers, and around August 17 for American Crystal Sugar Company producers. As such, growers will need to determine which fields or parts of fields they will harvest early and make sure that they use fungicides in those fields with appropriate pre-harvest intervals.

How long should the sugarbeet crop be protected from Cercospora leaf spot?

Growers are encouraged to scout and protect fields with fungicides through the end of September. If fields are not protected in September, Cercospora leaf spot can result in rapid defoliation which can cause a 2% reduction in sucrose concentration or a reduction in payment of about $300 to $360 per acre! There are several fungicides (Tin, Proline, Copper, Priaxor) which can be safely applied until mid- or late-September (in North Dakota and Minnesota) to provide protection from the very damaging leaf spot fungus.

 

Mohamed Khan

Extension Sugarbeet Specialist

NDSU & U of MN

701-231-8596

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