ISSUE #366

366 - Season Long Cercospora Control Critical To Profitability

7/10/1999

Cercospora leafspot has reached near epidemic conditions in Minnesota and North Dakota in 1997 and 1998. Cercospora infection of sugarbeet leaves produces circular spots about 1/8 inch in diameter with ash gray centers and dark brown to reddish purple brown borders.

The most common source of the fungus is infected beet debris in the field or adjacent fields of 1997 or 1998. The spores are spread by wind, water and insects. The fungus may also infect redroot pigweed, lambsquarters, mallow, and bindweed. The disease develops most rapidly at temperatures of 68-790F and relative humidities of 90-100%. Night temperatures above 600F favor disease development. Cercospora leafspot severity has increased dramatically during September on three separate years in the 1990’s. It’s essential to maintain disease control at less than economic loss levels through September. Figure 1 shows the disease cycle of Cercospora leafspot.

Figure 1: Disease cycle of Cercospora leaf spot of sugarbeet

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Figure 2: Cercospora Leaf Spot Damage Rating

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A key to a high sugar content crop and high payment per ton is keeping your plant leaf canopy nearly disease free. Figure 2 shows the low percentage of leaf tissue that has to be infected to cause economic loss from your sugarbeet crop. Keep in mind that toxins produced from lesions on the leaf cause much greater yield loss than the percentage of leaf area covered by spots might indicate.

Frequently Asked Questions About Cercospora

Q1. Frequent periods of high humidity and warm temperature are necessary to cause a severe problem?

A1. One day of suitable weather every 10 days can cause a very severe disease problem.

Q2. Leafspots show up on the leaves as soon as infection occurs?

A2. Spots on leaves appear in as few as 5 days after infection if conditions are ideal, but may not be visible for 10 days or more if weather is less suitable for disease development.

Q3. Hot dry weather for a two-week period stops further disease development?

A3. The Cercospora fungus remains slowly active on the leaf during dry hot weather. The disease resumes rapid development with the first suitable weather for infection.

Q4. Additions of fertilizer nitrogen in excess of recommended levels will reduce Cercospora severity?

A4. Research by Dr. Larry Smith, Allan Cattanach and Mark Bredehoeft found 30-lb./acre excess nitrogen reduced crop quality and had no effect on Cercospora leafspot severity in 1999.

Q5. Applications of Cercospora fungicides following hail on sugarbeets will hasten crop recovery?

A5. Research at NDSU and U of MN found no benefit from fungicides, nitrogen or phosphate fertilizer, micronutrients, or sulfur for speeding crop recovery after hail damage.

Q6. Tillage practices in the crop rotation have no effect on Cercospora leafspot?

A6. Use of deeper tillage practices like moldboard plowing after harvest can reduce innoculum carryover and severity of Cercospora.

Q7. Sugarbeet varieties immune to Cercospora leafspot are available to growers?

A7. Varieties immune to Cercospora have not been developed. Approved varieties with a very low leafspot rating (about 4.3) may need one less fungicide application than more susceptible varieties with a leafspot rating over 5.0. Grower Idea Contest Entries Needed Contact your agriculturist to get assistance in documenting new ideas suitable for entry in the 1999 contest. Take pictures and make notes now. Share your ideas with fellow growers so everyone benefits.

“Remember The 120 Or Less”