Over the past 18 years the first fungicide spray for Cercospora leaf spot occurs between July 1 and July 25. Within the next two weeks most fields will be at row closure- a time when Cercospora leaf spot commonly first appears. Timing of the first spray is critical to successful control. Cercospora leaf spot caused by the fungus Cercospora beticola, is a serious problem for both sugarbeet producers and processors in Montana.
Losses in research plots over the past 16 years have ranged from 0.5-3 tons per acre and 0.5-1.5{6b02cb02835b82b7f756ddf6717aaab7139b350de274ea97f5b53eb230607107} lower sugar. Losses in the Sidney area have been greatly reduced by the use of less susceptible varieties (KWS scores of 5 or less). Greater levels of impurities and sugar loss to molasses have been common where this disease is uncontrolled.
In addition, where Cercospora was not well controlled in the field, storage pile deterioration is more rapid. Losses are caused by loss of photosynthetic leaf area and toxicity from toxins produced by the Cercospora fungus.
Over the past 16 years a well-timed fungicide program increased income by $0-350 per acre as the result of spending $30-70/A for fungicide plus application. In recent years on moderately resistant varieties 1-2 sprays have yielded gross returns of $50-200/A with a properly timed first spray.
Timing of the first spray is dependent on two factors; 1) favorable weather for sporulation and infection by the Cercospora fungus and 2) presence of the disease in a given field. Development of this disease requires overwintered inoculum in the form of infected leaves and petioles from last years crop or infected weeds.
Significant inoculum should be present this year from infections the occurred in late August through early October of 2011 particularly in river bottom fields. Spores are spread by winds and splashing rain, with wind transport generally less than 100-150 yards. Sporulation and infection are generally favored by warm (75-90 0F) temperatures where relative humidity in the leaf canopy is greater than 90{6b02cb02835b82b7f756ddf6717aaab7139b350de274ea97f5b53eb230607107} for more than 10-12 hours.
Infection and sporulation can occur at lower temperatures but require longer periods of relative humidity above 90{6b02cb02835b82b7f756ddf6717aaab7139b350de274ea97f5b53eb230607107} for infections to occur. The optimum conditions for spore production are 86F and 12 hours of relative humidity above 90{6b02cb02835b82b7f756ddf6717aaab7139b350de274ea97f5b53eb230607107}. The optimum conditions for infection are 76F and 100{6b02cb02835b82b7f756ddf6717aaab7139b350de274ea97f5b53eb230607107} relative humidity for 8 hours.
As a rule of thumb night temperatures above 60 F are required for significant sporulation and infection. The predictive model developed by Shane and Teng at the University of Minnesota is model is the basis for the computer prediction system used by Sidney Sugars and Western Sugar fieldmen.
This information is available either via “hotlines” or websites provided by the sugar companies. This system has proven to be very valuable to identify when the epidemic will likely start and when infection periods occur.
Use of this model and use of moderately resistant varieties has resulted in dramatic reductions of fungicide use without losses from Cercospora. Spraying should be based on the presence of the disease in a given field. Initial scouting should be done in fields bordering last years sugarbeet fields since the fungus survives between beet crops in undecayed infected leaves and the spores blow in the wind, commonly less than 100
yards.
Because of the requirement for high humidity for infection and sporulation,
disease development will often start in areas protected from the wind or near water. Field borders and these areas should be scouted first to identify initial infections. Losses are determined by the time of infection and infection intensity. Where beets have been planted back on 2010 beet ground disease risk is very high and extra scouting is encouraged.
Click Here to learn more about the Cercospora Leaf Spot Disease Cycle.
Source: Montana State University
Posted by Russell Nemetz
