Cooperative Extension Service Communications and Technology Department 3354 1000 E. University Ave. Laramie, WY 82071 (307) 766-6342 • fax (307) 766-3998 • www.uwyo.edu

 

For Immediate Release

Story Contact:

Gary Franc: (307) 766-2397

 

Contact: Steven L. Miller, Senior Editor

Phone: (307) 766-6342

E-mail: slmiller@uwyo.edu

Archived News Site www.uwyo.edu/agadmin/news/news.htm

 

Date: May 25, 2006

 

UW plant pathologist recommends head start against cercospora leaf spot

 

            Sometimes not seeing is believing in a producer’s battle against cercospora leaf spot (CLS) in sugar beets.

            Waiting to apply fungicides until symptoms of the profit-eating disease to occur can be costly. As growers peer down the road to harvest, the disease may already have a 10- to 20-day head start.

            Gary Franc, a University of Wyoming Cooperative Extension Service plant pathologist, recommends that producers trip the disease at the start by using a model that predicts the onset of the disease and spraying before symptoms are seen.

            Symptoms can occur 10 to 20 days after infection. The period of time between infection and when symptoms appear is the latency period of a disease.

            “Due to latency, making the first fungicide application when the disease first appears already misses the most effective time for the first application and allows a much greater population to be present that now must be suppressed,” said Franc, a professor in the College of Agriculture’s Department of Plant Sciences. “The first fungicide should have been applied 10 to 20 days earlier than scouting indicated.”

            The disease prediction model developed in the Red River Valley in North Dakota and modified for other sugar beet production areas is useful for alerting growers when environmental conditions are favorable for infection. “The model assumes the cercospora leaf spot pathogen is present, and the sugar beet crop is susceptible,” noted Franc.

            “If you have faith in the model, it is telling you to spray oftentimes when you see nothing out there. That’s tough to get across because it is counterintuitive to growers. How do you convince growers the most effective spraying time for the first fungicide application is when you really don’t see the disease?”

            CLS attacks older leaves first and forms circular spots with a purplish-brown border and gray-tan center. With favorable environmental conditions, such as high humidity and warm evening temperatures, the disease literally explodes and the spots merge. Leaves die and remain attached to the crown.

            The damage is not just from the loss of photosynthesis, said Franc. The fungus exudes toxins into the leaf causing large amounts of damage. The plant expends energy for leaf production rather than producing sugar to be stored in the beet root. Once lesions appear, the fungus can redistribute itself, with up to 10 times the amount of disease appearing in a week or two under favorable environmental conditions.

            There is almost a direct relationship to the percentage of disease present at harvest and loss in profits. One percent observable disease resulted in approximately one-half to 1-percent loss in profits in several studies, Franc said. He noted studies suggest a 30- percent decrease of recoverable sucrose was fairly common under moderate disease conditions in some production areas.

            “Putting fungicide on at the proper time delays the onset of the disease and will slow the rate of the disease progression,” said Franc.

            Growers have indicated there has been resistance of the fungus to some pesticides, and surveys have verified this has happened, according to Franc.

             Franc recommends a four-pronged plan for reducing overall reliance on fungicide: integrated pest management eliminating or reducing field residue, crop rotation, planting resistant varieties, and appropriate-use fungicides following the disease model guidelines.

            Franc suggests tank mixing or alternating fungicides with different modes of action to delay the selection for resistant isolates of the pathogen. If fungicide resistance management is to be effective, each fungicide partner needs to provide effective disease suppression if they were used alone, he said.

            On the Web: http://www.uwyo.edu/UWplant/faculty/G_Franc.asp

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