Randy Nelson

USING SPORE TRAPS FOR CERCOSPORA BETICOLA IN SUGARBEET

temperature of 25 to 35 o C and prolonged periods of high relative humidity or free moisture on leaves (Windels et al., 1998). Due to limited levels of resistance in commercial sugarbeet varieties grown in North Dakota and Minnesota, numerous fungicide applications are necessary to obtain acceptable levels of protection against Cercospora leaf spot. In order to assist growers in making economic fungicide applications, Shane and Teng (1984) developed the Cercospora prediction model. The model is comprised of percent disease severity based on field monitoring, and Cercospora advisory (a daily infection value based on weather conditions) (Shane and Teng, 1984, Windels et al., 1998). Since disease monitoring is a laborious process and requires a trained person, most growers rely on the Cercospora advisory component of the model to determine timing of fungicide application once symptoms are observed in the field. Since Cercospora leaf spot is a polycyclic disease, timely application of the first fungicide application is crucial in reducing the rate of infection, and contributes to effective season long disease control. Spore traps can be used to determine the timing of spore dispersal, and the number of spores dispersed. The information could be most useful for advising growers to apply the first fungicide application when Cercospora conidia are first dispersed, rather than the current practice of applying the first application when symptoms are present. The use of protectant fungicides preventatively would provide better disease control and result in less selection pressure for the development of resistant C. beticola isolates. MATERIALS AND METHOD