| Cercospora leaf spot (CLS), caused by Cercospora beticola, is the most economically important foliar disease of sugarbeet in Minnesota and North Dakota. Seasonal survival, mechanism of dispersal, and primary infection site of C. beticola was studied. Seasonal survival of C. beticola was investigated by burying infected sugarbeet leaf residue at depths of 0, 10, and 20 cm for 10, 22, and 34 months. Inoculum left on the soil surface at the 0 cm depth survived the longest (22 months) compared to that buried at the 10 (10 months) and 20 cm (10 months) depths. Significantly higher disease severity was observed on exposed plants than caged plants with or without ground cover, suggesting that wind was the major factor for dispersal of C. beticola inoculum from the primary sources. Sugarbeet leaves, roots, and stems were inoculated to determine the primary infection site for C. beticola. Cercospora leaf spot symptoms were observed only on plants that were leaf inoculated, suggesting that leaves were the primary site of infection for C. beticola.; Temporal distribution was studied by trapping conidia with volumetric spore traps daily from July through September 13, 2003 and 2004. Several peaks of conidia per cubic meter air were observed during the season, which had significant correlation with mean temperature when daily hours of RH were greater than 87% and 1 d after a rainfall period. This research showed that C. beticola conidial concentration determined by trapping can be used to estimate potential disease severity in a disease-forecasting model.; Research was conducted to improve the CLS management model. Fungicide application significantly reduced maximum disease severity (ymax) and area under disease progress curve (AUDPC) when compared with the untreated control. The use of RH greater than 87% for calculating daily infection values (DIVs) was validated. All sugarbeet growers in Minnesota and North Dakota can use one recommendation for application of fungicides; that is, the first fungicide should be applied at disease onset, and subsequent applications should be made based on DIVs and disease severity. Conidial dispersal can also be used as a decision making tool to initiate preventative fungicide application. |
