A new integrated pest management (IPM) model for cercospora leaf spot of sugar beets in the Po Valley

Cercospora leaf spot (CLS), caused by the fungus Cercospora beticola, is the most economically important foliar disease of sugar beets in Italy. It can be controlled with the integrated use of resistant varieties, cultural practices and foliar fungicides. Environmental conditions strongly influence the activity of C. beticola in the field and can be used to guide fungicide applications once the relationships are understood. To assist growers in making profitable decisions regarding the application of foliar fungicides for CLS control, a predictive model, developed by Minnesota and North Dakota State University was adjusted to Italian conditions and evaluated. The model was designed to predict the time of sugar beet infection by C. beticola was likely to occur based on hourly temperature and relative humidity data. CLS control is currently based on treatments scheduled using a “calendar” program. This system recommends to start fungicide applications on fixed date and continue regularly every 18-20 days. Three years of field trial evaluation of the CLS prediction model at several experimental sites, compared with the calendar program, has resulted in a savings of two fungicide treatments (corresponding to 170 €/ha) without a significant loss of yield. For control of CLS, multiple applications of the same fungicide during a growing season are unfortunately common. This practice provides the target fungus with the conditions to evolve resistance. Sensitivity assays of C. beticola to DMI and QoI fungicides were carried out on isolates collected in 2009 and 2010 in northern Italy by the University of Bologna and North Dakota State University respectively. Isolates principally came from trial plot but also from commercial sugar beet fields and were tested towards tetraconazole and difenoconazole, pyraclostrobin and tri-floxystrobin using radial growth and spore germination assays. All samples collected in 2009 showed EC50 values for QoI ranging from 0.0008 to 0.2195 μg/ml. The percent range with EC50 values >1 for isolates collected in 2010 for tetraconazole was 44%, for difenoconazole 84%, for pyraclostrobin 27% and for trifloxystrobin 34%. From a samples of isolates with EC50 values >1 μg/ml, the range of EC50 values (μg/ml) for tetraconazole was 3.4-70.0, for difenoconazole 2.0-69.5, for pyraclostrobin 1.5-43.6, and for trifloxystrobin 3.8-77.1