Sugar beet is regularly attacked by leaf diseases in Germany. Cerospora beticola, Ramularia beticola and Erysiphe beta all affect assimilation and reduce sugar beet yield and quality. During the past years, Cercospora as the most important pathogen also caused infections in areas of low infection risk. Several agronomic measures can be used to control infestation in heavily affected areas, such as a wider crop rotation or the cultivation of varieties with a reduced susceptibility to Cerco¬spora. Efficient systemic fungicides should only be applied on failing of all other measures and according to thresholds as low pathogen attack rarely induces significant damage. A fungicide application taking into account damage thresholds is the most important tool to reduce the risk of pesticide application in sugar beet. From 2001 on, a nationwide three–years research project started at the Institut für Zuckerrübenforschung (IfZ) Göttingen (funded by Deutsche Bundesstiftung Umwelt) in cooperation with Landesanstalt für Pflanzenbau und Pflanzenschutz (LPP) Mainz and including numerous regional partners. The aim of the project is to develop a complex crop protection system within the context of an ecologically sound sugar beet cultivation. The system integrates threshold values, forecasting models, tolerance-levels of sugar beet varieties and fungicide effects according to the criteria of integrated plant protection and optimises the control of fungal leaf diseases on sugar beet. Disease severity, yield and quality data were obtained from comprehensive field trials and a nationwide monitoring. At about 40 % of trial sites, disease severity on beets was higher than 20 % at harvest. Beside typical infestation areas (Rheingraben, Niederbayern), several sites in dryer areas of mid Germany were strongly attacked, too. In all cropping regions and on numerous sites, disease severities of below 20 % at harvest were found, caused by a low infestation with Cercospora. One reason of the different degrees of disease serverity is the slow spread of Cercospora on sites with a lowered infestation pressure. The development of disease incidence correlated strongly with final disease serverity. In Germany, sugar beet growers are requested to observe fields when disease thesholds are reached. Present thresholds for fungicide application up to mid-august were inappropriate at sites with lower infestation, as infestation often increased more slowly here compared to areas with high infection pressure. Weekly increases of disease incidence are recorded by leaf disease monitoring. This data can be used to improve thesholds, including the weekly progress of disease. Disease incidence on sites with low infection pressure nevertheless increased 15 %, which is comparable to increases at high risk sites (week 31 to 32), although the increase was retarded three weeks. In addition to fixed thresholds, endangered sites could be better identified by assessing the weekly differences in disease incidence. Thereby, information on epidemic progress could add to decision making, improving the actual model.
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