3.1.15
CONCEPTION AND METHOD OF AN OPTIMAL THRESHOLD-ORIENTED CONTROL OF CERCOSPORA BETICOLA ON SUGAR BEETS -- THE IPM SUGAR BEET MODEL

P WOLF, FJ WEIS and J-A VERREET

Institute of Phytopathology, Kiel University, Hermann-Rodewaid-Str. 9, 24118 Kiel, Germany

Background and objectives
In cultivable areas with high rainfall and average temperatures greater than 20C Cercospora beticola is the most important leaf blotch on sugar beets in the world. Leaf and sugar beet losses at levels of 50-60 and 10-30%, respectively, can be demonstrated. Depending on the infestation, this leads to losses of sugar yield of the order of 50%. The annual losses of yield and quality due to leaf blotch and the existing uncertainty of the exact diagnosis of Cercospora symptoms led in practice to the use of inflexible fungicide strategies and preventive routine measures. These are seldom associated with optimum biological and epidemic pathogen control with positive economical results. Within the framework of research based on nation-wide studies over many years, a threshold-based integrated pest management system - IPM sugar beet model - has been developed for pathogen control.

Materials and methods
Different sensible threshold values (limits on the pathogen population) connected with fungicides have been controlled regarding the biological-epidemic and economical effectiveness. The examinations verify the higher functionality of sensible threshold values (lower threshold in the pathogen population). By using the IPM conception, which assumes an exact diagnosis (qualitative=pathogen, qualitative=severity of the infestation), the location-specific and seasonal losses could be completely cushioned by a threshold-oriented treatment.

Results and conclusion
The IPM sugar beet model has been available since 1996 (Cercospora situation in 1996 - areas alerted: southern Bavaria 80%, Franconia 100%, Baden-Wurttemberg 80%, Rhineland-Palatinate 100%, southern Hessen 75%; in nothern Hessen and northern Rhine-Westphalia no alerts were issued). For simple use in a wide practice, academic threshold values (first application in the vegetation period from 15 June to 15 August (i) = 50% incidence of infection; second application from 15 July to 30 August and 15 September, respectively (ii) = 0.4% severity of infection) have been deduced from the data. For the practical use in the epidemic period (i) the threshold value is reached if five leaves out of 100 randomly chosen show a Cercospora-specific leaf blotch. Second threshold value in the epidemic pehod (ii) is reached if 60 leaves out of 100 randomly chosen show a Cercospora-specific leaf blotch. The different steps of development (visual material, control thresholds, proof of biological epidemic and economical effectiveness) and the realization in a wide practice have been demonstrated.

References
1. Verreet JA et al., 1996. In Proceedings of the 59th IIRB Congress, February 1996, pp. 55-69.
2. Wolf P et al., 1995. Journal of Plant Diseases and Protection 102, 574-585.