Development of a field biotest using artificial inoculation to evaluate resistance and yield effects in sugar beet cultivars against Cercospora beticola

Cercospora beticola resistance and disease yield loss relationships in sugar beet cultivars are best characterised under field conditions with heavy natural infection; this does not occur regularly under German climatic conditions. Since Cercospora resistance reduces the rate of pathogen development, high yield loss was observed in studies using artificial inoculation. Our study, therefore aimed to optimise inoculum density to obtain cultivar differentiation, which correlates to natural infection. In 2005 and 2006, field trials were carried out to determine the effect of different inoculum densities on Cercospora resistance of three sugar beet cultivars possessing variable resistance. The epidemic progress and white sugar yield loss (WSYloss) were determined and their relationship evaluated. An optimal inoculum concentration range (between 10,000-20,000 infectious Cercospora units ml(-1) inoculum suspension) was determined which allowed maximum resistance parameter differentiation in terms of C. beticola disease severity (DS), area under the disease progress curve (AUDPC) and WSYloss. The correlation between AUDPC and WSYloss was identical for all cultivars independent of the resistance level, demonstrating that tolerant reactions of the cultivars under study were not detectable. This study provides evidence that even under optimal inoculum levels necessary to obtain maximum differentiation between cultivars, climatic conditions are important for disease management, but remain unpredictable, indicating that artificial inoculation needs to be optimised, but that single field locations are not sufficient and reliable to evaluate Cercospora resistance.