Impact of Cercospora Leaf Spot on Root Weight, Sugar Yield, and Purity of Beta vulgaris. W. W. Shane, Department of Plant Pathology, University of Minnesota, St. Paul 55108. P. S. Teng, Department of Plant Pathology, University of Minnesota, St. Paul 55108. Plant Dis. 76:812-820. Accepted for publication 22 March 1992. Copyright 1992 The American Phytopathological Society. DOI: 10.1094/PD-76-0812.

The impact of Cercospora leaf spot, caused by Cercospora beticola, on the yield and quality of sugar beets (Beta vulgaris) was studied in one field trial in 1982 and two trials in 1983. Fungicides were applied to cultivars of various disease susceptibilities to obtain different levels of disease severity. Plants were rated for disease severity with a spot-percentage scale according to the number of lesions per leaf at intensities <3% and with standard area diagrams at intensities ?3%. A disease severity of 50% approximately 10 days before harvest corresponded to relative dollar losses of 43, 27, and 27% for trials 1–3, respectively. Reduced payment per hectare was sometimes evident for epidemics of >3% severity by mid-September. Increased concentration of impurities and decreased sugar concentration in beet roots and decreased root weight generally were correlated with disease severity. Beets with higher concentrations of impurities yielded less pure sugar and more unrefined sugar (molasses) in the extraction process. In diseased plots, sugar loss to molasses had a minor impact on dollar return compared with losses attributed to reductions in root weight and sugar content.

Keyword(s): disease management, epidemics.