Six experiments were conducted in commercial fields, selected at Kafr El-Sheikh and Sharkyia Governorates, under natural
infection stress during 2010/11, 2011/12 and 2012/13 growing seasons. These experiments were carried out to determine the relationship between Cercospora leaf spot (CLS) severity and sugar beet yield, sugar content and juice quality in roots of cv. Oscar Poly (susceptible cultivar). 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%. Decreases in root weight, percentages, in sucrose, quality and white sugar yield (WSY) and increase in non-soluble solids with increasing severities were detected across the full range of severities (1-7%). Significant decreases in root weight, percentages of sucrose, quality and WSY at or above 20% severity were observed. The regression analyses showed significant linear relationship (P < 0.05) between disease severity and root yield in kg, percentages of sucrose, quality, WSY and white sugar yield loss. The relationships also showed a high negative significantly correlated between disease severity and root yield/ kg, percentage of sucrose, quality and WSY. While, positive correlation was found between disease severity and potassium, sodium, alpha amino acids (mM) and white sugar yield loss.