Four experiments were conducted during 2004/2005, 2005/2006, 2006/2007 and 2007/2008 seasons at Sakha Agricultural Research Station, Kafr El-Sheikh, Egypt to study the effect of six plant extracts on controlling powdery mildew disease of sugar beet caused by Erysiphe polygoni, and its effect on different characters. Six plant extracts were used i.e. Solanum nigrum, Pancratinum maritimum, Melia azedarach, Anthemis nobilis, Ammi visnaga and Mentha piperita in different concentrations, i.e. 1000, 3000 and 5000 ppm in the screen house on two sugar beet cvs. viz. Sultan and Glorious, under artificial inoculation by conidio-spores of E. polygoni combinations between M. azedarach andThiovit 80 were used, ½ M. azedarach + ½ Thiovit 80, ¾ M. azedarach + ¼ Thiovit 80 and ¼ M. azedarach + ¾ Thiovit 80. Those experiments were conducted in the screen house during 2004/21005 and 2005/2006. The combination was applied 3 days before and after inoculation. Data show that M. azedarach at the level of 5000 ppm gave the best disease control before and after inoculation for powdery mildew followed by Ammi visnaga. Data showed also that, mixing fungicide with plant extract lead to increasing efficiency of plant extract when ¼ plant extract + ¾ Thiovit 80, ½ plant extract + ½ Thiovit 80, ¾ plant extract + ¼ Thiovit 80, efficiency were 93.3, 89.9 and 84.5% before inoculation and 86, 77.5 and 69.9% after inoculation in comparison with the efficiency of Thiovit 80 before inoculation (94.7) and after inoculation (90.1%) for Sultan cv. while efficiency for Glorious the date showed the same trend as 93.5, 90.2 and 84.4% before inoculation, on the other hand, it recorded 87.4, 78.9 and 70.1% after inoculation for the three combinations, respectively. While, Thiovit 80 recorded efficiency of 95.6% before inoculation and 91.2% after inoculation. In the field experiment, M. azedarach (5000 ppm) and the combination ¾ M. azedarach + ¼ Thiovit 80, was used because it has high efficiency for controlling powdery mildew disease as well as to reduce pollution and costs. During 2006/2007, 2007/2008 seasons, disease severity (%) was reduced to more than 50% by utilization of ¾ M. azedarach + ¼ Thiovit 80, efficiency reached to 68.1% for Sultan and 70.1 % for Glorious during 2006/2007. Data also showed that chlorophyll content was high, 67.5 and 77.4 for Sultan and Glorious cvs. All studied characters were affected, like, root weight, TSS%, sucrose (%), purity and increase percentages. Increase % in root weight 61.1 and 58.8% for Sultan and Glorious, respectively. While, increase % in sucrose; was 62.6 and 37.6% for Sultan and Glorious, respectively. Phenolic compounds showed negative correlation between disease severity and phenolic compounds. AUDPC affected with disease severity (%), when disease severity increased, AUDPC increased. So, this research paper pointed out to the possibility of replacing fungicides by plant extracts of for disease control or of foliage diseases of sugar beet, either alone or mixed with Thiovit 80 to reduce the costs and environmental pollution as well as preventing residual effect of the fungicide on produced sugar.