Powdey mildew disease of sugar beet incited by Erysiphe betae (Vanha) Weltzien is an important disease of sugar beet ( Beta vulgaris var vulgaris ). Very little information is available on weather or how the pathogen survives. Experiments  were conducted to study the role disease transmission by chard (Beta vulgaris var cicla) that found as a common weed beet in sugar beet fields in Egypt.   Powdery mildew infection and cleistothecia of the pathogen were abundantly  occurred  on both chard and sugar beet plants.  . Microscopic observation during the sugar beet growing season indicated that by May more than 90% of the cleistocthia had generated under field conditions. In the several sample taken from sugar beet and chard plants from two different locations, the dimensions of conidia, cleistothecia and asci confirm that the isolates are typically belong to E. betae. Cleistothecia were formed on seed clusters of chard plants and 65% of cleistothecia contained intact ascospores. When chard seeds infected with cleistothecia planted in greenhouse with sugar beet under aseptic conditions, 30% of the plants examined developed powdery mildew symptoms. The percentage of mature cleistothcia on sugar beet leaves were 44- 45% and 47-52% in chard plants. The pathogenicity of ascospores from stored cleistothecia with different inoculation methods either by detached leaf method or by whole plant spray were succefull and lead to 45-50 % disease incidence in detached leaves under laboratory conditions while disease incidence was 55-75% in whole plant spray method in March and April 2007 trials