Abstract: Background and Objectives: Farmers in many big cities have been using sewage polluted water for irrigation of crops. That increase the heavy metal contents of soils that affect on plant growth parameters and which makes the plant weak against any infection. The study was conducted to study the impact of the irrigation by drainage water mixed with untreated sewage water on the susceptibility of Giza 3 cultivar of Vicia faba to infection by Botrytis fabae pathogen in Egypt. Materials and Methods: Pathogen was isolated from infected Vicia fabe cultivars in Egypt. The drainage water mixed with untreated sewage water samples were collected from study area. Giza 3 was irrigated with drainage water mixed with untreated sewage water and infection with Botrytis fabae under greenhouse. Disease severity, enzyme activities, level of genes expression, percentage of total number of abnormalities cell, electrolytic leakage, growth parameters and mitotic index were determined. Results: The disease severity, enzyme activities (POD and PPO), level of POD and PPO gene expression, percentage of total number of abnormalities cell, electrolytic leakage (EC) and photosynthetic rate were increased. While photosynthesis pigments (chlorophyll a, b and the carotene), the fresh, dry weight of shoot and root and mitotic index were decreased. Conclusion: The irrigation by drainage water mixed with untreated sewage water increase the susceptibility of Vicia faba (Giza 3) cultivar to infection by Botrytis fabae.