Foliar application of indol acetic acid (IAA) and/or indol butyric acid (IBA) to onion cv. Giza 20 under field conditions, significantly reduced the incidence of either downy mildew or purple blotch diseases than the control. Selenium and potassium hydrogen carbonate (KHCO3), also affected the incidence of both diseases. However, spraying potassium hydrogen phosphate (K2HPO4) showed high efficacy in reducing downy mildew and purple blotch diseases. Yield of bulbs was increased by 93.5 and 53.3% than the control in response to the application of IAA (400 ppm) at 2010 and 2011 growing seasons, respectively. In the same respect, selenium (50 ppm) resulted 85.5 and 45.3% yield increase than control. Phenolic compounds were increased as compared to the control in response to the different tested treatments. The highest increment was noticed when IBA was applied. This was also recorded in total sugars, while KHCO3 gave the least increase of the total sugars than the control. Galben-Mancozeb was more effective than other treatments in reducing both diseases.